The Enigma of Potassium for Tree Nut Growers
By Dan Cook, Agronomist
For almond and pistachio growers, one of the most critical, consequential, and confusing decisions when making a nutrient management plan is potassium fertilization. It’s critical, due to the importance of potassium in several physiological processes, and especially its role in maximizing nut size and total yield. It’s consequential because potassium can be a significant portion of a grower’s fertilizer budget. Unfortunately, it can also be confusing to decide what fertilizer to use, how much to use, and when to apply it.
In an effort to differentiate products, fertilizer marketing often focuses on the ideas of “solubility”, “availability” and “efficiency”. Sometimes there are trial results included to prove that the product is better than the competition. These types of messages aren’t necessarily dishonest or purposefully misleading, but they are oftentimes an oversimplification. The truth is that crop production is complex, there any number of tens to hundreds of factors that can play a significant part in helping to grow a successful crop. Potassium nutrition is one of those factors, but even potassium availability and uptake itself can be influenced by climate, soil properties, water dynamics, and more. There are no guarantees that results from a research trial performed across the street will translate to any particular orchard in any given season. The best we can do in agronomy and crop production is to gather as much information as possible and use that information to make the best decisions that we can, then evaluate the results and try again.
A few notes on potassium…
The agronomic saying goes, “The most important nutrient is the one that is limiting.” While that can certainly be true. When it comes to nut production the two most significant nutrients, nutrient 1a and 1b, are definitely nitrogen and potassium. While we’ll leave a detailed discussion of nitrogen for another time, I think it’s fair to say that more than any other nutrient, we all know when a crop is under-fertilized with nitrogen. Potassium on the other hand is kind of an enigma. I’ve had discussions with growers who have no faith that potassium fertilizers add value to their program. I’ve also worked with growers that believe that good yields require several hundred pounds of potassium be applied annually. As a research agronomist, I’ve designed research trials where I expect to see a potassium response and I don’t, and then the following year, don’t expect to see one, and I do. So, what’s a grower to do?
Where can potassium fertilizer go?
With nutrients, I like to draw a picture in mind of all the possible places where that nutrient can go. This can be helpful in predicting when I may or may not see a response. Luckily with potassium, there are no physical transformations that occur and so it’s fairly easy to visualize where potassium fertilizer can go. Generally speaking, it will go into one of three places:
- Potassium will be taken up by the plant
- Potassium can move downward with the water (i.e. leaching)
- Potassium can be held up in the soil
Obviously, #1 is our ultimate goal, we want as much of our applied potassium as possible to get into the plant. The worst-case scenario is #2, that we lose the potassium below the root zone and lost. While the nature of potassium makes it less prone to leaching losses compared to nutrients like nitrogen, undoubtedly some portion of our potassium fertilizer will be lost to leaching. This is especially true in sandier soils.
The most interesting and complex place that potassium will go is…nowhere. What I mean is that it stays put in the soil. Best case scenario, the potassium is caught on the exchange sites of the soil clay and organic matter and will be readily available to the plant. More nefarious, however, is when certain types of clay minerals pull the potassium into interlayer spaces and lock them away from the plant (i.e. potassium fixation). The reality is that many fields where our almonds and pistachio are planted could potentially be made up of clay minerals that will potentially fix that potassium removing it from availability to the crop. In places where potassium fixation occurs, the worst way to apply potassium is broadcast across the field at timings when the crop doesn’t need it (for example, in dormancy).
A “Program Approach” to Potassium
For decades, common recommendations stated that potassium additions were unnecessary. Over time, as high demand potassium crops have mined potassium from the soil, the need for potassium fertilizers is much more accepted. A common question for almond and pistachio growers is whether they should apply potassium in dormancy or in-season? As an agronomist, my go to answer to this and almost any other question is “It depends!”
Each grower needs to assess their operation’s equipment and capabilities, production goals, and the conditions and properties of each field to be able to decide which application methods fit best. In some cases, growers can be very successful with potassium applications in dormancy. With the rise in micro-irrigation systems across almond and pistachio growing areas, in-season applications through fertigation are very convenient and effective. Still other growers choose a combination of both, following soil analysis recommendations and supplementing when needed with in-season applications based on leaf tissue samples.
Tessenderlo Kerley provides potassium solutions that can be used in any number of situations. For decades, almond and pistachio growers have chosen to use our liquid potassium product KTS®, a potassium thiosulfate fertilizer. KTS can be conveniently used for application into irrigation systems, and can be blended with other crop nutrients to provide nutrient blends that are customized to specific crops at specific growth stages. During the last 10 years or so, growers have also discovered K-Row 23®, another liquid potassium and sulfur product that allows for higher application rates to give grower’s flexibility to apply potassium quickly when needed. Tessenderlo Kerley is also a producer of dry granular potassium sulfate (SOP), branded as GranuPotasse®. GranuPotasse is ideal for dormant applications of potassium during the winter months. We also manufacture an exceptionally high-quality solution grade SOP product called SoluPotasse®.
Each of these products can be included in a potassium program. The approach should be catered to the individual farm, depending on preferences and conditions. The goal of course is to maximize productivity and economic sustainability to the grower. In 2022, we initiated a multi-year research project on almonds in collaboration with the University of California Cooperative Extension. The trial was conducted at the Nickels Soil Laboratory in Arbuckle, CA. The goal of the project was to look at different potassium fertilization strategies, including dry potassium sulfate applications (GranuPotasse), in-season applications of KTS (5 applications total throughout the growing season), and a combination of both. As we initiated the trial, we saw only slight benefits from applying any potassium fertilizer in the first two seasons. During 2024 we started to observe a separation in yield between fertilized and unfertilized plots. As trends continue, we expect to see the gap in yield widen.
| Treatment | % Leaf K (Summer 2024) | Yield 2024 (lbs/acre) | Cum yield 2022-2024 (lbs/acre) |
|---|---|---|---|
| Control (No Applied K) | 1.67 ab | 2147 a | 5487 a |
| GranuPotasse (125 lbs K₂O/A) | 1.75 abc | 2232 a | 5746 a |
| KTS (125 lbs K₂O/A) | 2.00 c | 2730 a | 6030 a |
| GranuPotasse + KTS (each 125 lbs K₂O/A, 250 lbs/A total) | 1.96 bc | 2457 a | 5754 a |
The story continues, as we have seen the potassium levels in the soil start to build. Each year, when soil samples were taken at the end of the season, we have seen the separation in soil extractable K increase. Building soil K, helps to ensure enough potassium is available to the tree at essential growth stages.
| Treatment | Extractable K (ppm) | ||
|---|---|---|---|
| 2022 | 2023 | 2024 | |
| Control (No Applied K) | 65 bc | 58 a | 58 a |
| GranuPotasse (125 lbs K₂O/A) | 119 d | 108 b | 122 ab |
| KTS (125 lbs K₂O/A) | 107 cd | 120 b | 164 b |
| GranuPotasse + KTS (each 125 lbs K₂O/A, 250 lbs/A total) | 108 d | 151 b | 324 c |
We plan to continue this trial work into the future, and hope to see if building soil K results in continuing to sustain higher yields.
Concluding Thoughts
As I alluded to before, the purpose of trial results should not be to convince you that one product is better than the rest. Rather trial results should be one data point, in many, that can help an almond or pistachio grower decide the practices and strategies that can be adopted to improve an operation. Developing potassium fertilization plans does require some nuance. Potassium will remain a critical nutrient for nut trees, but it doesn’t have to be confusing. Gathering information from soil and tissue samples, consulting advisors and university extension personnel, and working with fertilizer dealers and manufacturers can all be helpful in making potassium fertilization decisions.