Soil

Soil is the basic component of rangeland ecosystems and is associated with nearly all ecosystem processes. It provides a medium to support plant growth and is the home for innumerable insects and microorganisms. In terms of human life-spans, it is a nonrenewable resource and should be treated as such. Soil is a product of parent material, climate, living organisms, topography and time. The soil formation processes work slowly, especially in arid and semiarid climates. It is believed to take several hundred years to replace an inch of top soil lost by erosion. No management decision should be made without a careful consideration of its impact on the soil.

The type of soil present in a range management unit is important to both the kind and amount of forage produced and the type of management that is possible or appropriate. The chemical and physical characteristics of a soil determine: its ability to furnish plant nutrients; the rate and depth of water penetration; and the amount of water the soil can hold and its availability to plants. Fine-textured soils, especially without plant or residue cover, tend to reduce water infiltration. Coarse-textured soils may have high infiltration rates but dry to deeper depths than do the fine-textured soils.

Idaho has many different kinds of soil which reflect a wide array of parent material and varying climates. Geological events over great periods of time have provided many kinds of parent material most abundant of which are of igneous derivation -- basalt, lava, granite, etc. Idaho has soils that are both formed in place and on weathered materials such as sand and silt that has been transported by wind or water from its original location.

Idaho has a great diversity of climates. Annual precipitation ranges from as little as 6 inches along the Snake River south of Boise to over 60 inches at the higher elevations in northern Idaho. Temperatures also vary over a wide range. Water and temperature both have a major impact on the soil formation processes.

Living organisms including plants, grazing animals, insects, microorganisms and man all affect the development of soil. Plants, through the process of photosynthesis, capture energy from the sun and make it available to other living organisms In addition to their energy-providing role, plants are the main barrier between soil and the erosive forces of wind and water. The roots help hold the soil in place, and the foliage, both live and dead reduce the impact of falling rain drops and slow the movement of surface water and increase its infiltration into the soil. When too much vegetation is removed (by grazing, fires, drought, insects, etc.), soil erosion can become a major problem. Microorganisms play an important role in recycling nutrients, improving soil tilth and enhancing infiltration.

Topography influences soil in many ways. As slope increases, runoff and subsequent soil erosion increase. On steep slopes, soil can actually creep downward under the force of gravity. Soils on steep slopes are generally shallow and tend to be fragile. In the valleys and flat plains adjacent to hillsides, soil accumulates often developing deep fertile soil profiles. South facing slopes absorb more of the sun's energy than other slopes providing a warmer habitat for plants and other organisms, but they are usually drier because of earlier snow melt and higher evapotranspiration.

Soil stability is a major criteria for determining rangeland health:

"Soil degradation, primarily through accelerated erosion by wind and water, causes a direct and often irreversible loss of rangeland health. Soil degradation not only damages the soil itself but also disrupts nutrient cycling, water infiltration, seed germination, seedling development, and other ecological processes that are important components of rangeland ecosystems. In addition, soil degradation damages watersheds, which leads to further degradation of rangeland ecosystems as well as water pollution. Indicators of soil stability and watershed function should be central to the evaluation of rangeland health.

"The indicators selected to assess soil stability and watershed function should relate to two fundamental processes: (1) soil erosion by wind and water and (2) infiltration or capture of precipitation. The development of predictive models that estimate rates of soil loss and infiltration coupled with the establishment of acceptable rates of soil erosion could help to quantify soil stability and watershed function for an evaluation of rangeland health. Reliable predictive models are being developed but do not yet exist for rangelands.

"Soil surface characteristics are currently the best available indicators of soil stability and watershed function. Soil surface characteristics, such as presence of rills and gullies or pedestaling of plants, have been widely used as indicators of the degree of soil movement and the condition of the soil surface. Soil surface characteristics also give partial evidence of the magnitude of infiltration or runoff from a site. An evaluation of soil stability and watershed function, as determined by the use of soil surface characteristics as indicators of soil erosion and runoff, should become a fundamental component of all inventorying and monitoring programs for federal and nonfederal rangelands."

Rangeland Health: New Methods to Classify, Inventory and Monitor Rangelands, Committee on Rangeland Classification, Board on Agriculture, and National Research Council, National Academies Press, Washington D.C., 1994. 180 p.

Fertilization is not a common practice on rangelands. Because of limiting soil water and lack of response by native plant species, range fertilization is generally not economically feasible. Overflow or high water table meadows and possibly some crested wheatgrass or other such seedings might be exceptions. It should be noted that fertilization, particularly with nitrogen, often stimulates the growth and spread of weedy plant species.

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