Effects of organic matter amendments on soil nutrient availability and tree nutrient status in California almond

Organic matter amendments (OMA) offer a viable option to supplement or partially substitute fertilizers. We examined the effects of composted manure and green waste compost on soil nutrient availability and tree nutrient status in a nonbearing almond orchard during 2016 and 2017. Leaf N, P and K concentrations were not significantly different between OMA sources and the control during both years. During year one, leaf nutrients were nearly equal or lower than the control. While by year two all leaf nutrients from OMA sources increased to a greater extent than the control, although not significantly different. Changes in soil organic matter measured by soil organic carbon (SOC) and total nitrogen (TN) determine if OMA sources are accumulating or depleting in orchard soil. In both years, SOC from the top soil (0-10 cm) was significantly greater for composted manure and green waste compost compared to the unamended control. The same trends were apparent for TN but, without any significant differences between composted manure and the unamended control in year two. Overall, SOC and TN from composted manure and the control decreased from year one to two, while the same parameters for the green water compost remained unaffected. There were no significant differences in inorganic N during both years between OMA sources and the control however, inorganic N from OMA sources increased relative to the control in year two. Soil inorganic P was nearly significantly different in year one (p=0.07) between the composted manure and control due to greater P concentrations from composted manure compared to the control. Exchangeable K from soils amended with composted manure were significantly greater than green waste compost and control during both years. Our results demonstrate OMA sources increase plant available P and K and changes in soil organic matter from OMA use may impact both P and K availability over the short and long-term. As a whole, these results demonstrate nutrients from OMA sources require from one to two years to become available to trees.