Compost spreaders are working in almond orchards in this area. The addition of composted organic matter – often composted yard waste — to soil should generally benefit soil physical and chemical properties. However, it isn’t a source of short-term plant available nitrogen. Research by UC Davis specialists Tim Hartz and Jeff Mitchell has shown that composted yard waste, when mixed with sand and held under constant, optimum moisture and temperature (the optimum conditions for mineralization of organic nitrogen) produces a very little plant available nitrogen (2% of total nitrogen in the compost) in the short run (12-24 weeks). When compost is broadcast on the soil surface and not incorporated, plant available nitrogen should be even less. Dr. Hartz did conclude that manures and composts had value in long-term soil building in vegetable crop production, where those materials are incorporated into the soil. [Untreated/uncomposted manure use in almonds is a food quality risk and not supported by the almond industry.]
Why bring up compost use? Growers are willing to pay for it and seem happy with the results. Here’s why. If California agriculture ends up in a regulatory situation where nutrient budgets are required of growers, it should be recognized that the short term nitrogen benefits from composted yard waste are almost non-existent and the long term benefits are unknown.