Issue link: https://www.e-digitaleditions.com/i/1202547
A L M O N D O R C H A R D 2 0 2 5 G O A L S ZERO WASTE 12 "You don't see any nitrogen uptake, you don't see distinguishable demand, until you get to flower development in late February or early March, when the temperatures rise and the days get longer," Navarra said. That delay in nitrogen uptake creates the opportunity for the leftover nitrogen from the previous cole crop to mineralize – that is, convert from organic nitrogen to more plant-available forms like ammonium and nitrate. Nitrate is mobile and moves with water in the soil, however, so this valuable nutrient can be lost as winter rains or irrigation progresses if there are no actively growing plants to take it up. "As soon as that first irrigation goes on, it's kind of like the start of the race for mineralization because of the activity in the soil," Navarra said. That's where ground almond shells come in. By introducing more carbon to the soil via the shells and creating a higher carbon-to-nitrogen ratio, microbes that immobilize nitrogen and delay re- mineralization are encouraged. This buys time for the strawberries to be ready to take up the nitrogen. The goal, Navarra said, is to achieve "synchrony" between the time the nitrogen is available and when the plants need it. Both researchers noted that the rate of almond shell application is critical. Too much carbon in the soil can tie up the nitrogen from plant residues for too long, making them unavailable when the next crop needs them. While a five-ton-per-acre application of almond shells appeared to work well in 2018, researchers will continue to study application rate and timing. Muramoto said it may not be necessary to apply carbon-rich amendments between every cole-to-strawberry crop rotation if enough residual carbon remains in the soil. Replication may lead to increased relevancy While the 2018 trial took place on heavy clay soils, the new trial, also held in the Watsonville area, will happen on lighter soils. Of course, every winter season is unique, but researchers hope to see that despite different conditions improved yields and favorable nitrogen mineralization trends still occur. If growers can receive some assurance that the nitrogen from the cole crop will still be there when their strawberries need it, Muramoto said, that could give them the confidence to apply less additional nitrogen fertilizer, a decision with both economic and regulatory implications. Navarra said strawberry growers today apply 100 to 150 pounds of nitrogen fertilizer per acre, pre-plant, to their crops, in addition to the nitrogen that is available from the previous crop residue. While that is less nitrogen than has been applied in the past, growers remain under increasing regulatory pressure to reduce nitrate leaching and increase nitrogen use efficiency in their crops. Adding carbon-rich soil amendments, such as almond shells, could help growers deal with tightening restrictions. "Nitrogen budgeting is becoming more and more important," Navarra said. "As new agriculture water quality orders come out, nitrogen budgets are becoming more and more restrictive." While Muramoto specializes in organic crops – he is the first UC Cooperative Extension specialist dedicated to organic agriculture – he noted that using high-carbon soil amendments like ground almond shells could be useful for both organic and conventionally grown strawberries. With more research the technique may prove beneficial in other crops and other regions, such as the Central Valley. Almonds remain committed to Zero Waste Goal Supporting efforts to find new and better uses for almond coproducts – including hulls, shells and trees – remains an important priority for the California almond industry as it strives to reach its Almond Orchard 2025 Goal of achieving zero waste by ensuring everything grown in the orchard is put to optimal use, both economically and environmentally. "California almond growers and processors have always found uses for these coproducts and are constantly on the lookout for more beneficial outlets," said Guangwei Huang, associate director of Food Research and Technology at ABC. "The changing market for these materials has led the California almond community to focus its research investment on new uses for coproducts, addressing needs across multiple industries including other sectors of agriculture," Huang said. "We are excited by this research and its potential to help ensure the future of California almond orchards and strawberry fields, as well as the hardworking individuals who make up both industries." During the trials, researchers use one-meter square of PVC pipe to help measure the exact amount of almond shells applied to the soil.