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.