Soil Organic Matter
Inherent Factors Affecting Soil Organic Matter
Inherent factors affecting soil organic matter include climate, soil properties, topography, and vegetation. Climatic conditions, such as rainfall and temperature, and soil moisture and aeration (oxygen levels) affect the rate of organic matter decomposition. Organic matter decomposes faster in warm, humid climates and slower in cool, dry climates. It also decomposes faster when the soil is well aerated (higher oxygen level) and much slower when the soil is saturated (lower oxygen level). Soils that support grass vegetation (prairie) commonly have at least twice as much organic matter as those that support forest vegetation.
Temperature
High average temperatures promote plant growth. However, organic matter decays more rapidly at higher temperatures, so soils in warmer climates tend to contain less organic matter than those in cooler climates. As a simple guide, organic matter is generated faster than decay when soil temperatures are below 77 degrees F (25°C), and decay almost stops below 41 degrees F (5°C).
Soil Moisture and Water Saturation
Soils in arid climates usually have low amounts of organic matter. In a very dry climate, such as a desert, there is little growth of vegetation. Decomposition is also low because of low amounts of organic inputs and low microorganism activity when the soil is dry. Soil organic matter levels generally increase as average annual precipitation increases.
Soil Texture
While climate affects soil organic matter over broad geographic areas, soil texture is often responsible for marked differences in soil organic matter within a local landscape. Under aerobic conditions, fine-textured soils, containing high percentages of clay and silt, tend to have naturally higher amounts of soil organic matter than coarse-textured sands or sandy loams.
Salinity and Acidity
Salinity, toxicity, and extremes in soil pH (acid or alkaline) result in poor biomass production and, thus in reduced additions of organic matter to the soil. For example, pH affects humus formation in two ways: decomposition, and biomass production. In strongly acid or highly alkaline soils, the growing conditions for microorganisms are poor, resulting in low levels of biological oxidation of organic matter.
Soil Organisms
Organic matter consists of numerous compounds which vary greatly in their ease of decomposition. Different groups of organisms have different capacities in the process of decomposition. The product of one group can be the source of food for others to further break down organic material. Added organic matter will be colonized and decomposed by a community of organisms that will change over time as the composition of the residue changes.
Vegetation and Biomass Production
Vegetation has a profound bearing on the soil organic matter level because different vegetation produces different amount of biomass and of varying decomposability. Forest and grassland soils are particularly rich in organic matter. Soils that form under grassland vegetation generally contain more and a deeper distribution of organic matter than soils that form under forest vegetation.
Topography
Organic matter accumulation is often favored at the bottom of hills. There are two reasons for this accumulation: conditions are wetter than at mid- or upper-slope positions, and organic matter is transported to the lowest point in the landscape through runoff and erosion.
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