watershed function, integrity of nutrient cycles and energy flow, and presence of functioning recovery mechanisms.

The process of rangeland change is complex, and multiple criteria should be used to determine whether rangelands are healthy, at risk, or unhealthy. No single criterion alone will be a sufficient basis for this determination. The committee recommends a three-phase approach for assessing rangeland health. Phase 1 is an evaluation of soft stability and watershed function. Phase 2 is an evaluation of the functioning of nutrient cycles and energy flows. Phase 3 is an evaluation of the probability that recovery mechanisms will occur on the rangeland being assessed.

SOIL STABILITY AND WATERSHED FUNCTION

The physical, chemical, and biological processes that occur in rangeland soils supply plants with nutrients and water. Microorganisms in the soil break down plant litter, releasing nitrogen, phosphorus, and other nutrients essential to plant growth. The texture, structure, and porosity of soil determine how much rain is captured and how much runs off during a storm. Soils are storehouses of water and nutrients for plants to draw on when they need them. The soft is a living system that is inextricably linked to nutrient cycles, energy flows, and other ecological processes of rangeland ecosystems.

Soil Degradation

There are three principal processes involved in soil degradation: physical, chemical, and biological. These processes are closely linked, and modification of one unavoidably alters the others. Physical degradation results in the deterioration of the physical properties of soils through compaction, wind or water erosion, deposition of sediments, and loss of soil structure. Biological degradation occurs when there is a reduction in the organic matter content of the soil, a decline in the amount of carbon stored as biomass, and a depression in the activity and diversity of the organisms living in the soft. Chemical degradation includes nutrient depletion, shifts toward extremes in the pH of the soft, increases in salt concentration, and contamination by toxic substances such as heavy metals (Lal and Stewart, 1990). Summaries of these phenomena and interactions can be found in basic soils texts (for example, Brady [1990], Foth [1990], Miller and Donahue [1990], and Singer and Munns [1987]).

SOIL EROSION BY WIND AND WATER

Soft erosion by wind and water is a major factor in the process of soft degradation on rangelands and has been recognized as such for a long



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