New studies show that almond husks and husks release potassium into the soil when applied topically as organic matter amendments in the orchard. The release of potassium from hulls and shells is strongly influenced by water application and is not initially limited by the rate of decomposition.
This practice can help increase soil exchangeable potassium with benefits for July leaf potassium values. In the long term, hull and hull amendments could be used with other soil health practices such as reduced soil disturbance to increase soil organic matter content while reducing dust and erosion. . Hulls and shells are often available locally and can be returned to the orchard inexpensively.
High concentrations of potassium distinguish cockles and shells from other organic matter amendments. For many almond growers, potassium (K) accounts for a large portion of fertilizer costs. Potassium export in almonds can range from 236 to 347 lb/ac K.
However, it is important to consider the contribution of K from the soil: if the exchangeable potassium of the soil is between 100 and 150 ppm, a K application of approximately 100 lbs/acre can satisfy the K demand of the crops. Almond growers in the Sacramento Valley typically use about 400 lb/ac K2SO4 annually for mature orchards.
Returning cockles and shellfish to the orchard could help cover the necessary K inputs. The total amount of K released depends on the application rate, the initial K concentration of amendments, and total rainfall and irrigation.
Three commercial almond orchards near Woodland, Davis and Tracy are holding field trials with shell and hull amendments. In these three separate field trials with different shell-shell materials and different irrigation systems, soil exchangeable potassium was significantly higher under the shell-shell amendments after about 1.5 to 2 inches of water (irrigation and precipitation).
Each amendment treatment was applied to entire rows of at least 40 trees and compared to a control row without amendment. Treatments were replicated in randomized full block designs with 4 blocks each containing all treatments.
Since the field sites differed in soil type (clay, silty clay loam, and loam), irrigation approach (two drip irrigated orchards, one micro-sprinkler), and nutrient management, each field site was approached as a unique case study with different application rates tailored to meet the interests and goals of different growers.
Application rates ranged from 2.5 to 9.5 tonnes/acre. At each site, the growers’ application of mineral potassium was consistent across all treatments.
Experimental design board. Each yellow bar represents a Nonpareil line that has received treatment. Each treatment was randomized and repeated in four blocks in areas of the orchard that had a consistent soil type. Click on the image to enlarge
Approximately 35-70% of the total K in the shell and hull materials was released within the first 3 inches of water application, which significantly increased the exchangeable K of the soil under the amendments at all sites . After this initial steep release curve, the release of K was more gradual. Based on these trials so far, total K release can vary between 77-95% from fall application through the following summer.
These results are supported by numerous previous studies that conclude that water application is the primary driver of K release from crop residues into the soil, making K available for uptake by plants.
These results can help growers estimate the amount of potassium likely to be made available in the soil from an application of hulls, hulls, or a mixture of hulls and hulls.
For example, an application of 5 fresh tons of hull-hull mix with 2.37% K and 13% moisture has the potential to deliver up to 208 lbs of K (250 lbs of K2O). To calculate the K2O equivalent and compare with fertilizers, it is important to know that each pound of K is equivalent to 1.2 lb K2O.
2.37% K * 0.01 * 2,000 lbs of shell-shell mix = 47.4 lbs K per ton of shell-shell mix, where 0.01 is a conversion factor allowing us to use %K to find lb K in 1 ton of mix
47.4 lbs K per ton x 5 tons x 0.87 dry weight = 206 lbs K x 1.21 = ~250 lbs K2O, where 0.87 is the dry weight fraction since the moisture content is 13%
If 85% of this total K is released from amendments into the total soil over a season, this equates to approximately 175 lb K (212 lb K2O).
Table 1. Initial Mean Potassium in Hull, Mix, and Hull Amendments. Source: %K averages from unpublished data from Andrews, 3 field sites, 2019-2022.
Growers can adjust the application rate to achieve site-specific K targets, such as increasing levels of exchangeable K in the soil, matching the amount of K exported at harvest, or increasing levels of K in the July leaves. See Table 1 above for differences in K concentration in July leaves between control trees and trees amended with hulls and hulls.
So far, none of the three field trials have found an effect of potential immobilization of soil microbial nitrogen on July leaf nitrogen. Avoiding extremely high levels of these amendments might be advisable to reduce the potential for high K uptake competing with magnesium uptake. After one season, hulls decompose ~20-30% by dry weight, while hulls and hull-shell mixtures tend to decompose ~40-50%.
The trash in the harvest yield samples was weighed and separated to compare the shell-and-shell trash to all other trash. Hull-hull waste only made up about 2.4-10.6% of all yield sample waste – hull-hull amendments left in the driveway likely filter yield samples when picked up because of their light weight and small size.
Applying hulls and hulls as soil surface amendments above tree roots can increase soil exchangeable potassium in the tree row. Hulls and shells have different bulk density, K concentration, release characteristics, and interactions with soil properties than mineral K fertilizers. Existing recommendations suggest that mineral K may be line banded.
Table 2. Review of July K-sheet results from three field trials sampled in 2021. Trees amended with hull-hull mixture in loam soil at 8 tons/acre showed a
July leaf K compared to control trees. July leaf K increased at other sites to a lesser degree. Typically, 1.4-2.7% K is a sufficient range for Almond July Leaf K values (FREP CDFA).
These trials indicate that almond shell amendments on the soil surface release potassium under water application (drip and micro), increase levels of exchangeable potassium in the soil, and increase potassium values of July tree leaves in varying degrees. This research is ongoing: researchers are studying the short- and long-term effects of this practice on the K cycle, soil-plant water dynamics, soil health and crop productivity.
We have yet to answer the question of whether these amendments provide a mulching effect to maintain soil moisture storage and reduce evaporation from the soil surface relative to bare soil, and the associated potential effects on the soil. water status of trees over time.
We are also investigating the effects of this practice on components of soil health, including changes in the composition and function of the soil microbial community, potential improvements in soil fertility parameters (in addition to potassium) and potential improvements in soil physical properties that benefit water and soil quality. nutrient retention.
Further long-term research is needed to understand how best to use this practice to increase yield in different settings, such as in potassium-deficient orchards.
Overall, almond shells and hulls make up about 70% of the crop weight leaving the field at harvest. After processing, billions of pounds of these materials remain every year, even after some have been sold to the dairy industry. Instead of leaving the field for good, applying hulls and husks recycles K-value back into the orchard.
Nearby orchards can be a convenient outlet for processors who need a destination for cockle and shellfish stocks, which could help prevent biomass buildup and mulch fires. These materials can be surface applied each year in the fall to tree roots for nutrients or to driveways as a mulch.
Although this practice is compatible with ground harvesting, aboveground harvesting equipment could help maintain this layer of mulch on the soil surface for longer periods of time, release a higher fraction of K amendment and create an organic layer on the soil surface.
Hulls and shells can be used alongside other soil health practices, such as compost applications, cover crops, and whole orchard recycling. Hulls and side-spread husks can help concentrate K on tree roots, while husks and diffusion husks can provide carbon inputs across the entire soil surface, including the driveway. Growers can adjust application strategies to meet the needs of their orchard.
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