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Understanding and Leveraging the Nitrogen Cycle for Informed Almond Fertility Decision-Making

By
Matt Comrey | Technical Nutrition Agronomist, Wilbur-Ellis Agribusiness
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Figure 1. Schematic of nitrogen cycling including ammonia volatilization in the soil/plant/atmosphere environment.

There is no doubt that almond producers have had to navigate numerous challenges over the past several years. Drought conditions, declining nut prices and regional frost events have forced growers to implement a renewed focus on input management as it relates to agronomic efficiency. Agronomic efficiency can be improved through the implementation of the 4R’s of Nutrient Stewardship, which include Right Source, Right Rate, Right Time and Right Place. Improved grower understanding of various nutrient cycles and soil nutrient loss mechanisms are allowing growers to incorporate a sophisticated layer of nutrient management that has evolved over recent years. By understanding and leveraging both the nitrogen cycle and the 4R’s, almond producers can make informed fertility decisions that maximize nitrogen use efficiency (NUE), allowing them to successfully navigate both economic and environmental headwinds.

Factors Impacting the Nitrogen Cycle
Nitrogen management is important, not only due to product cost but also because of the many loss mechanisms associated with less-than-ideal soil conditions. Almonds require a large amount of annual nitrogen to produce a commercial yield. CDFA suggests that on average, 68 lbs N are removed per 1,000 pounds of kernels with another 30+ lbs needed to maintain adequate growth (depending on tree age.) Almonds rely on large amounts of N to build both amino acids and proteins as well as for the formation of chlorophyll. To adequately supply the appropriate amount of N in an efficient manner, growers will need to understand how environmental factors impact the nitrogen cycle (Figure 1) and ensure that applications are available during periods of peak uptake and demand.

Figure 2. Nitrogen allocation in various tissues. Almond trees are quite good at remobilizing N resources from perennial plant parts to annual tissues during the bloom period through fruit set.

Almond trees are quite good at remobilizing N resources from perennial plant parts to annual tissues during the bloom period through fruit set. Figure 2 depicts this remobilization concept with total N remaining constant through March 12, accumulating in annual tissues while decreasing in perennial tissues. The graphic suggests that there is little uptake from applied N during this period but does not necessarily mean growers should avoid applications at this timing. Depending on soil and other environmental conditions, application timing can vary greatly during this period.

Table 1. Seasonal nitrogen allocations can be a valuable guide to timing applications but lack some of the nuance needed to understand the right rate as well as the right source to consider.

Proper application timing not only requires understanding crop uptake demand, but also how soil conditions impact nutrient form. Because common N fertilizers used by almond producers contain different combinations of urea, ammonium and/or nitrate, different considerations related to application timing are needed. The majority of almond N uptake is in the nitrate form. This requires N applied in either the ammonium or urea form to largely be converted to nitrate, making it more available for uptake. The conversion reactions such as nitrification (conversion of ammonium to nitrate) are governed by soil moisture and soil temperature. Growers should also consider rain forecasts when making early N applications because spring rain events can possibly leach nitrate out of the rootzone, resulting in not only an environmental impact but an economic impact as well.

Figure 3. Relationship between nitrification rates and soil temperature. The speed at which ammonium is converted to nitrate can be dramatically different depending on soil temperature.

Figure 3 illustrates the relationship between nitrification rates and soil temperature. The speed at which ammonium is converted to nitrate can be dramatically different depending on soil temperature. Figure 4 is a graph generated from Wilbur Ellis’ Probe Schedule depicting soil temperatures of a mid-textured loam soil at different depths from mid-February through mid-March. Shallow soil temperatures during this period were approaching and at times exceeding 60 degrees F. When we use this information with the nitrification table, we see that it can take up to three weeks for 50% of our applied ammonium to be converted to nitrate. Heavier, cooler soils experience much slower nitrification than coarser textured soils, meaning growers should take their soil conditions, weather forecasts and N source into consideration when timing applications.

Reconciling Data into Nitrogen Application Decisions
A general rule of thumb for managing N throughout the almond growing season involves allocating large portions of the N budget to applications during four major timings. Table 1 outlines seasonal nitrogen allocations, which can be a valuable guide to timing applications but lack some of the nuance needed to understand the right rate as well as the right source to consider.

Figure 4. Soil temperatures of a mid-textured loam soil at different depths from mid-February through mid-March. Shallow soil temperatures during this period were approaching and at times exceeding 60 degrees F.

Many growers and consultants sample almond leaves throughout various time periods of the season to understand the status of various nutrients including N. It is not uncommon for leaf tissue samples to be evaluated for N status monthly starting in April through mid-July. July leaf tissue samples can be an excellent tool for growers to adjust or implement postharvest fertility applications to address any nutrient shortages prior to the remobilization of these nutrient resources to storage organs. If leaf N levels during the mid-July period are inadequate, then almond trees may not be able store enough N for next year’s bloom period. This means growers and consultants alike should consider 2022’s July tissue levels as an important data point when developing the spring fertility plan for 2023.

Another important consideration when developing the spring fertility plan is the overall yield of the previous year’s crop. When I discuss postharvest fertility with growers, I describe it as the final opportunity of the season to reconcile fertility inputs with crop exports. For most of the season, growers making most fertility decisions are guided via certain assumptions about crop load. Growers can make a pretty good assessment of crop load by late spring and into early summer, adjust their fertility accordingly and then monitor via leaf tissue analysis and/or field observations throughout the remainder of the season. Once crop starts to get harvested and loads are delivered to the processor, a clearer picture of final yield develops. If the postharvest fertility program was shorted/omitted or if yield was dramatically reduced via frost or some other phenomenon, spring fertility may need to be adjusted accordingly. Growers in the Sacramento Valley that experienced extreme crop loss from freezing temperatures in 2022 will likely need to work with their advisors and adjust spring fertility to feed what is likely to be a large crop in 2023.

Almond growers have the difficult task of reconciling an enormous amount of data to make fertility decisions for their operation. These data points include lag measures such as previous year’s July nutrition levels and/or yield to lead measures like current year’s estimated N usage based on yield goals. Growers are likely to experience economic pressure through elevated input costs and weak nut prices as well as regulatory pressure from state and environmental agencies. N management will continue to be a focus throughout California, and growers who can effectively leverage their understanding of the nitrogen cycle as well as the 4R’s will be better positioned for the coming 2023 season.

Phosphate vs Phosphite Part Two: Can Phosphites Serve the Function of a Biostimulant?

By
JW Lemons | CCA, CPAg, Verdesian Life Sciences
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Table 1. Beneficial effects of phosphite (Phi) as a biostimulator in vegetable crops.

Phosphite has been a controversial topic for years. Its use and benefits are argued in hundreds of research papers across the world’s scientific communities. Is it a fertilizer, a biostimulant or a fungicide? These questions are discussed in multiple university research results. I believe that if we look carefully, we can conclude phosphite serves all three functions. As with everything we do with chemicals and nutrition, we need to be aware of possible negative effects. We also need to determine how we are using the phosphite materials and the results we are seeking.

Phosphite (PO33-; Phi), a reduced form of phosphate (PO43-; Pi), is widely marketed as either a fungicide or fertilizer or sometimes as a biostimulant. Because CCAs, growers and distributors see the product marketed in all three ways, this can cause confusion. Each designated use will most likely be tied to the phenology (growth stage) of each treated crop. Timing is critical to determine which effect we are seeking. In this article, we broach the controversial topic of Phi as a biostimulant.


In Research
Many researchers believe that Phi does not attribute to the growth and health of the plant. They believe that the positive response to Phi is attributable to the disease suppression by Phi. Lovatt and Mikkelsen (2006) emphasized Phi, if used at appropriate rates, can provide stimulation to plants that may not occur with Pi. They suggested Phi is more than just a fungicide; for example, it increases floral intensity, yield, fruit size and total soluble solids. In addition, combinations of Phi and Pi ions are believed to be more effective than either Pi or Phi alone in plant assimilation (Foster et al. 1998; Young 2004).


With all my studies and visits with research teams, I believe Pi is becoming accepted by agriculture as a biostimulator. Even without a consensus on its physiological function across agriculture and horticulture, too many positive effects are evident. As long as there is sufficient Pi present, Phi continues to have a positive effect on plant metabolism. Today, Phi is emerging as a potential inductor of beneficial metabolic responses in plants as it has demonstrated its effectiveness against different stress factors and has improved crop yield and quality. Important progress has been made in the field of Phi uptake, transport and subcellular localization, and there is still a more in-depth understanding of the fundamental processes behind the effects of Phi on plant metabolism needed.

Moor et al. (2009) found the application of Phi does not affect strawberry growth or yield compared to traditional Pi fertilization, although it does increase fruit quality by activating the synthesis of ascorbic acid and anthocyanins. Similarly, Estrada-Ortiz et al. (2013) found beneficial effects of Phi on strawberry fruit quality and induction of plant defense mechanisms (Estrada-Ortiz et al., 2011, Estrada-Ortiz et al., 2012), which has also been reported by Rickard (2000) in several crop species and cultivars. Likewise, Glinicki et al. (2010) reported beneficial effects of Phi on the growth parameters of three strawberry cultivars.

Table 2. Beneficial effects of phosphite (Phi) as a biostimulator in fruit crops.

As a Certified Crop Consultant and Certified Professional Agronomist, based on research studies and trials, I believe that phosphites are biostimulants. Couple that with the ability of phosphite to enter and translocate within a plant and you have a formidable tool. The following statement and sited research results confirms this:

“Phosphite has been found to display systemic effects and high chemical stability in plant tissues, though it also shows great mobility throughout the whole plant. This mobility facilitates the penetration and transport of the foliar-applied Phi to the rest of the plant, including the roots.” – Smillie et al. 1989, Brunings et al. 2015

By definition, “A plant biostimulant is any substance or microorganism applied to plants with the aim to enhance nutrition efficiency, abiotic stress tolerance and/or crop quality traits, regardless of its nutrient content.” By extension, plant biostimulants also designate commercial products containing mixtures of such substances and/or microorganisms. Research projects and results are too numerous to quote them all.

Alternative Nitrogen Management Strategies for California Rice Systems

By
Bruce Linquist | UCCE Rice Specialist, Karla Cornelio | Graduate Student Researcher, UC Davis and Telha Rehman | Graduate Student Researcher, UC Davis
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Aqua-ammonia being applied to a rice field. General findings from research show that aqua or urea applied to a dry soil before flooding resulted in the highest yields at harvest (all photos courtesy B. Linquist.)

Most rice in California is established and grown in what is referred to as a water-seeded system. In this system, after the field is tilled and leveled, most (70% to 100%) of the nitrogen (N) is applied as aqua-ammonia (aqua) injected 3 to 4 inches below the soil surface. After this, the field is flooded and then aerial seeded by airplane. The field stays flooded, resulting in anaerobic (low oxygen) soil conditions. The deep placement of fertilizer and the anaerobic soil conditions protect the fertilizer N from gaseous losses due to ammonia volatilization or nitrification/denitrification. Very little of this preplant N fertilizer is taken up by the crop during the first month, so it needs to be well protected by the flood layer. Using this practice, fertilizer N is used very efficiently. In addition to this preplant aqua-N, some starter N (containing P, K and other nutrients) are applied early in the season, and a top-dress N may be applied mid-season.

What if Aqua is Not Available?
While this strategy has proven to be efficient and effective, there are situations where it may not be possible to apply aqua. For example, in recent years, supply chain issues have limited the availability of aqua. Also, heavy rain following seedbed preparation (but before fertilizer application) makes it difficult and inefficient to apply aqua. It is not advisable to apply aqua into a moist soil. Not only is it difficult for the equipment on wet clayey soils, but the aqua will start to nitrify (convert from NH4 to NO3) in moist soils, resulting in nitrate accumulation which will denitrify (NO3 lost as N2 gas to atmosphere) when the field is flooded.

So, what is a grower to do if aqua is not available? We evaluated six products applied at different times (19 treatments) at two locations in 2020 and 2021. The products included aqua, urea, ammonium sulfate and three enhanced efficiency N (EENF) fertilizers (Super U, Agrotain or Agrocote). We applied the products at a single dose at various times including preflood, one day, one week and two weeks after flooding. We also had a split application where the N rate was split and applied at four different times during the season.

Some general findings were that aqua or urea applied to a dry soil before flooding resulted in the highest yields at harvest. This confirms other earlier findings we have made. Ideally, if urea is used, it should be worked into the soil with a harrow before flooding, although if urea is applied to a cloddy soil, the urea granules tend to fall deeper into the soil via gaps between clods. It is important that urea is applied to dry soil so that the N fertilizer does not start to nitrify before field is flooded and to avoid ammonium volatilization.

Our second major finding was if a preflood application to a dry soil is not possible, the next best scenario is to split the N rate. The total N rate split at 15%, 35%, 35% and 15% at three, four, five and six weeks, respectively, after planting will likely give the best results. The idea behind split applications is that the fertilizer is being applied when the crop needs it most. This results in rapid fertilizer N uptake and lowers the chances for significant N losses. However, the fertilizer N is being broadcast into the flood water, and this may result in some volatilization and denitrification losses. At one of our sites, the data suggested if one were to split the N, the total N rate may need to be increased compared to aqua or urea applied before flooding.

Third, using the EENFs, such as Super U, Agrotain or Agrocote, had no benefit over applying urea alone. They did not improve yields, and in some cases, yields were reduced. These are expensive fertilizers, and we found no additional benefit to their use.
Finally, using ammonium sulfate did not affect yields compared to using urea. In our studies, we applied sulfate fertilizer to make sure S was not limiting. In soils deficient in S, applications of ammonium sulfate may be of benefit.

Testing the Green Seeker in the field. This instrument measures the Normalized Difference Vegetation Index of the canopy and can help decide whether a top-dress N application is needed if N deficiency is observed or expected.

Decide if Top-Dress N is Necessary
We recommend applying the full rate of N at the beginning of the season in the aqua and starter fertilizer application. Regardless of the basal strategy, including those just discussed, top-dress N applications are needed if the plant is N deficient or you suspect it will become so. At PI (45 to 55 days after planting), the crop should be accessed to determine if a top-dress of N fertilizer is necessary. A good assessment is important because not applying N when needed can lead to a reduction in yield; however, applying N fertilizer when it is not needed can lead to lodging, delayed maturity, increased incidence of disease and reduced yields. The GreenSeeker (or similar instrument), which measures the NDVI (Normalized Difference Vegetation Index) of the canopy, is a tool we have tested for this purpose. The best way to do this is to develop a Sufficiency Index (SI). The sufficiency index is calculated using the NDVI reading from the field test area divided by the NDVI from an enriched N strip (representing a crop with unlimited N.) The N-enriched strip is an area where extra N was added to the field (could be done by overlapping an area with an aqua rig or a small area where you intentionally added extra N.) The SI will be a number between 0 and 1. The lower the SI, the more N-deficient. For example, if the N-enriched strip gave an NDVI value of 75 and the field test area gave an NDVI value of 69, the SI would be 0.92 (69/75 = 0.92). Our research has indicated it is cost-effective to apply a top-dress of N fertilizer when the SI is below 0.95, although this depends on the price of rice and cost of applying fertilizer. This could also be done using a drone which would cover a larger area. If using a drone, we recommend using a camera that can measure NDRE (Normalized Difference Red Edge). We have found NDRE to be more precise than NDVI when quantifying the nitrogen status of a crop.

More on this can be found on our website at https://rice.ucanr.edu/.

Pulling the Trigger for the Start of Irrigation in the Spring: Too Much Too Soon for Walnuts?

By
Ken Shackel | Plant Sciences Department, UC Davis, Allan Fulton | UCCE Soil and Water Advisor, Emeritus,, Kari Arnold | UCCE Orchard and Vineyard Advisor, Stanislaus County, Bruce Lampinen | UCCE Specialist, UC Davis and Luke Milliron | UCCE Orchard Advisor, Butte, Glenn and Tehama Counties
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Figure 1. An ailing tree at the Stanislaus site in 2018 showed signs of deterioration. Although the trunk was somewhat sunken at the soil line and necrosis was forming under the bark (center photo), samples were collected multiple times, but no Phytophthora spp. were isolated/found. This tree happened to be included in the delayed irrigation treatment and during the passing of three years appears to be recovering, specifically showing greater shade under the tree canopy at midday since the beginning of the trial in 2018 (photos by K. Arnold.)

Walnuts are generally regarded as very sensitive to water stress. Severe stress and defoliation can occur when irrigation is reduced in the summer or discontinued entirely for harvest. Since walnuts depend on stored soil moisture during this time, growers were historically advised to start irrigation early in the spring to save deep soil moisture ‘in the bank’ for use later in the season. However, research findings in a Red Bluff, Calif. walnut orchard have seriously challenged this conventional wisdom. In fact, trees that were given an early start of irrigation (late April) showed more water stress at harvest than trees that were given a delayed start of irrigation (late May/early June). Surprisingly, this occurred even though the delayed start trees received substantially less water (about 28 inches throughout the growing season) than the early start trees (about 38 inches). The Red Bluff orchard is on a deep silt-loam/fine sandy-loam soil. However, similar results are being found in one Stanislaus County orchard on heavier clay soil and one orchard in western Tehama County on stratified soils with gravelly subsoils and much lower water holding capacity.

Using the Right Tool
In many commercial orchards, in-season tree water stress is monitored by measuring midday stem water potential (SWP) using a pressure chamber (a.k.a. “pressure bomb,” see sacvalleyorchards.com/manuals). This same tool could be used, specifically, to decide when to start irrigation in the spring with the appropriate information on this subject. As a starting point, there is a reference level of SWP that is expected for a fully irrigated (non-stressed) walnut tree, which is called the “baseline” SWP. For more information about baseline SWP and how to obtain this value for a particular location, day and time, we suggest the following websites:

Baseline and advanced interpretation explained: sacvalleyorchards.com/manuals/stem-water-potential/using-baseline-swp-for-precise-interpretation/

Baseline values calculated for you at: informatics.plantsciences.ucdavis.edu/brooke_jacobs/index.php

Using the Tool to Trigger the Start of Irrigation
We began testing in 2014 in a nine-year-old commercial Chandler/Paradox orchard planted at 18 x 28 feet (86 trees per acre) on a deep, well-drained silt-loam/fine sandy-loam soil near Red Bluff. The test continued through 2019. The design of the experiment was simple: we compared control trees given 100% irrigation (see below) starting about 30 days after leaf-out to trees that were not irrigated until a trigger level of SWP was reached. We tested five trigger levels for the start of irrigation: a grower control (typically starting irrigation while the trees were still near baseline SWP), or 1, 2, 3 or 4 bars drier than baseline SWP.

We divided the field into 4 rows x 11 tree plots and had five individual plots for each trigger level. In total, the test consisted of 12.5 acres. Starting after leaf-out (about the third week of April), we measured the SWP of two middle trees in each plot every three or four days. When the average of those trees reached the trigger on two consecutive dates, we opened the sprinkler control valves to the tree rows in that plot. From then on, the plot was irrigated whenever the control plots and the rest of the orchard were irrigated.

Figure 2. Summary of average orchard water requirement (ET-rain) and applied irrigation for all delayed irrigation tests to date (2014-20). Daily CIMIS values for orchard water requirements were calculated beginning on April 1 based on current walnut crop coefficients, for each site and year, and averaged. Irrigation applied to all delayed treatments for each site and year were averaged for 10-day periods over the same seasons.

Initial Results in 2014
We expected that a 1- or 2-bar trigger might cause mild water stress with minimal effect on the trees, but the 3- or 4-bar triggers would show some detrimental effects. However, we were not sure how long of a delay would result from waiting to start irrigation using any of these trigger levels. We were also unsure if trees with late triggers would always be ‘behind’ in their water needs and experience severe water stress at harvest because we could not apply a ‘catch-up’ irrigation to any of the delayed trees.

In 2014, the 1-bar trigger occurred about the same time as the grower control, but much to our surprise, waiting for the 2-bar trigger gave one to two months of delay (depending on the plot), with the 3- and 4-bar triggers giving slightly longer delays (Table 1). Longer delays also resulted in less irrigation. In 2014, the control trees received 100% of calculated evapotranspiration (ET, see anrcatalog.ucanr.edu/ pdf/8533.pdf), whereas the 1- through 4-bar trees ranged from 89% to 66% of this value, respectively (Table 1).

Table 1. Irrigation start dates, seasonal irrigation applied (in inches and as the equivalent percent of irrigation requirement, calculated from ET minus in-season rainfall) and crop yield for each of the irrigation treatments imposed in the first year of the study (2014).

There were some negative effects on crop yield, with the 4-bar trigger reducing yield by about 10% (Table 1, see page 16), but there were also some positive signs. For instance, at harvest in October, the 2-, 3- and 4-bar triggers had a healthier canopy appearance than the controls. This matched our SWP measurements, which indicated that the delayed trees were less stressed than the controls (Table 2). This was the most surprising result from the first year of the study; during the delay period (May, June), the longer delays were associated with more stressed (more negative) SWP values as expected, with the controls being closest to the baseline. However, by harvest, the opposite was the case, with the controls being furthest from the baseline (Table 2).

Table 2. Average SWP measured in May and June 2014 when irrigation was being delayed in most of the treatments, and average SWP in October around harvest (October 17, 2014). Also shown are the baseline SWP values for the same time periods.

Trial results for 2015-18
Due to the overall improved appearance of trees in the delayed plots at harvest compared to the controls, the grower’s standard (control) irrigation start time in the entire orchard, including our control plots, was gradually delayed each year after 2014. Water applications in the orchard and the control plots became substantially less than 100% of the seasonal irrigation need (Table 3, see page 16). Yields also generally improved across treatments compared to 2014, even though canopy size as measured by midsummer ground shaded area has remained stable at 86%. Even with the changes over time that occurred in the control trees, delays associated with a 1- to 4-bar trigger showed small but consistent improvements in percent edible yield and relative value as well as substantial savings in water (Table 3). There were also indications of small but consistent increases in nut load. However, since nut load is determined by many factors, ongoing research in additional orchards is being conducted to determine if this effect is consistent.

Table 3. Average irrigation start date (and equivalent days after leaf-out), seasonal irrigation applied in inches (and equivalent percent of the seasonal irrigation requirement, as in Table 1), yield, percent edible yield, relative value and crop relative value (and equivalent percent of the control treatment.) Relative value is an index combining the two main economic drivers of walnut value (percent edible yield and kernel color), and crop relative value is Yield x Relative value.

Soil Moisture Storage and Possible Implication for Root Health
The soil in this location is a deep, well-drained silt-loam/fine sandy-loam, and soil moisture measurements have indicated that the trees in this orchard have access to at least 10 feet of stored soil moisture. In most years, rainfall is also sufficient to refill this soil profile. Hence, using the pressure chamber to determine when to start irrigating has enabled the grower to take maximum advantage of this soil moisture resource, potentially improving soil aeration and overall root health. This may be one of the reasons the delayed trees appeared healthier and less stressed around harvest compared to the controls. Answering this question with greater confidence will require more research focused on the root system.

Taking the Practice Beyond Red Bluff
It is also important to test the delayed irrigation approach on different soil types. Because this project was conducted in a relatively high rainfall area in the Sacramento Valley, extending these dramatic results to other areas within the state with differing rainfall and soils should be done with caution. We currently have two different trials underway to further test the merits of delaying the start of irrigation in walnut (a second site in Stanislaus County on heavier clay soil and a third trial in western Tehama County on stratified soils, with gravelly subsoils and much lower water holding capacity.) Both trials are a smaller-scale version of the Red Bluff trial.

In the Stanislaus County orchard consisting of Chandler on Vlach, results after three years suggest that similar benefits of delaying the first irrigation may be possible in this higher-clay-content soil site. Some ailing trees have shown partial recovery in the delay treatment, indicating the possibility of too much water being applied too early (Figure 1,see page 14). Yield at the Stanislaus site was not affected when irrigation was withheld until readings of 2 bars drier than baseline.

After two years, results from the western Tehama County test on soils with lower water holding capacity and soil layers that may restrict root depth suggest there may still be some benefit of delaying irrigation in terms of less tree stress at harvest, reduced water costs, and improved edible kernel. However, because of the lower water holding capacity of the soils, the delay may only be about one to two weeks with water savings of about 4 inches.

A key feature of using SWP to manage irrigation is that it provides growers with an orchard-specific measurement of tree water stress and hence allows them to safely take advantage of the existing soil moisture resource, regardless of soil depth, type and quantity of the stored soil moisture. Using SWP to delay the start of irrigation resulted in healthier-looking, less-water-stressed trees at harvest, challenging the conventional wisdom that an early start to irrigation is beneficial because it allows the saving of deep soil moisture ‘in the bank’ for use later in the season. Quite possibly, keeping this savings account too full in the spring may cause more problems than it solves.

The benefits of waiting to irrigate in spring until trees read 2 to 3 bars drier than the baseline despite the stark differences between these three sites is a powerful testament to the value of using the pressure chamber. Once growers use the pressure chamber to trigger the start of irrigation, they can continue to trigger irrigations throughout the season by waiting for SWP readings of 2 to 3 bars drier than the fully watered baseline.

Baseline and other information for interpreting SWP readings can be found at sacvalleyorchards.com/manuals/stem-water-potential/pressure-chamber-advanced-interpretation-in-walnut/.

These trials are also challenging the conventional wisdom that we must irrigate to keep up with ET to have healthy and high-yielding walnut orchards (Figure 2, see page 14). Stay tuned as these two new trials continue to add to our collection of experiences.

Research Updates on Integrated Pest Management for Citrus Mealybug in California Citrus

By
Sandipa Gautam, Ph.D., | Area Citrus IPM Advisor, Fresno, Madera, Tulare and Kern Counties
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Figure 1. Adult female citrus mealybug showing amber-colored eggs. Note the distinctly segmented body with a vertical line through the mid-section, a characteristic feature of citrus mealybug (photo by S. Gautam.) 2022-09-30T23:08:37Z

Citrus mealybug infestations continue to increase in the San Joaquin Valley (SJV) making this species an emerging concern for citrus growers. Although known to be present in California for more than 50 years, it was kept in check by natural enemies (Ebeling 1959) and was not a pest of concern until recently. As such, citrus mealybug is an understudied pest in California citrus systems, and little is known about its biology and field ecology, monitoring and threshold, and control options, making management challenging. In 2022, we initiated research studies to improve knowledge about field ecology, monitoring and management of mealybug in SJV citrus.

Citrus mealybug, Planococcus citri, is a soft-bodied, oval, flat and distinctly segmented insect covered with white mealy wax (Figure 1). This small polyphagous sap-sucking insect prefers to live in between fruit clusters or under the thick leaf canopy, therefore avoiding detection when population is low. Infestation causes chlorosis, leaf drop and stunted growth, while direct feeding damage results in tissue discoloration around feeding spots, fruit deformity and contamination of foliage and fruit with honeydew and sooty mold (Figure 2), resulting in loss of yield and marketability of fruit.

Figure 2. Navel orange infested with citrus mealybug. Infestations are usually found protected under the thick canopy of leaves. Also note the sooty mold on leaves (photo by David Haviland, UCCE.)

Adult female citrus mealybugs are ~3 millimeters long and have distinctly segmented bodies covered with white mealy wax. A female can lay up to 600 eggs in a lifetime, in a group of 5 to 20, deposited and protected within cottony ovisacs (Figure 1). These eggs hatch in 3 to 10 days depending on temperature and crawlers disperse to different parts of the plant looking for feeding sites. All life stages are mobile, and they spread by crawling from tree to tree, carried by ants, blown by wind and picked up by birds, machinery or labor crews. Immatures and adult females feed on leaves and fruits. Males have wings, are short-lived and do not feed.

Field Ecology for Monitoring
Citrus mealybug overwinters as eggs or crawlers in protected areas of the tree including fruit. Greenhouse research has shown eight complete generations in one year (Myers 1932). To generate information on seasonal activity and the number of generations in the SJV, we monitored citrus mealybug in 13 trees in a block known to have mealybug infestations in 2021. Double-sided sticky tapes were wrapped around the trunk and four main inner branches. Traps were replaced weekly, and the number of mealybugs caught on the traps was counted every week. In addition, males were monitored using a pheromone trap.

We found that mealybugs were present within the tree canopy throughout the season, but the areas where they aggregated changed as the season progressed. Overwintering eggs/crawlers serve as an inoculum for a new season. The first-generation activity starts in March as eggs hatch and crawlers move from the overwintering sites (cracks and crevices on trunk, leaves protected under thick canopy, fruit or from the ground) to new leaves and then to fruit. When fruit is present, most of the infestation was found on fruit. Males caught on the trap card showed six distinct peaks that preceded the crawler emergence, suggesting six complete generations of mealybug in the valley (Figure 3). This seasonal occurrence pattern can be used to scout for mealybug.

Figure 3. Citrus mealybug population in a citrus block. Adult and crawlers on sticky tape trap count (left Y axis) overlayed on mealybug males/trap (right Y axis). Flight peaks at the arrowheads suggests a new generation.

January to March: fruits or leaves. Mealybugs like protected areas. Check inside the tree canopy, fruit (navels and under the calyces) or leaves (underside) for overwintering adults, egg sacs and crawlers. Presence of sooty mold and white mealy wax is an indicator (Figure 2).

April to June: crawlers emerge and start moving into new flush or fruit. Egg masses, crawlers on the trunk and inner branches.

July to December: on fruit, feed and multiply. Overlapping generations can be present.
Monitoring can also be done by using a pheromone lure and traps. Place lure and traps by March 15, before the first-generation activity starts and monitor throughout the season. If you catch males on trap, scout trees for infestation and make pesticide applications when needed.

Management Options
Citrus mealybug is naturally regulated by natural enemies and predaceous insects. But recent outbreaks suggest that this ecological balance is shifting. Although very little is known about mealybug management in citrus systems, incorporating cultural, biological and chemical tools is encouraged as researchers continue to study the seasonal occurrence, efficacy and timing of pesticide applications for improved management.

Cultural control
Female and nymphal mealybugs are wingless and spread to new areas by humans, equipment, wind or birds. Sanitize equipment before moving to new areas of the grove. Pruning to open tree canopy and hedging trees to prevent canopy overlap will help slow the crawler movement and increase natural enemy activities.

Biological control
Several natural enemies are identified as effective biocontrol agents for controlling mealybug (Grafton-Cardwell et al. 2021).

Mealybug destroyer, Cryptolemus montrouzieri, is known to be a highly effective predator and is most effective when population pressure is high. This predator does not survive cold winters. It is readily available from commercial suppliers and can be released in orchards where mealybug was a problem in the previous year.

Anagyrus spp. and Coccidoxenoides have been found in mealybugs collected from the SJV, suggesting natural infestations. Future research will investigate presence of natural enemies and their impact on mealybug.

Chemical control
Although UC IPM guidelines have only one product recommended for mealybug control in citrus systems, several insecticides are recommended for controlling mealybug in grapes, pistachios and greenhouses (Haviland et al. 2019). Different species of mealybug may respond differently to insecticide applications. Moreover, different seasonal phenology of pests in separate cropping systems warrants crop-specific pesticide efficacy trials. Two field trials conducted in summer 2022 showed that one application significantly reduced mealybug infestation but did not provide long-term solution (Gautam 2022). If your block was infested, plan on an early season control, targeting the first-generation crawlers in late March/early April. Monitor populations and make a second application using a different group of insecticides within two weeks of the first application, targeting the newly hatched crawlers. Continue monitoring and use mealybug-effective materials.

Pheromone lure is not available as a management choice but is available for monitoring.

Other Factors
Natural enemies are relatively more susceptible to broad-spectrum insecticides such as organophosphates, carbamates, pyrethroids and neonicotinoids in groves with mealybug infestations, so reduce/limit the use of these chemicals. Spirotetramat is relatively non-toxic to mealybug destroyers (Grafton-Cardwell 2021).

Ant control: ants help in mealybug dispersion within the tree canopy and between trees and groves. They also defend mealybug colonies from natural enemies (Figure 4). Use effective ant control methods.

Figure 4. Citrus mealybug infestation in mandarin. Note the colonies at the cluster, bleached feeding spots and deformed fruit surface. Also note ants attending mealybugs (photo by S. Gautam.)

Prevent the spread of mealybug by informing the pruning/harvesting crew.
Strip and dispose of the fruit infested with mealybug as fruits may harbor mealybug for next season’s infestation.

Talk to neighboring growers about this threat.

The University of California and Fresno State University researchers are working on a citrus research board-funded project to improve knowledge on biology, seasonal phenology and management of citrus mealybug. Stay informed on research outcomes by participating in grower seminars, newsletter publications and UC IPM guidelines updates.

References
Ebeling, W. 1959. Subtropical fruit pests: Biology and control of citrus pests. Pp. 135-229. University of California, Division of Agricultural Sciences
Gautam, S.G., 2023. Core IPM Research Update: Working toward citrus mealybug IPM. Citrograph, 14: 32-36.
Grafton-Cardwell E.E., Baldwin R.A., Becker J.O., Eskalen A., Lovatt C.J., Rios S., Adaskaveg J.E., Faber B.A., Haviland D.R., Hembree K.J., Morse J.G., Westerdahl B.B. 2021. Revised continuously. UC IPM Pest Management Guidelines: Citrus. UC ANR Publication 3441. Davis, CA.
Haviland D.R., Bettiga L.J., Varela L.G., Baldwin R.A., Roncoroni J.A., Smith R.J., Westerdahl B.B., Bentley W.J., Daane K.M., Ferris H., Gubler W.D., Hembree K.J., Ingels C.A., Wunderlich L.R., Zalom F.G., Zasada I. 2019. Revised continuously. UC IPM Pest Management Guidelines: Grape. UC ANR Publication 3448. Davis, CA.
Myers, L.E. Two Economic Greenhouse Mealybugs of Mississippi, Journal of Economic Entomology, Volume 25, Issue 4, 1 August 1932, Pages 891–896, doi.org/10.1093/jee/25.4.891

Sensor-Controlled Sprayers for Specialty Crop Production

By
Brent Warneke | Faculty Research Assistant, Department of Horticulture, Oregon State University
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Figure 1. A classic axial fan air-blast sprayer (photo by B. Warneke.)

Many of the pesticide application technologies used in specialty crop production today are based on axial fan air-blast sprayers (Figure 1). Air-blast sprayers were first developed in the 1950s when orchard trees were 20 feet tall or more; today orchard trees of many crops are 7 to 13 feet tall. Air-blast sprayers are versatile and reliable and can be modified to fit numerous types of crops. Despite their popularity, standard axial fan air-blast sprayers have long had a reputation for inefficient application characteristics. For example, spray landing on the ground from radial air-blast sprayers can be from 20% to 40% of total applied spray volume. To improve application efficiency, sensor-controlled spray systems were designed in the 1980s as a way to reduce labor costs and pesticide waste. Sensor-controlled spray systems are receiving renewed interest as their reliability has improved and more options have become available.

Sensor Sprayer Types
There are two main types of sensor sprayers:

  • On/off sensor sprayers operate by automatically turning on individual nozzles or sections of nozzles on the spray boom when plants are sensed. Likewise, when no object is sensed, the spray will be turned off (Figure 2a).
  • Crop-adapting sprayers are similar in that the sensor is used to turn the sprayer on and off as it passes by plants. However, they go a step further by adjusting application volume, air volume or a combination of those two (Figure 2b).

Sensor controlled sprayers typically have an override where the user can bypass the sensor system and spray as they would with a standard sprayer if, for example, the sensor components of the sprayer are not working.

Figure 2. Illustration of on/off sensor sprayers (a) and canopy-adapting sensor sprayers (b). Sensors are illustrated with small red ovals and sensor field of view illustrated with grey shaded areas (illustration by B. Warneke.)

Sensors
Crop sensing systems are the “eyes” of the sprayer and determine crop shape by emitting and receiving signals. There are four basic sensor types used in sensor sprayers: infrared, ultrasonic, plant fluorescence and LiDAR. Each one of these sensors emit their own specific signal aimed at the plants that then bounce off the plants and return to the sensor. Some sensor types require multiple sensors to resolve plant structure characteristics.

Infrared (IR) sensors detect IR signals reflected from the plants and can be used in on/off sensor sprayers. Humidity and temperature do not interfere with IR sensing accuracy. However, low light conditions such as during dawn and dusk can interfere with an IR sensor. IR sensors are unable to accurately resolve plant structure. IR sensors are usually used in canopy spraying to trigger the release of spray from the whole side of a boom when a plant canopy is detected. These systems can also be used for herbicide sprayers. In that case, sensors are aimed at the plant trunks and turn off the sprayer as they pass the trunk, or they can trigger spray directly at the trunk to specifically target suckers.

Ultrasonic sensors emit high-frequency sound waves to measure objects. This process is similar to how bats and dolphins use echolocation to navigate and search for food. When arranged in an array (usually three sensors per side of the sprayer), ultrasonic sensors can detect objects with approximately 4-inch resolution. This allows for calculation of canopy volume with similar accuracy to taking manual measurements. Despite their ability to resolve plant structures, ultrasonic sensors are usually used in on/off sensor sprayers. The initial patents on ultrasonic sensors have expired, so continued off-patent development has improved their quality and capability while reducing costs. Comparatively, ultrasonic sensors are more expensive than IR sensors but less expensive than laser sensors.

Plant fluorescence sensors emit a beam of visible light and detect the light reflected back to the sensor from the plant. These sensors have a spatial resolution of about 4 square inches and can detect a plant area as small as 0.40 square inches. Plant fluorescence sensors can collect plant structure data while in the field that can be stored and used for further planning. These sensors are typically used on “weed-seeing” herbicide sprayers to detect green weed tissue contrasted with the soil surface. They have also been integrated into on/off sensor-controlled canopy sprayers to trigger the spray when a green canopy is sensed.

Laser sensors used to characterize plants are termed LiDAR (light detection and ranging) sensors. These sensors have a laser beam that is directed in an arc around the sensor by a spinning mirror inside the sensor housing. The laser beam is then reflected by any surface it hits, relaying distance and structure information to the sensor. Compared to other sensors, LiDAR most accurately measures crop structure, with resolution to several hundreths of an inch. This gives LiDAR sensors the capability of being used for other tasks while spraying, such as measuring canopy vigor. LiDAR sensors are typically used on crop-adapting sensor sprayers, where only one sensor is needed for each sprayer. These sensors are the most expensive of those listed.

One other sensor is needed for sensor-controlled canopy sprayers. Currently, retrofitted sensor spraying systems are not directly connected to the speedometer on the tractor, so a separate speed sensor is needed to convey the sprayer ground speed to the sensor system. A few examples of speed sensor types used are wheel bolt sensors and radar sensors that use Doppler technology. Maintaining accurate speed detection is critical to ensure spray is released on target.

Sensor Sprayer Efficacy and Efficiency
Insect and disease control with sensor-controlled sprayers has been shown to be similar to that of standard sprayers. Using a LiDAR-based variable rate sprayer has resulted in equivalent control of insect pests, such as codling moth, oriental fruit moth and spotted wing drosophila, to that of an air-blast sprayer. Diseases such as powdery mildew, apple scab, brown rot, anthracnose and mummy berry have also been controlled equal to that of an air-blast sprayer. Ultrasonic sensor-controlled systems have resulted in similar control to standard sprayers on difficult-to-control pests such as apple rust mite and pear psylla and diseases such as apple scab and apple powdery mildew. The amount of spray saved when using sensor sprayers varies, but generally, spray volume savings in the range of 20% to 70% can be seen depending on the sensor system used and the crop. Higher savings typically correspond to more precise sensors such as LiDAR in crops with more variable canopies, while lower-volume savings occur with less precise sensors and less variable crops.

There are a host of other benefits of sensor sprayers beyond spray volume savings. Each spray tank goes further so less time is spent spraying, decreasing the need to refill the sprayer and thus saving on labor. A reduction in the number of sprayer fill-ups required leads to less fuel costs and water savings in addition to a lower amount of wear on the tractor. When spray operations can be completed quicker, it can also make it easier to fit sprays into critical application windows.

Environmental Benefits of Using Sensor Sprayers
Spray drift is greatly reduced with sensor-controlled sprayers. Some studies have shown 23% to 45% of the applied pesticide volume can drift off target using standard air-blast sprayers in orchards. Ultrasonic sensor systems can reduce ground deposition by 70% compared to a standard air-blast sprayer in orchards. Canopy-adapting sprayers can be even more effective at reducing drift in orchards, in one study by 70% to 100% compared to a standard air-blast sprayer depending on the tree growth stage. Lower chemical load on non-target locations also helps decrease the rate of development of pesticide resistance. Other considerations include less pesticide contamination of surface and groundwater and lower chances of exposure on non-target organisms such as beneficial insect populations and livestock.

One Step Further: Autonomous Sensor-Controlled Air-Blast Sprayers
Sensor-controlled sprayers help increase spray application efficiency, but there still needs to be an educated applicator driving the tractor and operating the sprayer. Agricultural labor has become less reliable and more expensive, so some manufacturers are producing autonomous sprayers that can be monitored remotely during a spray process. Global Unmanned Spray Systems (GUSS), Jacto and Hol Spraying Systems, in partnership with AgXeed (Figure 3, see page 6), are three manufacturers that are making fully autonomous self-propelled sprayers for specialty crop production. All three manufacturers are integrating sensors onto the autonomous sprayers to do variable rate spraying. Autonomous sprayers offer lower labor requirements and more efficiency than manned sprayer applications and represent the future of spraying in specialty crops.

Figure 3. The Hol Spraying Systems/AgXeed autonomous sprayer and robot. This unit is equipped with plant fluorescence sensors (photo by Amanda Dooney.)

This work was provided in part by the USDA-ARS Integration of Intelligent Spray Technology into IPM Programs in Specialty Crop Production (Project Number 58-5082-2-010).

Resources
Warneke, B. W., Zhu, H., Nackley, L. L., Pscheidt, J. W.(2020) ‘Canopy spray application technology in specialty crops: a slowly evolving landscape’, Pest Management Science, 77(5), pp. 2157–2164. doi: 10.1002/ps.6167.
Nackley, L. L., Warneke, B., Fessler, L., Pscheidt, J. W., Lockwood, D., Wright, W. C., Sun, X., Fulcher, A. (2021) ‘Variable-rate Spray Technology Optimizes Pesticide Application by Adjusting for Seasonal Shifts in Deciduous Perennial Crops’, HortTechnology, 31(4), pp. 479–489. doi: 10.21273/horttech04794-21.
Giles, D. K., Klassen, P., Niederholzer, F., Downey, D. (2011) ‘“Smart” sprayer technology provides environmental and economic benefits in California orchards’, California Agriculture, 65(2), pp. 85–89. doi: 10.3733/ca.v065n02p85.
Chen, L., Wallhead, M., Reding, M., Horst, L., Zhu, H. (2020) ‘Control of Insect Pests and Diseases in an Ohio Fruit Farm with a Laser-guided Intelligent Sprayer’, HortTechnology, 30(2), pp. 168–175. doi: 10.21273/horttech04497-19.

Unwrapping the Possibility of Watermelon Grafting

By
Zheng Wang
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Figure 1. A grafted watermelon plant is a recombined physical hybrid from two different plants. The lower part of a grafted plant comes from a rootstock seedling, which was bred for superior root traits, while the upper part is the regular commercial watermelon plant for producing fruit.

You will probably not surprise to see the grafting of fruit and nut tree crops and well understand the needs to do so. But when I am going to discuss watermelon grafting here, I am sure many of you may pause and ask, “Can watermelon graft? What is that? Why graft watermelons? Is it still sweet after grafting? Is it still watermelon?” 

Why Graft Watermelons?

The main purpose of grafting watermelons is to deliver plant traits to farms much faster than the traditional breeding programs. It is NOT designed to replace breeding and other well-documented techniques of developing new cultivars; rather, it offers growers and consultants a quicker and unique way to solve production problems. In contrast to the traditional breeding system, a grafted watermelon seedling comes from the recombination of two different plant species into one physical (not genetic) hybrid (Figure 1). The phenotypic traits that are brought to farms after grafting include soilborne disease resistance, abiotic stress tolerance, greater nutrient and water use efficiency, higher fruit yield and better fruit quality. 

Do I Need Grafted Watermelons?

This is a question you should always ask before making a decision. The following are some of the benefits from using grafted watermelons. If you have the related issues but with limited solutions, you may consider grafting as your “icebreaker”. Again, please be mindful that these benefits may not be observed at each farm. Using grafting must be proceeded in a case-by-case protocol.

Grafted watermelon can have a stronger resistance to soilborne diseases (e.g., Fusarium wilt, Fusarium crown and root rot, Verticillium wilt, etc.)

Grafted watermelon can produce a higher fruit yield under soilborne pathogenic pressure than non-grafted plants.

Grafted watermelon can produce a higher fruit yield under low-disease or disease-free conditions. 

Grafted watermelon can use water and nutrients differently compared to non-grafted plants. 

Grafted watermelon may have better fruit quality than non-grafted plants.  

Where to Find Rootstock Information and How to Graft?  

A majority of the rootstock varieties can be found at vegetablegrafting.org/resources/rootstock-tables/cucurbit-rootstocks/. Please note that 1.) not all cucurbit rootstocks can be grafted onto watermelons; 2.) information about the disease resistance of each rootstock was collected from the seed company; and 3.) the rootstock table may be updated as new or old rootstocks emerge or disappear. In addition to the table, you can also seek help from rootstock seed suppliers, nurseries that work on grafting watermelons, extension advisors and your trusted growers. For information about grafting nurseries, feel free to give me a call or email me (209-525-6822, zzwwang@ucanr.edu) and I will give you a list. If you need to know the grafting methods, check out the grafting manual at vegetablegrafting.org/resources/grafting-manual/. 

Possible Challenges and Pitfalls

Using grafted watermelons can be challenging. Such challenges can be serious under some, though rare, circumstances. 

Cost

In the past, growers only dealt with scion, but now they must think about rootstocks. Paying extra money for the rootstock seeds and producing grafted transplants will definitely increase the production cost. Currently, the most effective way in balancing the cost is to plant grafted watermelons in a wider spacing while still managing them for a higher fruit yield. Nowadays in California, grafted watermelons should be planted in a 4- to 5-foot in-row spacing, which is equivalent to 1,200 to 1,500 plants per acre compared to 2,200 plants per acre for the non-grafted plants. 

Extended vegetative growth and delayed harvest

Due to their stronger rootability and growth vigor, grafted watermelons usually produce more and larger canopies than regular plants, possibly resulting in delayed fruit maturity. Also, the typical exterior changes that indicate fruit maturity (e.g., a dried tendril and leaflet) on non-grafted watermelons may not be true for grafted plants. An early harvest that causes a lower sugar content can occur frequently in grafted fields. From our past observations as well as research from other areas, a possible 7- to 10-day delay from non-grafted plants could be reasonable for harvesting grafted watermelons.

Figure 2. Cumulative fruit yield for the two grafted watermelon combinations (RS1 and RS2) compared to the non-grafted scion (SC). Note the similar yields in the firsAt harvest.

 

Yield advantage at later harvests 

If you don’t see a higher yield at your first harvest or even a lower yield, be patient and the longer harvest window of grafted plants will give you the subsequent melons. Figures 2 and 3 were from my previous trials in 2021 and 2022 and indicated the yield superiority in the subsequent harvests for grafted plants (Figure 2) and the duration for grafted plants to maintain canopy compared to non-grafted plants (Figure 3). 

Figure 3. Canopy coverage for watermelons grafted onto rootstocks Cobalt, Flexifort and RS841 compared to the non-grafted watermelons (NonG). Note: the field was transplanted on May 17, 2022. The first harvest was made on August 9, 2022.

 

Different responses to irrigation and fertilization 

With the changes of growth characteristics, you may need to consider adjusting your irrigation and fertilization to meet the needs of grafted watermelons. Figure 4 was also from a field trial in 2022 and showed the different canopy regrowth after supplying with irrigation after each harvest between grafted and non-grafted watermelons. It is obvious to see that the canopy of non-grafted watermelons was consistently lower than the grafted plants since the start of fruit harvest on mid-July 2022 (Figure 4). In the meantime, grafted watermelons may take up nitrogen differently from the non-grafted watermelons as well. Figure 5 was a trial I conducted in 2021 which demonstrated the different patterns of nitrogen uptake between grafted and non-grafted watermelons, especially at the last part of the growth cycle.

Figure 4. Seasonal canopy coverage between grafted watermelons grafted onto Camelforce and Cobalt rootstocks and the nongrafted control. Note the fluctuation of canopy regrowth curves due to continuous irrigation and fertilization after each harvest since mid-July 2022.

 

Scion-rootstock incompatibility and change of fruit exterior and interior quality 

This is a complex question. In many cases, you will not be able to detect the incompatibility until the plants are transplanted or even when fruits are harvested and cut open. Symptoms of incompatibility early in the season could have 1.) poor early growth compared to other combinations and the non-grafted control; 2.) death after a few days of transplanting; 3.) incomplete removal of rootstock shoot part at grafting (Figure 6); and 4.) deformed plants. Of course, some of the symptoms may look similar to pathogen infestations. Symptoms of grafting incompatibility at a later stage of the growth cycle usually include a much lower final yield compared to others and a significant reduction in fruit quality (e.g., smell, flesh color, sweetness and hollow heart). For fruit quality, most noticeable changes after grafting that you may see or taste include thicker rinds, bigger fruit and firmer flesh. Therefore, preparing accordingly to meet your needs and your customer’s preference is important.

Figure 5. Cumulative N uptake for the two rootstock-scion combinations (RS1 and RS2) and the non-grafted control (SC). Note the timing of the first (93 DAT), second (104 DAT), third (114 DAT) and fourth harvests (166 DAT, unmarked).

Figure 6. Rootstock shoot protruded from the graft union, indicating an incomplete removal of rootstock shoot at grafting.

The Five Rs of Nutrition in the Vineyard

By
Sean Jacobs | Senior Agronomist, Technical Sales and Marketing Representative, Agro-K
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Maximizing nutrient uptake and energy conversion of vines begins at budbreak and leads to larger, higher-quality crops (all photos by S. Jacobs.)

Whether your grapes are destined for wine, raisins, juice or the table, the job of the grapevine is to capture energy from the sun and use it to convert CO2 and H2O to carbohydrates and O2. Maximizing this process begins at budbreak and leads to larger, higher-quality crops. To have the greatest influence, a wholistic understanding of plant nutrition and crop production need to be engaged to ensure that every dime of fertilizer applied best serves its intended purpose. This is where the Five Rs of Plant Nutrition enters the decision-making process. 

By combining knowledge from plant physiology, soil science, microbiology and chemistry, core principles of plant nutrition arise that affect fertilizer use efficiency, yield and quality. The Five Rs provide this science-driven approach in a memorable way that helps to confirm or guide nutritional decisions. A series of checks, the Five Rs ensure that the applied nutrients get into the plant when and where they are needed with minimal unintended nutrient interactions or losses. When framed as a question, the Five Rs can be stated as:

For my application, is this the:

  • Right Nutrient?
  • Right Time/Crop Stage?
  • Right Form?
  • Right Nutrient Mix?
  • Right Place in the Plant?

The order of the Five Rs is easily re-organized to fit a given scenario, and when focusing on a specific growth period, considering the right time/crop stage first has the most value.

A combination of both foliar and soil-based applications for most nutrients will be necessary to meet the quantities and timing required for high-level production.

Right Time/Crop Stage

Early spring is a critical period for grapevine growth and development as the foundational support components of the plant and crop (xylem, phloem, initial leaves and new roots) are created. This timing is also very challenging because the ability of the plant to obtain and move the needed nutrients from the soil is hindered by low soil temperature and low atmospheric evaporative draw.

Cold soils inhibit microbial growth and function, preventing the cycling of nutrients from plant-unavailable to plant-available forms. Similarly, mineral solubility in the soil-water solution is also reduced as soil temperatures decline. 

Evapotranspiration is the primary mechanism for the movement of nutrients between the soil and the plant. When air temperatures decrease, so does the amount of moisture a given volume of air can hold as does the rate that the air can cause evaporation. Coupled with the small surface area of new growth, very low levels of evapotranspiration result. 

The classical thought that soil nutrient stores alone can support optimal plant growth and development early on is brought into doubt, and other fertility decisions need to be considered. Other non-evapotranspirational mechanisms exist within the plant to mobilize and move stored nutrients but should be viewed purely as supplemental.

Plant nutrient demands fluctuate over the season, and nutrient-to-nutrient ratios shift subsequently.

Right Place in the Plant

Just as important as what nutrient to apply and when to apply it is identifying the most efficient or appropriate place in the plant for the application. As addressed above, early spring conditions result in the soil being a poor nutrient source with low delivery efficiency. Yet, nearly all nutrients found in or used by the plant over a growing season were sourced from the soil via the roots. So then why is “Right place in the plant” a core principle? When nutrient demand timing and nutrient delivery limitations meet, the end goal of maximizing economic yield focuses less on ‘where is the nutrient found?’ and more on ‘where are the nutrients needed and what is the best way to deliver them?’ Hint: The answer isn’t always via soil application.

For most nutrients, what is found in or applied to the soil meets the volume requirements of the plant, but often soil conditions during the demand period or interactions with other nutrients limit their availability. Even in soils with perfect nutrient conditions, periods still exist where only foliar nutrient applications can meet the limited window of nutrient demand. Post-budbreak is such a time and pre-bloom, bud differentiation, set and berry development, post-veraison and postharvest periods all see benefits from timely foliar applications. During these periods, foliar nutrient applications can achieve results that the soil cannot deliver. And to go a step further, foliarly applied nutrients can uniquely alter nutrient ratios or balances within the plant in ways that cannot be achieved economically, or potentially at all, through the soil. 

However, just as the soil cannot deliver all nutrients at the right time, the volume of macronutrients needed cannot be delivered solely through foliar feeding. In the end, some combination of both foliar and soil-based applications for most nutrients will be necessary to meet the quantities and timing required for high-level production.

Right Form(ulation)

The soil and leaf environments to which nutrients are applied are extremely harsh, albeit in almost opposite ways. This article will not go into specifics, but generally, foliar applications dry quickly, are exposed to comparatively high oxygen concentrations and wide-ranging temperatures, and are bombarded with solar radiation. The soil, on the other hand, is very chemically and physically active at the molecular scale and is teaming with life that needs many of the nutrients for its life cycle that our plants require.

Nutrient products are available as various compounds that can be organized roughly into four formulation groups: insoluble salts, soluble salts, chelates and complexes. Ideal use scenarios can generally be defined for each formulation group, and nutrient stability and uptake performance rely heavily upon where (right place) they are applied. 

The main points to understand are 1.) there is no “magic bullet” formulation possessing very high performance as a nutrient delivery vehicle in both soil and foliar applications; and 2.) something else is always competing with the plant to acquire applied nutrients or acting against the formulation of the nutrient, decreasing its availability to the plant. 

Insoluble salts: carbonates, oxides, hydroxides 

Nearly or completely insoluble in aqueous solutions but micronize well. Foliarly, they coat tissues like paint and are highly effective reflectors/blockers of sunlight. As such, they are often used to prevent sunburn or sunscald of plant tissues. Foliar nutrient delivery performance is very poor. Slow conversion to plant-available forms in the soil results in poor performance.

Soluble salts: sulfates, nitrates, acetates, chlorides 

Soil applications usually perform very poorly, but low cost can offset the inefficiency. Foliar performance is poor as uptake is slow, and excess accumulation of the companion anions (SO4, NO3, C2H3O2, Cl) elicit stress responses in the plant or are otherwise problematic.

Chelates: EDTA, EDDHA, EDDHSA, citric acid, amino acids 

Synthetic, EDTA-type and similar are generally large, highly water-soluble materials that perform exceptionally well in soil applications. They are toxic to plants and soil organisms, however, and can solubilize heavy metals in the soil, causing accumulation in plant tissues. They are not great foliars.

Conversely, citric and amino acid based chelates perform well when applied foliarly and are less toxic. Stability is an issue in soil applications compared to EDTA-type chelates. 

Complexes: dextrose-lactose, mannitol, glucoheptonate, lignosulfonate 

Poor performers in soil applications, these naturally derived materials make average to exceptional foliar delivery vehicles. Molecular weight and size of the complex affects performance. The complexing compounds of some offer carbon skeletons that are easily assimilated by the plant once the nutrient is removed. 

Right Nutrient/Nutrient Mix

While not always grouped together, the right nutrient and the right nutrient mix are closely related. Plant nutrient demands fluctuate over the season, and nutrient-to-nutrient ratios shift subsequently. Nutrient-to-nutrient inhibitions, synergies, antagonisms and stimulations exist and must be accounted for as not all nutrients work well together in the plant at the same time.

In our budbreak to pre-bloom vineyard, demand for all nutrients except potassium are high, and calcium and phosphorus are both needed early on. But Ca and P antagonize each other, decreasing application and assimilation efficiency. So, which do we apply? If we assess the right time and right place components, foliar application of Ca makes sense since it moves exclusively with the transpiration stream and as is needed in the leaves to initiate cell division and develop cell walls. P on the other hand provides the energy needed for cell division and other growth functions to occur. Under ideal circumstances, we could apply P foliarly today and Ca foliarly in three to seven days and see the greatest benefit. Practicality doesn’t often allow for this type of application situation, and in our attempts to reduce the number of passes through the field, we must apply both simultaneously.

Tie-ups and antagonisms in real-world agriculture are inevitable and will occur in the spray tank and in the plant. But synergies and gains in efficiency and yield will occur from a little time, effort and application of the Five Rs into your nutritional program.

The Agronomy of Water

By
Karl Wyant
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A water sample can help quantify your risk of negative fertilizer interactions, which can help you choose the right kinds of formulations based on your on-farm water quality (photo by Vicky Boyd.)

Water ties all aspects of crop production together and is the largest input on most farm operations on a per-acre basis (both by volume and weight.) Aside from recent challenges with water quantity, we are also experiencing issues with quality, or the chemical and physical properties that characterize a water source.  In this article, I will describe how water quality is important to five categories of farm management: nitrogen management plans, fertilizer performance, predicting salinity issues, irrigation efficiency and distribution uniformity, and interactions with crop protection products (Figure 1). 

Given the long list of interactions that farm water quality can impact, I would like to make the case for the explicit consideration of what I call “the agronomy of water”, in which we are managing this input similarly to how we take soil and plant tissue tests. After reading this, my hope is to convince you of the utility derived from water sample data to help manage this important natural resource. Remember, a water sample can help inform the management strategy of the five aforementioned categories, often in the same analysis. Thus, a water sample can allow you to better manage five aspects of crop production for the price of one analysis. 

Figure 1. Crop yield is strongly influenced by the agronomy of water and the quality of applied irrigation and sprays. Water quality can be managed to influence several different areas of crop protection, as shown above.

 

Nitrogen Management Plans

All water sources contain dissolved ions in various quantities. Most often, we think of the ions that cause salinity issues and toxicities, which include sodium and chloride. However, due to recent changes in regulatory policy, we now must think about the nutrient content of water applied to field to help mitigate groundwater pollution risk. In this case, nitrate (NO3–) values can be measured in your irrigation water, converted to lbs/acre, and applied against your annual crop nitrogen budget. The benefit of a water quality analysis is that you can keep track of other important nutrients, which can help inform your overall management program. A water sample can help estimate the presence of nitrates in your irrigation water and contribute to an improved understanding regarding in-field nitrogen management compliance. 

Predictive Salinity Management 

Soil salinity is often caused by the application of irrigation water that is high in dissolved salts, particularly sodium, chlorine and boron. These ions can cause a loss in crop yield due to specific ion toxicities (e.g., boron toxicity) and, in the case of sodium, can drive a loss of soil structure, impairing drainage and impeding water penetration and infiltration. A water sample can help quantify your risk of salt damage to your fields and crops and help you formulate a proactive reclamation plan. 

Negative Fertilizer Interactions 

Ok, so you have the water, now you just apply it to the crop, right? Not so fast! Ions in the water can interact with each other and form solid precipitates (chalky or cottage cheese like substances). For example, water that is high in calcium and/or has a high pH can negatively interact with the phosphate in your fertilizer program. When calcium and phosphate come together, they form a mineral called apatite that is highly insoluble. This can have a severe impact on the liquid blends you are applying to your crops. I have seen cases where one ranch has tremendous success running a certain blend but the ranch down the road cannot use the same formulation due to interactions with local water quality. Along the same lines, when phosphate and calcium form precipitates, the nutrients are no longer available for your crop to drive yield, which is a waste of input dollars. A water sample can help quantify your risk of negative fertilizer interactions, which can help you choose the right kinds of formulations based on your on-farm water quality. 

Irrigation Efficiency and Distribution Uniformity 

Continuing with the clogging theme, fertilizer precipitates, and other minerals, can also lodge themselves in drip emitters, sprinkler heads and other types of nozzles. Once these particles clog your system, you lose irrigation system efficiency. When your irrigation system is clogged, you now must use more water and power to deliver the same irrigation set to your thirsty crops than you would if your system had higher efficiency. Furthermore, the clogging of lines can impact some parts of the field more than others, causing an issue with the distribution uniformity of your applied water. A water sample can help you understand drip system clogging risk and formulate a plan to deal with it. Further fieldwork is required to quantify changes in distribution uniformity across a field, but it can be beneficial for improving overall irrigation efficiency.

Ag Chem Efficacy 

Now let’s turn to your crop protection program. An essential question to ask here is, “How good is your spray water quality?” The answer demonstrates the connection between your ag chem programs (e.g., pesticides, herbicides and fungicides) and the most common mechanism for conveyance (e.g., spray water). However, the constituents of the water can have severe impacts on the pest control capacity of your ag chem spray program (e.g., efficacy). A pesticide spray that controls a pest to a high degree per application has high efficacy. An ag chem spray that doesn’t quite do the job has low efficacy. Efficacy is strongly influenced by the interaction between the active ingredient in your applied pesticide and the water it is mixed with (e.g., spray tank, chemigation, etc.)  

Five main physical components of water can have a negative influence on efficacy: pH, bicarbonates, hardness, total dissolved solids and turbidity. For example, spray water that is characterized by alkaline pH (>7) can cause an issue called alkaline hydrolysis, a scenario where the pH causes the active ingredient in the crop protection product to lose its efficacy due to physical and chemical deterioration. In another example, certain ions in the spray water, called hardness, can tie up the active ingredient in your ag chem and render it unusable for pest control.    

Some pesticides are more strongly influenced by these components than others, and management programs should work to improve efficacy on a case-by-case basis. A water sample can help you determine how your ag chem sprays interact with water quality and put together a water conditioning and adjuvant plan to improve the overall activity and control of your pesticide programs. 

Take a Water Sample

The importance of understanding your water quality cannot be understated, and a water sampling program will help you form a solid foundation of “water” agronomy and will produce tangible benefits for your farm. Ironically, while water is often the largest farm input by volume and weight, a water sample can be one of the cheapest inputs (e.g., $/acre) around as one water source can serve many acres on a given ranch. However, many folks do not have a regular water sampling program in place to monitor changes in water quality. Talk to a Certified Crop Advisor today about starting a water sampling program and to improve the agronomy of water on your farm operation across the five categories described above. 

Resources

Irrigation Water Salinity and Crop Production, anrcatalog.ucanr.edu/pdf/8066.pdf 

The Impact of Water Quality on Pesticide Performance, extension.purdue.edu/extmedia/ppp/ppp-86.pdf 

Water Quality for Crop Production, ag.umass.edu/sites/ag.umass.edu/files/book/pdf/ghbmpwaterqualityforcropprod.pdf 

Agronomy of Water FieldLink (originally created by author), helenaagri.com/fieldlink/the-agronomy-of-water/

The Agronomy of Water Series (2 CCA credits available: Soil and Water), wrcca.org/continuinged

Five Tools For The Price Of One: The Case For Farm Water Testing https://nutrien-ekonomics.com/news/five-tools-for-the-price-of-one-the-case-for-farm-water-testing/

Adapting Solutions: A Novel Product for Soil Salinity Management

By
Raquel Gomez | M.Sc., Research Agronomist, AgroPlantae and Davide Ciceri | Ph.D., Senior R&D Scientist, AgroPlantae
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Figure 1. Almond (left) and pistachio (right) foliage yellowing and leaf margins with symptoms of salt toxicity (photos courtesy R. Gomez).

It is estimated that approximately 78 million acres in the western San Joaquin Valley are affected by soil salinity, with 30% of that acreage categorized as strongly or extremely saline (Scudiero et al. 2017). In soil, salinity refers to the presence of dissolvable ions like sodium, potassium, magnesium, calcium, chloride and nitrate. Salinity stress in crops occurs either from high concentration in the soil solution of specific ions like Na and Cl or mixes of several soluble salts. Geological processes in the San Joaquin Valley (accumulation of marine coastal alluvium) and low-quality irrigation water are major contributors to soil salinity (Corwin 2003). Additionally, as crops absorb water, and water evaporates from the soil surface, salts are left behind in the rootzone. Crops growing in saline conditions experience osmotic stress, diminished growth, yield and shortened lifespan (Figure 1). With little access to high-quality irrigation water and ongoing drought, there are few solutions to alleviate salinity. Updated management practices are required to face the situation at hand. 

Figure 1. Almond (top) and pistachio (bottom) foliage yellowing and leaf margins with symptoms of salt toxicity (photos courtesy R. Gomez).

The most traditional method to manage soil salinity is by application of calcium in the form of gypsum (CaSO4 ·2H2O). As CaSO4 breaks down it yields calcium (Ca) and sulfate (SO42-) ions. Calcium works to desorb Na from the soil particle surface while the remaining sulfate binds the loose Na to yield sodium sulfate (Na2SO4). In turn, the newly formed sodium sulphate can be leached and moved down the soil profile and out of the rootzone. Additionally, replacing Na with Ca flocculates the soil for improved water infiltration and pore space. However, the application of gypsum as a sole salinity manager has challenges. Gypsum may require specialized machinery and equipment to facilitate applications in water-saving drip line, and attention should be paid to water quality or Ca may precipitate and plug the irrigation system. Finally, it is not uncommon to see Ca stratification in the soil from years of broadcast applications that have not fully dissolved or incorporated.

The standard practice for salinity management is a 0.5- to 2-ton/ac application of gypsum in the fall in anticipation of fall/winter rain. This practice has become the feel-good practice of many growers to date. However, updating such a practice becomes necessary when considering the current effects of climate change and economic circumstances. 2021 marked the driest winter months in 100 years with record lows of snow and rainfall as well as the third year of extreme drought. No rainfall and low access to irrigation water means little of the surface-applied gypsum is dissolving in the soil. With less access to surface water, more reliance on ground water and decreasing crop prices, growers now more than ever should reassess their management practices. 

Modern methods for salinity management focus on high solubility products, ease of application and efficiency. CATION-EX5 PLUS is a salinity management product formulated by AgroPlantae. It is fully miscible with water and better suited for fertigation systems than traditional gypsum and contains other important elements to help rebalance soil, improve fertility and invigorate soil microbes. CATION-EX5 PLUS is 0-0-5, Ca 10%, S 8%, Co 0.10% and Mo 0.10%.

The following are results of a three-year study on soil salinity management in California’s Tulare Basin.  Treatments were designated Grower’s Standard or CATION-EX5 PLUS. The Grower’s Standard (GS) consisted of a yearly postharvest application of broadcast gypsum 95% at a rate of 1 ton/ac in an 80-acre block and multiple in-season solution-grade gypsum applications through the irrigation system. AgroPlantae’s CATION-EX5 PLUS (CTX) was applied at 11 gal/ac over 80 acres, split into five applications over the growing season. Soil samples were taken in a predetermined area of each treatment block in 12-inch increments until 5 ft depth was achieved. Soil samples were taken in the spring before the first irrigation occurred and again in the fall after the last in-season irrigation in each year of the study. The spring sample acted as a baseline for the year while the fall soil sample showed the changes that occurred in the soil during the span of the crop growing months.                                                                                                

CATION-EX5 PLUS Reduces Salinity Buildup

Soil Electrical Conductivity (EC) is the measure of soil salinity. EC is highly influenced by soil texture, i.e., sand, silt and clay. For example, clay-like soils attract and retain more positive charges (cations) and have higher EC values than other soil textures. Based on saturation percentages, the soil in both treatment blocks was determined to be clay-loam. Aside from soil texture, soil EC is also highly affected by inputs like irrigation water and fertilizer. 

In this study, a greater degree of variability was seen in GS treatment when compared to CATION-EX5 PLUS (Figure 2). In 2020, EC values were comparable amongst treatments. However, in 2021 and 2022, large increases in soil EC were seen in the GS treatment. EC increased 24%, 742% and 573% in the GS treatment for 2020, 2021 and 2022, respectively. The red dotted line represents the average EC for the soil profile. The large increase in values suggests the GS treatment is neither remediating nor alleviating water penetration or infiltration. In the CTX treated area, EC decreased on average by 14%, increased by 145% and 117% in 2020, 2021 and 2022, respectively. With the application of CATION-EX5 PLUS, soil is maintaining a manageable EC level and showing greater performance for managing soil salinity when compared to conventional gypsum (GS). The increases seen in the GS area call for an update to salinity management practices. 

Figure 2. CATION-EX5 PLUS effectively manages soil EC compared to the Grower’s Standard (1 ton/ac gypsum). Red dotted line shows the average EC of all depths.

 

Managing Soil Sodium and Chloride

Sodium chloride (NaCl) is the most common and problematic salt in irrigation water. Sodium in soil antagonizes uptake of other beneficial elements like K, disperses soil and can create chemical compaction.  

At the start of the experiment in 2020, soil sodium was comparable amongst treatments (Figure 3). In the GS treatment, a clear Na increase is seen from spring to fall in each experimental year. The red dotted line represents the average Na for the soil profile. Overall, the GS increased 53%, 654% and 429% soil Na concentration in 2020, 2021 and 2022, respectively. Comparatively, in the CATION-EX5 PLUS treated field, soil sodium decreased by 4%, increased by 162% and 82% in 2020, 2021 and 2022, respectively. An increase in soil Na cannot be avoided but it can be managed to reduce toxicity, osmotic stress and mitigate growth/yield. Additionally, in 2022 CTX treated soil is beginning to show an effect carry over rate. In 2022 CTX treated soils showed little fluctuation in soil Na. This alludes to the lasting effects of proper soil flocculation.   

Figure 3. Soil sodium is effectively reduced by CATION-EX5 PLUS when compared to standard management practices like gypsum. Red dotted line shows the average sodium of all depths.

Soil Cl is an essential element needed by plants for growth, however when too much Cl is available in the soil, toxicity occurs. Cl toxicity is common in poorly drained soil and areas being irrigated with ground water. Cl is negatively charged, so it may be leached through the soil profile with irrigation water. When irrigation water is the largest contributor of Cl, and soil Na creates an environment for poor soil drainage, management becomes more critical. When soil Cl exceeds 5 meq/L, a management protocol should be activated. 

In this trial, soil Cl began the experiment below the action threshold in both treatments (Figure 4). The red dotted line represents the average Cl for the soil profile. As the experiment progressed, Cl concentration built in the soil profile. In 2021, the GS increased the soil concentration about 2257% from spring to fall and about 697% in 2022. The CTX treatment had lower increases of 378% and 112% from spring to fall in 2021 and 2022, respectively. 

Figure 4. Soil chloride is effectively reduced by CATION-EX5 PLUS when compared to standard management practices like gypsum. Red dotted line shows the average chloride of all depths.

No soil infiltration measurements were collected for this study; however, the data suggest an increase in water movement and wetted area in the soil profile. This is corroborated with the reduction in Na and Cl as well as EC. CTX is improving soil structure and remediating soil salinity. 

The increases in soil EC and Na seen in the GS treatment call for an update to salinity management practices. Experimental data from this long-term and large-scale trial show CATION-EX5 PLUS effectively managing soil salinity, especially when compared to the standard yearly gypsum application. Modern agriculture puts great emphasis on efficiency, precise solutions and ease of use while still demanding results. CATION-EX5 PLUS offers results, efficiency, precision and ease of use in fertigation systems like those used in California’s Central Valley. 

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