California growers face significant scrutiny for their contribution over the years to nitrates in groundwater. Off-site movement applies not only to mineral fertilizers, but also to nitrogen coming from organic-matter sources, including composted manure, compost and mowing of cover crops where there is enough natural rainfall.
In the long run, recycling organic sources of nitrogen rather than fossil fuel–based N sources may be desirable from a sustainability standpoint. However, the complexity of the soil, organic-matter type, application locations and other variables do require more care to ensure nitrogen sources are applied in a way that truly benefits the orchard.
While there is not a lot of data for orchard crops, there are some general trends that might help guide growers’ decisions about managing organic matter to improve the conditions for nitrogen release, so it can be taken up by the tree and thus reduce the chance it will contribute to groundwater leaching.
The mineralization of nitrogen from organic matter depends on the activity of various microbes in the soil. Microbial activity varies due to a number of factors, including soil temperature, water content and type of organic matter.
In general, we know that mineralization doesn’t occur if the soil temperature is less than 50° F, and increases the warmer the soil. Mineralization is rapid in moist soils, but is reduced in soils that are too wet or too dry.