Early season leaf sampling can help you stay on your fertilization target
Phoebe Gordon, orchard systems advisor for Madera and Merced Counties
Did you know that there is a tool that will help you identify whether your orchard is deficient in nitrogen or potassium with enough time to fix things in THIS growing season? It is called early leaf sampling and it can help you fine tune your nitrogen and potassium fertilization program, especially in the case of an unusually good or bad crop set.
Before I get into the leaf sampling protocol, I want to explain a bit about nitrogen and potassium in soils, and the physiology of nitrogen and potassium in pistachio trees.
Nitrogen and potassium in soils
No matter in what form nitrogen is applied to the soil, it will eventually be converted into nitrate, a form that is vulnerable to leaching losses from the root zone. Because of this, understanding when nitrogen is needed by pistachios and how much is needed will allow you to minimize N losses and maximize your return on fertilizer investment.
Nitrogen can be applied to soils in many forms. It can be found in organic matter amendments (compost, manures, many organic fertilizers), conventional fertilizers, and irrigation water. Soil organic matter also releases nitrogen, as do any cover crops that are grown in the orchard and release nutrients as they break down over time.
Nitrogen that is incorporated into organic matter is the “original” source of nitrogen in ecosystems, and is the main way nitrogen is applied to certified organic orchards. Organic matter must be broken down by soil microbes, or mineralized, before it is available for uptake. The resulting form is ammonium. Urea, while not a certified organic product, is an organic compound because it contains a carbon atom. Ammonium is then nitrified – converted into nitrate. The nitrogen contained in ammonium, like organic matter, is immobile in soils, while nitrate is mobile. Urea, while an organic compound, is actually very mobile as it is small and uncharged and can move rapidly through the soil.
Mineralization and nitrification (conversion of ammonium into nitrate) are done by soil bacteria, which work fastest in warm, well aerated but moist soils. This means that, at least in deciduous orchard crops, conversion of nitrogen from immobile to mobile forms happens most rapidly when nitrogen is needed by plants – during the warm periods of the year.
Table 1: summary of forms of nitrogen and behavior in soils.
1There is evidence that plants can take up amino acids, however it is unlikely that they are a significant source of nitrogen as they would be rapidly mineralized by soil bacteria
2Urea is very mobile until it is mineralized into ammonium
3Some soil organic matter is very resistant to breakdown by soil bacteria and will remain stable for decades or longer. This can be a source of nitrogen, however we are referring to more easily broken down sources like composts, manures, and cover crops
Potassium is mostly immobile in soils as it can adsorb to negatively charged soil particles. The ability of a soil to hold onto potassium and other positively charged ions is called the Cation Exchange Capacity (CEC). It is generated by clays and organic matter. Because of this, the CEC tends to be small in sandy soils and large in clay soils. Potassium can be somewhat mobile in sandy soils that have a small CEC, but is generally considered to be mostly immobile in soils.
Potassium fertilization has been covered before here and won’t be discussed extensively in this article. In general, different potassium fertilizers have not been found to differ when it comes to plant uptake and usage – applying potassium in the wetted zone of your orchard is the most critical part of ensuring potassium fertilizer is available for uptake. This is because root activity is primarily in the wetted zone during the growing season. Banding in the same location annually can be very effective, however these bands should be placed carefully to ensure the fertilizer lands in the wetted zone. Fine roots that take up nutrients can be found at very shallow depths in microirrigated orchards, so a fall band that is moved into the soil via rain will provide potassium to your trees.
The specific form of potassium fertilizer does matter when it comes to fertigation. Unless you buy potassium fertilizer that is already in liquid form, most sources of potassium tend to be fairly insoluble and can take a lot of water to dissolve. While formulations that are better for injection tend to be more expensive than forms that are typically used for soil applications, I’ve heard from several folks that the expenses associated with banding often mean that fertigating, even if the fertilizer is more expensive, is more cost effective. Fertigated potassium will move more in the soil than banded potassium.
Potassium, like nitrogen, can be found in composts and manures. Unlike nitrogen, potassium is very available in carbon-based amendments like composts and manures because it is not bound up into permanent structures.
Since it will stay put in most soils, potassium can be applied the previous fall, though many growers have great success with potassium by treating it like nitrogen and fertigating it throughout the season. This is especially true in potassium fixing soils.
Nitrogen and potassium in pistachios
Nitrogen is critically important for almost all plant processes. It’s a part of DNA and amino acids, and therefore involved in basically everything a plant does. Because of this, it is needed in large quantities as plants grow and develop their fruit. Because of its mobility, applications should be split between fruit set and harvest.
Pistachio nitrogen uptake is dictated by plant need. In immature trees, this is primarily growth, and in mature trees, it is primarily the crop load. Mature trees also need some nitrogen to supply vegetative growth but it typically pales in comparison to the amount that is needed for the crop. Pistachios require 28 pounds of nitrogen per 1000 lbs of marketable yield and about 25-30 lbs to supply vegetative growth. Since pistachios are alternate bearing, their yield demand will be different for an ON or OFF year.
Image 1: leaf nutrient deficiencies in an ON branch, resulting from such strong demand from developing nuts that stored nitrogen and potassium are being pulled from leaves.
Nitrogen uptake in pistachios is primarily driven by crop load and will vary whether the orchard is in an ON or OFF status. In heavily alternate bearing rootstock-scion combinations like Kerman on P. atlantica (which hasn’t been planted in a very long time), nitrogen demand could be so strong in an ON year that nitrogen stores in plants would be tapped during nut fill, and uptake would be heavier the following spring as the trees recovered. Alternate bearing is less in Golden Hills/Lost Hills and with the higher vigor UCBI rootstocks, and so the depletion of nitrogen from other tissues in ON years is probably less.
It may be tempting to think that plant nitrogen status drives alternate bearing, but our current data suggests it is more likely carbohydrate starvation of buds during ON years (the developing fruit demands a lot of photosynthesized sugars, too!). Nitrogen may very well play a role, however only hedging and pruning have been found to reduce alternate bearing in existing orchards by changing the composition of ON and OFF branches that are present on individual trees.
Potassium is not incorporated into plant structures, for the most part. It is involved in cellular processes such as enzyme function, and critically, maintaining osmotic balances in cells. It is also needed in large quantities by nut crops. Pistachios require about 45 lbs of K (55 lbs of K2O) per 1000 lbs of marketable yield and 25-30 lbs of K2O to supply vegetative growth.
Potassium uptake in pistachios is fairly straightforward: over 90% of uptake happens during nut fill, regardless of the bearing status of the tree. Like nitrogen, uptake will be greater in an ON year than an OFF year. Ensuring that your potassium fertilizer has been applied before nut fill is critical to ensure that there is enough available in soils to supply plant demands.
Managing nitrogen in an orchard
Nitrogen and potassium fertilization should start with a good assessment of 1) your block’s yield potential, and 2) what you think the current season’s yield will be.
The yield potential is based off the yield history of a site. While pistachios can yield as much as 6000 lbs in an ON year, in many locations soil texture, soil chemistry, and water quality will limit the yield potential of specific sites. Accurately assessing the yield potential of an orchard is important for correctly assessing nitrogen fertilization rates. Aspirational nitrogen applications cannot help an orchard overcome a yield limitation such as high salinity or a restricted root zone.
Year to year events will also change the yield potential of a block – whether it is in an ON or OFF year, bloom conditions, and boron fertilization status are some examples. Unfortunately, accurately predicting yield at bloom is far more of an art than a science – you probably won’t land on exactly the right number but you’ll probably be in the general area. This, however, is not good enough for generating accurate fertilizer recommendations!
Early season leaf sampling will help you determine whether your fertilization programs are on track. Leaf sampling should occur approximately 35-45 days after full bloom (roughly early to mid-May). Sample 10 subterminal leaflets from each tree, and sample trees that are at least 25 yards apart. A full nutrient panel is required for the prediction tool. It is important to note that this tool predicts what your July leaf tissue values will be, and it is essential to still sample in July to check that your possible course corrections worked.
The Fruit and Nut Research and Information Center has an online tool that you can use to guide nitrogen and potassium fertilizer applications, and will allow you to enter your early leaf sampling results to guide later season applications. A step-by-step walkthough can be found below:
Figure 2: The pre-season tab and fertilization suggestions
The pre-season section (on the left of the photo) will allow you to enter your expected yields, any nitrogen adjustments that need to be made, and what you would like to apply to supply tree growth. Note that potassium is in the form of the element, not K2O.
Once you have your leaf tissue results, plug them into the “mid season update” tab, along with your updated predicted yield and fertilization thus far, and the tool will give you a graphical representation of your expected yields. If your early-season nitrogen levels indicate low levels in July, the tool will provide you with updated fertilization rates to get back on track.
Figure 3: The mid-season update tab showing suggested fertilization rates based off of adequate early season nitrogen values
Figure 4: The mid-season update tab showing suggested nitrogen and potassium fertilization rates based off of low leaf nitrogen levels
Notes on leaf sampling
Leaf sampling is an imperfect tool – plant nutrient status can vary widely across an orchard, and the recommended best sampling methods aim to get an “average” picture of a block. However, we all know that an orchard block is anything but uniform! Until we get better yield mapping and site-specific fertilization tools, most blocks will receive one rate of nitrogen and potassium. While one leaf sample for a block doesn’t capture variability, it does still fit with how we still manage our orchards.
If you are interested in zone-specific fertilization, we do have some limited tools: it is possible to apply foliar sprays or soil applications to particularly well yielding areas of the orchard – whether this is economically feasible will be determined by the specific costs of these practices in your operation and how much money you would save by cutting back fertilizer in areas that yield worse. For best effect, variable fertilization would also require additional leaf samples to ensure that you are on track with what the trees need.
The accuracy of leaf sampling is also vulnerable to poor sampling technique – bad sampling or storage practices will not give accurate values. I used to work in an agricultural testing lab and saw some wild things – leaves that were rotting and obviously had sat in the back of a truck for weeks, whole plants (including roots) pulled for a standard leaf tissue sample, and soil samples submitted in a used tennis shoe (?!?). While these are some of the extremes, a term used in computer science is applicable here: garbage in = garbage out.
Despite all of these issues, leaf sampling is the best tool we currently have for managing nitrogen and potassium fertilization, in addition to fertilizing based off of predicted yields. And, despite all its issues, it still is a useful tool for diagnosing deficiencies. I’ve diagnosed many an orchard with low yields as having boron deficiencies, and it can help with diagnostics in cases of potential herbicide drift!
Additional resources
https://fruitsandnuts.ucdavis.edu/nutrition-landing-page/pistachio-nutrition-information
https://www.cdfa.ca.gov/is/ffldrs/frep/FertilizationGuidelines/Pistachio.html