3.7: How Much Organic Matter Is Enough?

We already mentioned that 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.

Some research has been conducted to determine the levels of organic matter where the fine soil mineral particles become saturated, having adsorbed as many organic compounds as possible. This provides some guidance where the soil is in terms of the current versus the potential organic matter level and whether or not the soil is at an upper equilibrium level. It also tells us whether the soil has the potential to store more organic matter as part of a carbon farming effort (carbon is 58% of organic matter). In this calculation, a soil with 20% silt and clay, for example, can store a maximum of 3.6% organic matter, while a soil with 80% silt and clay can hold 6.1% organic matter. This does not include the additional particulate organic matter that may be either subject to rapid decomposition (active) or protected from decomposition by soil organisms inside small (micro) aggregates (part of the passive organic matter). However, the clay content and type of clays present influence the amount of organic matter particles “stored” inside micro-aggregates. 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. We now have a soil test that tells you early on whether you are moving your organic matter levels in the right direction. It determines the amount of organic matter thought to be the active portion, is more sensitive to soil management than total organic matter and is an early indicator for soil health improvement (see Chapter 23).

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