As mentioned earlier, soils with higher levels of fine silt and clay usually have higher levels of organic matter than those with a sandier texture. However, unlike plant nutrients or pH levels, there are few accepted guidelines for adequate organic matter content in particular agricultural soils. We do know some general guidelines. For example, 2% organic matter in a sandy soil is very good and difficult to reach, but in a clay soil 2% indicates a greatly depleted situation. The complexity of soil organic matter composition, including biological diversity of organisms, as well as the actual organic chemicals present, means that there is no simple interpretation for total soil organic matter tests. We also know that soils higher in silt and clay need more organic matter to produce sufficient water-stable aggregates to protect soil from erosion and compaction. For example, to have an aggregation similar to that of a soil with 16% clay and 2% organic matter, a soil with close to 50% clay may need around 6% organic matter.
Organic matter accumulation takes place slowly and is difficult to detect in the short term by measurements of total soil organic matter. However, even if you do not greatly increase soil organic matter (and it might take years to know how much of an effect is occurring), improved management practices such as adding organic materials, creating better rotations, and reducing tillage will help maintain the levels currently in the soil. And, perhaps more important, continuously adding a variety of residues results in plentiful supplies of “dead” organic matter—the relatively fresh particulate organic matter— that helps maintain soil health by providing food for soil organisms and promoting the formation of soil aggregates. A recently developed soil test that oxidizes part of the organic matter is thought to provide a measure of active carbon. It is more sensitive to soil management than total organic matter and is thereby an earlier indicator for soil health improvement. Interpretation of the test is currently an active research area. (See chapter 22.)
The question will be raised, How much organic matter should be assigned to the soil? No general formula can be given. Soils vary widely in character and quality. Some can endure a measure of organic deprivation… others cannot. On slopes, strongly erodible soils, or soils that have been eroded already, require more input than soils on level lands.—Hans Jenny, 1980