Does Soil Biology Control Nutrient Availability of Organic Matter Amendments in Orchards?.

California orchard systems offer a unique opportunity to investigate soil health benefits from integrated nutrient management practices. Almonds (Prunus dulcis [Mill] D.A. Webb) utilize large amounts of chemical fertilizer, microirrigation where water is applied unevenly across the orchard floor, and minimal or no tillage. In addition, California has the largest dairy industry in the U.S. and mandated green waste recycling programs in proximity to orchards. This combination has resulted in an opportunity to integrate organic matter amendments (OMA) into the nutrient management of orchards. We examined multiple OMA sources and the timing of OMA application on soil microbial biomass carbon (C) and nitrogen (N) and diversity using fumigation-extraction and sequencing techniques as well as nutrient availability using ion exchange resins (IER). All OMA treatments were applied in addition to the control rate of chemical fertilizer. After year one of our study, both composted manure and green waste compost OMA sources significantly increased (p < 0.05) total soil N and organic C (0 – 50 cm) compared to the control where chemical fertilizer was applied without OMA. Yet, in year two when the same rate of OMA was applied to treatment plots, the effect was no longer significant. Similarly, greater P inputs from composted manure and green waste compost resulted in significantly greater (p < 0.05) adsorption of PO₄^3--P surfaces of IER membranes (0 – 10 cm) compared to the control during year one. However, the effect was no longer significant during year two. Furthermore, we examined if similar effects occur for microbial biomass and diversity and if there is a connection between soil biology and nutrient availability in orchards. The influence of organic matter amendments on nutrient availability is shown however; the role of soil biology plays in orchard soil fertility over multiple growing seasons remains in question.