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.

Mineralization

The mineralization of nitrogen from organic matter depends on the activity of various microbes in the soil. Since soil microbial activity varies with a number of factors, including soil temperature, water content and type of organic matter, estimating how much nitrogen will be available and when is difficult. Factors that do have a significant impact include:

• Mineralization doesn’t occur if the soil temperature is less than 50 F and increases in relation to the temperature of the soil.

• Mineralization is rapid in moist soils but is reduced in soils that are too wet or too dry. This is a factor when placing materials in micro-irrigated orchards as there is wide variability in soil moisture content between the tree rows and orchard middles.

• If the soil is tilled, mineralization rates are increased. However, in most almond orchards there is very little tillage, thus the mineralization rates will be slower.

The type of organic matter and its carbon-to-nitrogen ratio also play a large role in the rate at which mineralization will occur. Almond Board-funded research in the 1990s found that vetch as a cover crop mineralized within two to four weeks of mowing.

Similar data has been seen for incorporated cover crops in vegetable production. Compost and composted manure tend to have higher carbon content and therefore don’t mineralize as quickly. They do, however, increase the organic matter content of the soil, which improves nutrient availability in the long run. Materials with high carbon-to-nitrogen ratio may tie up nitrogen for awhile.

As a reminder, if any form of manure is used, precautions should be taken to avoid contamination with pathogens. It is advised that only properly composted manure be used and it should be applied and incorporated in the fall after harvest. With this food-safety recommendation in mind, a higher carbon-to-nitrogen ratio composted manure may be useful to ensure a slower mineralization rate to avoid leaching with winter rains. Increasing the organic matter content of orchard soils can also help to reduce nutrient leaching in the long term.

In terms of sustainability in the long run, recycling organic sources of nitrogen rather than fossil fuel–based N sources may be desirable. However, such factors as the complexity of the soil, organic matter type, application locations, and other factors require more care to ensure that they are applied in a way that truly benefits the orchard.

For more on creating a nutrient budget for nitrogen fertilizers in almonds, go to AlmondBoard.com/farmpress25. General information on different sources of organic matter can be found in the publication Soil Fertility Management for Organic Crops at http://anrcatalog.ucdavis.edu/pdf/7249.pdf.

This is the third in a series on nitrogen fertilization in almonds. For more, see Part 1 and Part 2.