Organic matter content can be increased by mulch tillage (Beale et al. 1955). Over a 10-year period in South Carolina, corn grown by no-tillage in a vetch and rye mulch increased the soil organic matter, degree of soil aggregation, and stability of the soil structure in the Ap horizon. Frye et al. (1982) found that restoring the organic matter of an eroded soil did less to restore the productivity of the soil than increasing the available water holding capacity. However, loss of organic matter has been shown to affect the use and effectiveness of herbicides (Frye et al., 1985).

Frye et al. (1982) found that eroded soils in the humid-region (Southeastern U .S.) usually are more acid and have higher lime requirements than uneroded soils as a consequence of higher buffering capacity associated with the subsurface clay mixing with the plow layer as a result of erosion. When clayey subsoil materials of an eroded soil are incorporated into the plow layer by tillage, the moisture range at which the soil can be easily and safely tilled become narrower (Frye et al., 1985). If the soil is worked wet, soil structure tends to break down resulting in decreased pore space, aeration, infiltration and percolation, and increased bulk density. Soil compaction often becomes a problem. If tilled dry, the clayey subsoil becomes cloddy and difficult to work, thus, raising energy costs.

Increased interest in the relationship of soils and their use has resulted in the development of the concept of soil quality. Soil quality is not easy to define. Johnson et al. (1992) proposed that one way of defining soil quality or rather the loss of soil quality is in terms of soil degradation. They defined net soil degradation as the amount of degradation decreased by the amount of soil formation or restoration. Blaikie and Brookfield (1987) defined net soil degradation in the following way: Net soil degradation = (natural degradation + anthropogenic degradation) - (soil formation + restoration management). If net soil degradation is used as a measure of soil quality, then soil quality declines when there is net degradation or increases when there is restoration. Johnson et ai 11992) state that the loss or decline of soil quality can be caused by a variety of forces. The effects of these forces are reflected in the changes in physical, chemical, or biological properties of the soils system.

Soil quality is the capacity to function within ecosystem boundaries to sustain biological productivity, maintain environmental quality, and promote plant and animal health (Doran and Parklin, 1994). Karlen et al. (1997) suggested the definition for soil quality is 'the ability (or fitness) of a specific kind of soil to function, within natural or managed ecosystem boundaries, to sustain plant and animal productivity, maintain or enhance water and air quality, and support human health". When examined as part of the ecosystem, soil quality assessments provide an effective method for evaluating direct and indirect environmental impacts of human management decisions (Karlen et al., 1997). Sampling measuring and reporting the response of an individual soil parameter to a given perturbation or management practice is no longer sufficient. The soil resource must be recognized as a dynamic living system that emerges through a unique balance and interaction of its biological, chemical, and physical components.

Some researchers (Neil, 1979; Pierce et al., 1983) have developed indices based on functional relationships between yield and soil properties. The productivity index and soil tilth index quantified only the physical state of soil and did not consider the environmental aspects of soils. Larson and Pierce (1994) proposed three functions associated with a good quality soil. These included: soil ability to function as a medium for plant growth, to regulate and partition water flow through environment, and to serve as an environmental filter. To perform these functions, a high quality soil accepts, holds and releases the nutrients and water, promotes and sustains root growth, maintains suitable soil biotic habitat, and responds to management and resists degradation (Larson and Pierce, 1994).

The assessment of soil quality functions can be done by quantifying measurable soil attributes/indicators and creating soil functions (Larson and Pierce, 1994). Larson and Pierce (1994) suggested a minimum data set for measuring soil quality, which included total organic C, bulk density, nutrient availability, available water, particle size, depth of rooting, soil strength, pH and electrical conductivity as soil quality indicators. In the process of soil quality assessment, Doran and Parkins (1994) proposed the estimation of soil quality by estimating the functions suggested by Larson and Pierce (1994) using regression equations relating soil quality indicators and soil quality functions. Due to the lack of these needed regression equations, Karlen et al. (1994) used the standardized scoring function technique to quantify changes in soil quality. Karlen et al. (1994) selected three standardized scoring functions (SSF) for normalization of soil quality indicators. The numerical values of soil quality indicators were converted into a score on a scale of 0 to 1. The score for each soil quality indicator was calculated using lower threshold limits, baseline and upper threshold limits. Karlen et al. (1994) calculated the individual function rating s for accommodation of water entry, facilitation of water transfer, adsorption and delivery, resistance against degradation and support for plant growth. Later, they combined all of these function indexes into an overall soil quality index. Harris et al., (1996) developed a EXCEL spreadsheet to estimate the soil functions: (1) water relations (WR), (2) nutrient relations (NR), and (3) rooting relations (RR).

Soil protection has received much attention in the Netherlands. An Interim Soil Cleanup Act covering the period 1982-1988 dealt primarily with remedial action of locally contaminated areas and dump sites that pose a harmful effect to human health or the environment. The Soil Protection Act became law on January 1, 1987. It is the intention of the Act that soil use does not diminish its multi-functionality, i.e., its agricultural, groundwater recovery, ecological, mining and carrier function, primarily through prevention measures. The Act provides for a general level of protection through soil quality standards and goals and source reduction.

The Land Resource Research Center in Canada (Acton, 1989) proposed the development of a soil quality evaluation system for assessing agricultural sustainability in Canada. They adopted a Leopold definition of soil quality: the sustaining capability of a soil to accept, store and recycle water, nutrients and energy. Leopold considered a quality soil to have adequate depth for water storage and rooting, adequacy of organ-mineral colloids for moisture storage and retention of nutrients in reasonably available forms, absence of unsuitable chemical conditions such as acidity or salinity, a physical condition which promotes the infiltration of moisture and its storage, aeration and the unhindered development of roots. The soil must be capable of handling energy, both to accept, store and recycle the energy contained in organic matter which drive biological processes in the soil, and the dynamic energy of rain drops or wind-borne soil properties which might dislodge the soil. Canadians go on to explain that monitoring of soil quality implies developing a system to evaluate the changes to soil quality described above. It includes establishing criteria and standards for measurement of soil quality and the kind, degree, extent and probable cause of change from the natural to the current agroecosystem. It also involves monitoring trends in land use and farming practices to ascertain the relationship between these trends and change in soil quality. Most importantly, it involves developing the capability to predict changes to soil quality that could occur as a result of specific land use, cropping systems and tillage practices and, in turn, the impact of these changes to crop production potentials, the environment, and food quality.

In the United States, soil quality standards are used by the Forest Service as a means to maintain long-term soil productivity on National Forest System Lands (Griffith et al., 1992). In the Pacific Southwest Region, these standards are based on threshold values for soil cover, soil porosity, and organic matter. They are applied during the planning, implementation and evaluation phases of land management projects.

Larson and Stewart (1992) proposed a method for defining thresholds for soil removal for maintaining cropland productivity. They divided the soil properties changed as a result of erosion into irreplaceable and replaceable attributes. Irreplaceable attributes included water-holding capacity and rooting depth, while replaceable attributes included plant nutrients and organic matter. They suggested threshold values for potential productivity and vulnerability of the soil to damage from erosion for cropland soils. Olson (1992) suggested threshold values for physical properties of the surface and subsoil layers be defined as no (less than 1%), minor (1-15%) and major (more than 15%) reductions in productivity as reflected in corn yields.

Smith et al. (1993) state that soil quality is the most important factor for sustaining the global biosphere. They concluded that soil quality can be defined in several different ways including productivity, sustainability, environmental quality, and effects on human nutrition. They emphasize that to quantify soil quality, specific soil indicators must be measured spatially. The indicators are mainly soil properties whose values relate directly to soil quality but may also include policy, economic or environmental considerations. They proposed a method to integrate soil quality parameters into an index to produce soil quality maps on a landscape basis.

Erosion can impact the soil as a source or sink of CO2 CH4 and other greenhouse gases, depending on soil quality and the predominant pedospheric processes. (Bouwman, 1990; Lal et al., 1995a; b). Increase in soil organic carbon (SOC) and soil inorganic carbon (SIC) contents, due to best management practices (BMPs) and soil restorative measures, can make soil a net sink for atmospheric C and reduce or mitigate the greenhouse effect. Decrease in SOC and SIC contents, due to accelerated erosion and attendant soil degradation, can exacerbate the greenhouse effect due to emissions of CO^ and CH4 from soil to the atmosphere. Soil erosion leads to on-site depletion of SOC content due to transport of dissolved organic carbon (DOC) and paniculate organic carbon (POC) in runoff and eroded sediments. Total SOC displaced by erosion over the global terrestrial ecosystems is estimated at 5.7 Pg/yr (Lal, 1995).