Based on various surveys, a good number of almond growers supplement their fertilization programs with organic matter sources, including composted manure, compost, and, where there is enough natural rainfall, mowing of cover crops. The addition of organic matter to agricultural soils can be beneficial in terms of soil water holding capacity, nutrient holding capacity, and overall soil tilth. However, the management of organic matter sources of nitrogen is not straightforward in terms of ensuring most of the nitrogen does not move off-site, in particular is not leached into groundwater.

California growers face significant scrutiny for their contribution over the years to nitrates in groundwater. The California legislature required the State Water Board to commission a study on how best to improve drinking water quality for communities with high nitrates in their well water.  The legislation required the study to focus on the Tulare and Salinas basins as models for the issue of nitrate in groundwater. The study by a team of UC Davis researchers was scheduled to be released March 13 and is likely to significantly raise the profile of this issue.

The legislation also requires the Central Valley Regional Water Board to implement any recommendations on how to improve drinking water quality in terms of nitrates within two years of the report. Previews of the report indicate that nitrogen fertilizers are a main source of the nitrates from years of use, in addition to cattle/dairy operations, and in some localized areas, septic systems. 

This article was written prior to the report’s release and so exact recommendations were not clear. What is clear, however, is the issue of off-site movement applies not only to mineral fertilizers but also to nitrogen coming from organic matter sources, including incorporated cover crops, green manures, compost and composted manures. If poorly timed, organic matter sources of nitrogen can also contribute to nitrates available for leaching.

Thus, almond growers who use some organic matter to contribute nitrogen to their trees should consider how their form of organic matter and the timing of the application may increase or decrease the chance that nitrogen will leach into groundwater. While there is not a lot of data for orchard crops, there are some general trends based on studies in vegetable crops that might help guide growers’ decisions about how to manage organic matter to improve the conditions for nitrogen release when it can be taken up by the tree.