Use of Almond Hull and Shell as Organic Matter Amendments in Advanced Orchard Management

August 2020-July 2021

Data that is being collected from all field sites will be analyzed this Fall with further results anticipated by the end of 2021. All plans outlined in the original submitted timeline chart have been fulfilled since August 2020 with the exception of implementation of catch frame harvest at the Woodland site last Fall and survey distribution. The catch frame harvest treatment will be implemented in August 2021 and the survey will be distributed at the Almond Conference 2021. 

August 2021-current

Across all three field sites, the percent potassium remaining in the different hull/shell amendment sources decreased over time, indicating potassium released into soil solution. Corresponding substantial increases in soil exchangeable potassium were found at all three sites, regardless of irrigation type. The potassium solubilization curves from these field sites resemble similar potassium release curves from other crop residues as described in the review paper, Andrews et al. 2021.  Ms. Andrews is currently working with a statistician to develop a model for estimating percent potassium released form these amendments as a direct function of water applied (irrigation and rainfall).  Potassium release is characterized by a dramatic reduction in K within the first 3 inches of applied water, followed by a more gradual reduction over time. Overall, around 30-50% of initial K was released within the first 3 inches of water, and then around 80-95% of total K was released by the following summer. Generally, from Fall until the following summer, shells decomposed around 20-30% total, the shell-based compost at Bullseye decomposed by around 50%, hull/shell mix decomposed around 50%, hulls decomposed around 60%. When allowed to remain on the soil surface under catch frame harvest, the hull/shell mix at Westwind decomposed by 45% after 365 days. 

At Crown Nut Company, there were no statistically significant differences in July leaf values in 2020 or 2021. There were no signs of any potential nitrogen immobilization in the soil impacting July leaf N values. Average leaf K was slightly higher for the amended trees in July 2021, though nonsignificant. Amendments maintained yield in 2020 and 2021 as there were no differences in average dry kernel lb/ac or average % crackout. While a small percentage of amendment materials were found in yield samples, the grower/processor reported this did not interfere with processing. Hulls, mix, and shells released an estimated 103-135 total lb/ac of potassium at approximately $52-68/ac value. The grower reported that potassium sulfate fertilizer costs around $200 for 400 lb/ac, whereas the amendment materials are free for them. 

At Bullseye, from Fall 2020 application until the following summer, the shells released approximately 80% of total K while compost released approximately 60%. However, dry mass remaining was approximately 70% for shells and 50% for compost, illustrating that potassium release is not strongly limited by decomposition rate. The shells released a higher fraction of initial potassium, and the compost decomposed relatively more than the shells. Potassium release corresponded with significant increases in soil exchangeable potassium under both amendments from around one month (1.85 inches of water) following application onward. All July leaf nutrients were in the optimum range, and there were no significant differences between treatments. Average leaf potassium under shells was slightly higher, though nonsignificant. Shells and compost maintained yield as there were no significant differences in average dry kernel lb/ac or average percent crackout. 

At Westwind, the hull/shell mix amendment applied in Fall 2020 released around 95% of total initial potassium by June. This corresponded with significantly higher soil exchangeable potassium particularly in winter and spring. After one year, the amendment decomposed only by around half of its dry weight. Average July leaf nutrients in 2021 showed statistically significantly higher leaf potassium and lower leaf magnesium under the amendments. However, all macronutrients fell well within the recommended range, except nitrogen which was similarly deficient in both amended and control trees. Stem water potential was significantly less negative for amendment trees when measured six days after irrigation events in April, June, and July, but not when measured only 3 days after irrigation. This suggests that the amendment can reduce tree drought stress particularly during the later period of an irrigation cycle. Collaboration with irrigation scientists using Electrical Resistivity Tomography imaging indicated that the amendment provided a mulching effect by increasing irrigation infiltration rate and reducing evaporation from the soil surface. Average kernel lb/ac yield and average percent crackout were both slightly higher for amended catch frame harvested trees, though not statistically significant. Issues with the catch frame equipment illustrated the importance of working with effective equipment that is adjusted appropriately for almond trees and experienced operators who can ensure efficacy in the future.

At Westwind, baseline microbial biomass carbon in Fall 2020 was similar across all treatment rows. By Fall 2021 we saw significant (p<0.001) treatment effects. Control rows had the least microbial biomass carbon, with the unamended off-ground harvest rows having slightly more. The rows which had received the almond hull and shell amendment had the most microbial biomass carbon, with the amended off-ground harvest rows having even more than their amended on-ground harvest counterparts. Statistical analysis showed that when amendment and harvest type were analyzed separately for Fall 2021, the effect of amendment was highly significant (p<0.0001) while the effect of harvest type was not yet significant (p=0.1486). In the intermediate timepoint, Spring 2021, the reverse order of treatments was observed, with the control rows having the highest microbial biomass carbon and the amended rows having the lowest, though the treatment effect at this timepoint was not statistically significant. This intermediate step is likely the effect of temperature, with the amendment acting as a mulch to increase soil moisture retention and create cooler soil temperatures, especially in the spring. By the fall, however, the hypothesized treatment effects had emerged.

At Westwind, samples were taken for 16S and ITS shotgun sequencing at three time points and stored at -80 C. However, after contracting with a sequencing laboratory, we were told that they were unable to extract enough DNA to move forward with sequencing. Instead, microbial community composition was assessed using PLFA (phospholipid fatty acid analysis) in both the amendment organic layer and the top 0-10cm soil. In October 2021, the 1 year old amendment layer contained around double the microbial biomass compared to the 1 week old amendment layer. The original amendment had higher total bacteria and total fungi biomass, specifically higher actinomycete, saprophyte, mycorrhizae, and protozoan biomass, with a higher ratio of fungi to bacteria. Compared to the control soil, the soil microbial community under the combined amendment and catch frame treatments contained higher average total biomass, including both higher total bacteria and fungi biomass, specifically higher actinomycete, mycorrhizal, and protozoan biomass with a very low fungi to bacteria ratio. The PLFA data indicates that the amendment layer is full of microbial biomass, can support higher trophic levels such as protozoa, and is building a microbial food web over time. Compared to control soil, the soil with the amendment and catch frame contained higher soil microbial biomass after one year and Principal Components Analysis indicated that higher bacterial biomass in particular distinguished it from the control soil. There was a slight increase in arbuscular mycorrhizal biomass under the amendment as well.