ยากต่อการตัดสินใจ

ยากต่อการตัดสินใจ

ผลลัพธ์ (อังกฤษ) 1:

Difficult to decide.

ผลลัพธ์ (อังกฤษ) 2:

Difficult decisions

ผลลัพธ์ (อังกฤษ) 3:

It is not easy to dramatically increase the organic matter content of soils or to maintain elevated levels once they are. Reached. It requires a sustained effort that includes a number of approaches that add organic materials to soils and minimize. Losses. It is especially difficult to raise the organic matter content of soils that are very well aerated such as, coarse. Sands because the, potential for aggregation (which protects particles of organic matter), is limited as are the fine minerals. That form protective bonds with organic matter. Soil organic matter levels can be maintained with lower additions of organic. Residues in high-clay-content soils with restricted aeration than in coarse-textured soils because of the slower, decomposition. Organic matter can be increased much more readily in soils that have become depleted of organic matter than in soils that. Already have a good amount of organic matter with respect to their texture and drainage condition.Figure3.6.When you change practices on a soil depleted in organic matter perhaps one, that has been intensively row-cropped for years. And has lost a lot of its original aggregation organic matter, will increase slowly as diagrammed, in Figure 3.6. At first. Any free mineral surfaces that are available for forming bonds with organic matter will form organic-mineral bonds. Small. Aggregates will also form around particles of organic matter. Then larger aggregates will form made up, of the smaller aggregates. And held by a variety of means - frequently by mycorrhizal fungi and small roots. Once all possible mineral sites have been. Occupied by organic molecules and all of the small aggregates have been formed around organic, matter particles organic. Matter accumulates mainly as free particles - within the larger aggregates or completely unaffiliated with minerals. This. Is referred to as free particulate organic matter. After you have followed similar soil-building practices (for, example. Cover cropping or applying manures), for some years the soil will come into equilibrium with your management and the total. Amount of soil organic matter will not change from year to year. In a sense the soil, is "saturated with organic matter." As long as your practices don 't change. All the sites that protect organic matter (chemical bonding sites on clays and physically. Protected sites inside small aggregates), are occupied and only free particles of organic matter (POM) can accumulate. But. Because there is little protection for the, free POM it tends to decompose relatively rapidly under normal (oxidized) conditions.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.