Potassium in Nut Crops: Plant Uses and Field Application
Phoebe Gordon, Orchard Crops Farm Advisor, Merced County
Franz J.A. Niederholzer, Orchard Systems Farm Advisor, Colusa County
Potassium in Plants and Soils
All tree crops need potassium, however its high cost, difficulty in management in some soils, and a sometimes apparent lack of a response to added K has resulted in incomplete adoption of potassium fertilization programs in orchard crops. It is an unusual nutrient in that it is not incorporated into plant tissues. Its role in the plant is primarily relegated to maintaining cellular ion balances, particularly in the stomata, and is an activator of many enzymes. Symptoms of potassium deficiency differ by species, but many symptoms are similar or the same: deficiency symptoms appear in the summer as the growing crop demands more potassium, which may be supplied from surrounding leaves and branches. Leaves will curl up, their color may become pale, leaves may become smaller, yield may decrease, and leaf margins may become necrotic. There may be more specific symptoms for each crop plant, but we won’t spend time going over each one in detail here.
| Crop | Critical Value (Percentage) | Adequate (Percentage) |
|---|---|---|
| Almonds | 1.0 | > 1.4 |
| Pistachios | 1.6 | 1.8 - 2.0 |
| Walnuts | 1.6 | > 1.2 |
Potassium is held in three different pools in the soil: most is found as a part of the primary minerals in soil and not plant available when considering an annual K fertility program. A smaller pool is bound to the cation exchange capacity, and an even smaller amount is found in the soil solution. The last two can be quantified by soil fertility tests, typically an ammonium acetate test.
Potassium is immobile in soils, as it binds to the cation exchange capacity (CEC) or can be fixed by soil minerals, both processes of which are described below. However, in coarse textured soils with low CECs, or in soils in which you apply other cations on top of a potassium application, (a gypsum application on top of a potassium application, for instance), potassium can move deeper into the soil. This can be beneficial in heavy-textured soils if you want potassium to move into the root zone, but in a low CEC sandy soil, fertilizer K can be leached out of the rootzone.
The CEC is a measure of the soil’s ability to hold onto positively charged ions. It is generated by weathering of soil particles, where negatively charged surfaces are exposed, among other factors. Because it is dependent on surface area, the only soil solids that meaningfully contribute to the CEC are clays and organic matter, both of which have large surface areas.
Cations, which are positively charged molecules that are released into solution when salts dissolve, are not permanently bound to the CEC. They are in flux with the soil solution, so large inputs of one type of cation (sodium, for instance) can result in the CEC being dominated by that cation. Similarly, intensive crop production without applications of fertilizer can result in the CEC being depleted of nutrients.
Potassium fixing soils
Potassium fixation is an issue on the eastern side of the Central Valley, primarily the Southern San Joaquin Valley. It occurs in soils derived from granitic parent material from the Sierra Nevada, which contains vermiculite. Soils on the west side of the valley are more likely to be derived from the coastal range, and they do not fix potassium as much as east-side soils. Potassium ions can become trapped between sheets of vermiculite, and once it is fixed it is unavailable for uptake by the plant.
This potassium will eventually be released, however, and become available for uptake but the rate of release is soil dependent. For our purposes, we’ll consider it unavailable for uptake in a growing season. This release makes the ammonium acetate soil test less reliable in potassium fixing soils, as the test does not reflect the potential for the release of fixed K. In potassium fixing soils, you either need to band potassium to overwhelm the fixing abilities of the soil, or apply potassium throughout the season, targeting when the trees need the nutrient the most. For example, in pistachios, this would be approximately between April and August, which is during fruit development and nut fill, when over 90% of the potassium needed by the trees is taken up. If potassium is applied outside of tree demand, it should be banded to overcome potassium fixation in the soil.
Research done showing positive responses to K
In the past, research looked at applying extremely high rates, such as 1500 lbs of KCl/acre, to remediate deficient orchards. However, results from the 80s in walnuts done by Olsen, Uriu, and Pearson indicated that it was better to maintain sufficiency levels than to wait until orchards showed deficiency symptoms to add potassium.
Other research has looked at comparing different sources of potassium. Two trials, one in almonds and one in pistachios, conducted during the 90s found that in general, the source of potassium did not matter, however mono potassium phosphate applied to almonds had better yields under microjet sprinklers in one of the three years of the trial. It is possible that the phosphorus contributed to increased yield, however soil levels for phosphorus in that study were adequate. The researchers did not report leaf values of phosphorus, so the authors of this article cannot confirm or deny the phosphorus response. More importantly, the researchers found a poor response to banded potassium under single line drip irrigation, as the potassium was banded outside of the wetted zone, which only allows uptake when soils are wet from winter rains, either very late in the fall or early in the spring.
In the pistachio trial, yield was greatest at 200 lbs K/acre. A higher rate, 300 lb K/acre, resulted in cation antagonism between the potassium, calcium, and magnesium. The soil type was a sandy loam, which has a more limited cation exchange capacity, and is more easily ‘flooded’ by a heavy application of potassium, so this rate may not cause antagonism in a heavier textured soil.
Potassium Fertilizers and Solubility
| Source | Formula | % K20 | Solubility@ 20 C◦ (grams/liter) | % K2O in a saturated solution |
|---|---|---|---|---|
| Potassium Chloride | KCl | > 63 | 255 | >16.1 |
| Potassium Sulfate | K2SO4 | >54 | 111 | >5.4 |
| Potassium Nitrate | KNO3 | >47 | 209 | 11.2 |
| Potassium Thiosulfate | K2S2O3 | >25 | complete | 25 |
Adapted from the Western Fertilizer Handbook, 9th Edition
Before we get into fertilizing your trees, it’s helpful to talk about solubility in potassium fertilizers, which can be an issue during fertigation. The solubility of a fertilizer determines how much potassium you can deliver over a certain period of time; fertilizers with high solubility require less water to dissolve. If, for some reason, you have a limited time frame for potash delivery, or you want to minimize the volume of fertilizer you need to dissolve, fertilizers with higher solubility may be for you. Just keep in mind that there’s no physiological reason why you can’t spread potassium applications throughout the growing season, especially if your orchard does not fix potassium. You can also combine fertigation with banding or micro-broadcasting.
The major reasons to choose a particular source of potassium fertilizer are price per pound of K, the presence of additional needed nutrients, whether you can leach with rain or high-quality irrigation water, and whether your orchard’s soil is suitable for leaching. Orchards in which
leaching may be difficult are those with heavy textures, clay pans or hard pans, or soils that already have reduced infiltration rates due to sodium. In deciduous tree crops, pistachios have the highest tolerance to soil and water salinity, however they too will eventually lose yield at soil salinity levels above 9 dS/m. The Sacramento Valley typically gets more rain than the San Joaquin Valley, and buildup of soil chlorides may not be as much a concern in normal rainfall years.The method of application, source of potassium, or solubility of a fertilizer do not change potassium’s fate in the soil. In most soils, potassium will end up in the CEC, and in a potassium fixing soil, potassium will eventually be fixed.
Fertilizing your trees
The best fertilization practices are to treat potassium like nitrogen and do yearly applications to replace what was lost through yield. The amount of potash removed per unit yield for common nut crops is below:
| Crop | Lbs K2O Removed | Harvest Unit |
|---|---|---|
| Almonds | 85-95 | 1000 lbs kernel weight |
| Pistachios | 29 | 1000 lbs marketable yield |
| Walnuts | 40-50 | 1 ton nuts |
Micro-broadcast
In orchards irrigated with microsprinklers, some growers and CCAs have moved to directed or micro-broadcast potassium fertilization that delivers dry potassium (usually potassium sulfate) in the tree row onto the soil wetted by the sprinklers. No fertilizer is spread on the soil in the drive row. Application timing is commonly in the fall.
Band
Banding potassium has been the standard in the industry for many years to overcome potassium fixation, and it works just as well in non-fixing soils. It works by saturating the fixation capacity of a small zone in the soil, ensuring there is potassium available for uptake. It is an appropriate application method as long as the band is in the wetted zone.
Fertigation
Fertigation has become more common, especially as irrigation technology has improved. Fertigation is appropriate for potassium fixing as well as non-fixing soils, provided you apply the potassium when the trees need it (during fruit development). You can avoid fixation of potassium through in-season fertigation.
Foliar sprays
Foliar potassium fertilization through an airblast sprayer can be an effective way to quickly deliver a limited amount of potassium into trees. However, given the large annual K budget for a mature orchard carrying a good crop, foliar potassium is best considered a supplemental program to soil applied K. For example, foliar applications in almonds and prunes using 20-30 lbs potassium nitrate (fertilizer material, not actual K2O)/per acre in 100 gallons per acre has been shown to increase leaf potassium levels over untreated trees. However, with spray application efficiency in the range of 75%, at best, the amount of K from a single application that reaches leaves is roughly 10 lbs K2O/acre/spray. This amount is only 3% of total crop K need assuming 100 lbs K2O per 1000 lbs of kernel crop
removed in a 3000 lb almond crop. In prunes, a crop with a smaller annual K budget than almonds, research showed that foliar potassium application (100 lbs KNO3/acre/year applied in 4 applications from April through July) could keep leaf K levels and yield at a par with a banded soil application of 360 lbs K2O/acre as KCl (600 lbs/acre MOP) applied the previous fall. The authors of this work cautioned that this work was done in a single orchard for only 3 years and so should be tested under other locations/crops before considered for wider application. In addition, the foliar K delivery rate in that trial was less than crop removal, suggesting that a “foliar K, only” program is not a long-term strategy.
Foliar potassium fertilizers come in many forms. There are bulk, dry materials such as potassium nitrate – alone or blended with other dry nutrients -- and specialty liquid materials formulated from sources including potassium acetate, potassium carbonate, potassium nitrate or others.
Rates for potassium nitrate, alone, or in a tank mix with pesticides, run from 10-30 lbs fertilizer/acre. Growers should be aware of the risk of phytotoxicity from higher rates and test application rates and practices in a particular orchard. An experienced PCA/CCA in the Yuba City area never recommended a potassium nitrate rate per acre above 20lbs material/acre in prunes to make eliminate the risk of phyto.
The specialty foliar K fertilizers often range in K2O content from 25-30% with max labeled rates of 4-6 quarts/acre. While these rates are most probably developed to minimize phyto, they also limit the potential K2O rate on the leaves to around 2.5-3 lbs K2O per acre per spray.
Foliar K fertilizer supplements soil applied K, but is not a sustainable, long term stand-alone K fertility program. Growers and PCA/CCAs should consider the level of K nutrition in the orchard and recognize the limited impact of foliar K application when deciding whether to spray K or not.