4.3 Soil and sustainability  (Page 2/8)

Soil forming factors

The fundamental factors that affect soil genesis can be categorized into five elements: climate, organisms, relief, parent material, and time. One could say that the landscape relief , climate , and organisms dictate the local soil environment, and act together to cause weathering and mixing of the soil parent material over time . The soil forming factors are interrelated and interdependent, but considered independently they provide a useful framework for discussion and categorization.

As soil is formed it often has distinct layers, which are formally described as "horizons." Upper horizons (labeled as the A and O horizons) are richer in organic material and so are important in plant growth, while deeper layers (such as the B and C horizons) retain more of the original features of the bedrock below.

Climate

The role of climate in soil development includes aspects of temperature and precipitation. Soils in very cold areas with permafrost conditions ( Gelisols ) tend to be shallow and weakly developed due to the short growing season. Organic rich surface horizons are common in low-lying areas due to limited chemical decomposition. In warm, tropical soils ( Ultisols, Oxisols ), other factors being equal, soils tend to be thicker, with extensive leaching and mineral alteration. In such climates, organic matter decomposition and chemical weathering occur at an accelerated rate.

Organisms

Animals, plants, and microorganisms all have important roles in soil development processes, in providing a supply of organic matter, and/or in nutrient cycling. Worms, nematodes, termites, ants, gophers, moles, crayfish, etc. all cause considerable mixing of soil and help to blend soil, aerate and lighten the soil by creating porosity, and create characteristic natural soil structure over time. Animal life, such as insects and mammals, can cause irregularities in the soil horizons.

Plant life provides much organic matter to soil and helps to recycle nutrients with uptake by roots in the subsurface. The type of plant life that occurs in a given area, such as types of trees or grasses, depends on the climate, along with parent material and soil type. So there are clearly feedbacks among the soil forming factors. With the annual dropping of leaves and needles, trees tend to add organic matter to soil surfaces, helping to create a thin, organic-rich A or O horizon over time. Grasses, on the other hand, have a considerable root mass, in addition to surficial organic material, that is released into the soil each fall for annuals and short-lived perennials. For this reason, grassland soils ( Mollisols ) have much thicker A horizons with higher organic matter contents, and are more agriculturally productive than forest soils. Grasses release organic matter to soils that is more rich in base cations, whereas leaf and needle litter result in release of acids into the soil.

Microorganisms aid in the oxidation of organic residues and in production of humus material. They also play a role in iron oxidation-reduction cycles, fine-grained mineral dissolution (providing nutrients to soil solutions), and mineral neoformation. New research is continually expanding our knowledge of the role of microorganisms in plant growth, nutrient cycling, and mineral transformations.