When management practices are used that deplete organic matter, the reverse of what is depicted in figure 3.6 occurs. First free POM is depleted, and then as aggregates are broken down physically protected organic matter becomes available to decomposers. What usually remains after many years of soil-depleting practices is the organic matter that is tightly held by clay mineral particles.
Assuming that the same management pattern has occurred for many years, a fairly simple model can be used to estimate the percent of organic matter in a soil. It allows us to see interesting trends that reflect the real world. To use this model you need to assume reasonable values for rates of addition of organic materials and SOM decomposition rates in the soil. Without going through the details (see the appendix for sample calculations), the estimated percent of organic matter in soils for various combinations of addition and decomposition rates indicates some dramatic differences (table 3.2). It takes about 5,000 pounds of organic residues added annually to a sandy loam soil (with an estimated decomposition rate of 3% per year) to result eventually in a soil with 1.7% organic matter. On the other hand, 7,500 pounds of residues added annually to a well-drained, coarse-textured soil (with a soil organic matter mineralization, or decomposition, rate of 5% per year) are estimated to result after many years in only 1.5% soil organic matter.