Soil Organic Matter

Soil organic matter (SOM) is a small but critical component of soils. SOM is continuously being produced by plants and animals and broken down by soil microbes that use it as a source of energy. As such it provides food for a diverse population of microbes in the soil and this helps prevent any one type of organism, such as a plant pathogen, from dominating. As microbes break down SOM, nutrients are released which are available for plant growth. This process is called mineralization and can provide some or all of the nutrients needed for successful crop production. Soil microbes are most active in warm soils (over 70°F) that are moist, but well aerated, with a pH between 6 and 7 (ideal conditions for most fruit crops). Mineralization of nutrients will proceed rapidly under these conditions.

SOM also improves soil structure. It binds individual soil particles together into aggregates. This makes soil friable, allowing for good drainage, aeration, and root growth. SOM also improves the moisture holding capacity of soils. SOM is also the chief contributor to cation exchange capacity in New England soils.

Adding to Soil Organic Matter

Using compost is an effective way to add organic matter to the soil. Small fruit growers can make compost on the farm although most don’t have enough raw materials to satisfy their needs. Some bring in additional materials such as municipal yard wastes to compost on site. Others purchase compost from the increasing number of commercial composters. Regardless of the source, compost should be finished before use. Finished compost has no recognizable bits of matter and will not heat up after turning. Compost should also be tested for nutrient content. Finished compost should have a low ammonium content, high nitrate level and a pH near neutral. Repeated use of a compost high in a particular element could cause a nutrient imbalance. For more information, obtain a copy of Berry Soil and Nutrient Management – A Guide for Educators and Growers (see Resources in Appendices at the end of this publication).

Animal manure is an excellent source of nutrients and organic matter. About half of the nitrogen in fresh dairy manure and 75% of the nitrogen in poultry manure is in the form of ammonia. Ammonia is subject to loss through volatilization if not incorporated immediately after spreading. In the soil, ammonia is converted to nitrate and is available for plant use. However, nitrate is subject to leaching and large applications should generally be avoided. There are times when readily available N is needed, but fresh manure should be applied only with caution. Many people prefer to compost manure before field application to stabilie the N contained in manure. Manure can be mixed with other materials for composting. Manure samples can be analyzed by several of the laboratories listed under Soil Testing

Manure often contains human pathogens and therefore application practices that avoid transferring pathogens to produce should be utilized when using manure in food production systems. See more on safe application practices in the Produce Safety section.

Cover crops are used by most growers to protect soil from erosion and to take up unused N. Cover crops also contribute to SOM when they are plowed down, although SOM varies considerably among crops (see Cover Crop section).

Carbon-To-Nitrogen Ratio

Organic matter is broken down by microbes which use carbon (C) for energy. They also have a need for nitrogen. Microbes have a requirement of about one N atom for each 25 C atoms. This is a carbon-to-nitrogen ratio (C:N) of 25:1 or 25. If the organic matter has a higher C:N (more C and less N), microbes will need more nitrogen and will take it from the soil. Microbes are more efficient than crops in obtaining N from the soil. If there is not enough nitrogen for both the microbes and the crop, the crop will not obtain what it needs. Eventually there will be a net gain in nitrogen, but crops can suffer in the short term. If organic matter with a high C:N is applied to soil shortly before planting a crop, additional N may be needed to assure the needs of both the microbe and the crop are met. Organic matter with a C:N of less than 25:1 (25) should not be a problem and in some cases can contribute N for crop use. See Table 5 for examples of C:Ns of some sources of organic matter.