Soil is the world's most vital component for food and fiber production: preservation of this critical natural resource is paramount for protecting the environment, and ensuring that current and future populations are healthy and well-fed. In many countries of the world, including the United States, soils are being degraded at an alarming rate by wind and water erosion, desertification, and salinization resulting from misuse and improper farming practices. Often the topsoil is lost at a rate that far exceeds the capacity of natural processes to regenerate it. With soil degradation, there is a concomitant decline in soil quality, i.e., the soil's capacity to produce healthy and nutritious crops, resist erosion, and reduce the impact of environmental stresses on plants. Many decades of research have consistently shown that the best means of improving and restoring soil quality and productivity is by proper and regular additions of organic materials mainly through the use of crop rotations, cover crops, crop residues, animal manures, composts, and reduced tillage.

Different chemical, physical, and biological properties of a soil interact in complex ways that determine its potential fitness or capacity to produce healthy and nutritious crops. The integration of these properties andine resulting level of productivity often is referred to as “soil quality.” Soil quality can be defined as an inherent attribute of a soil that is inferred from its specific characteristics and observations (e.g., compactability, erodibility, and fertility). The term also refers to the soil's structural integrity, which imparts resistance to erosion, and to the loss of plant nutrients and organic matter. Soil quality often is related to soil degradation, which can be defined as the time rate of change in soil quality.

Soil quality should not be limited to soil productivity, but should encompass environmental quality, human and animal health, and food safety and quality. There is inadequate reliable information on how changes in soil quality directly affect food quality, or indirectly affect human and animal health. In characterizing soil quality, biological properties have received less emphasis than chemical and physical properties, because their effects are difficult to measure, predict, or quantify. Improved soil quality often is indicated by increased infiltration, aeration, macropores, aggregate size, aggregate stability, and soil organic matter, and by decreased bulk density, soil resistance, erosion, and nutrient runoff. These are useful, but future research should seek to identify and quantify reliable and meaningful biological/ecological indicators of soil quality, such as total species diversity or genetic diversity of beneficial soil microorganisms, insects, and animals.

Because these biological/ecological indexes of soil quality are dynamic, they will require effective monitoring and assessment programs to develop appropriate databases for research and technology transfer. We need to know how such indexes are affected by management inputs, whether they can serve as early warning indicators of soil degradation, and how they relate to the sustainability of agricultural systems.

Our knowledge of soil is based primarily on quantitative analysis of isolated physical, chemical, and biological properties. However, the interaction of these quantitative aspects determines soil quality. Integrative tools are needed by researchers, farmers, regulators, and others to evaluate changes in soil quality from human activity at a local and global level. An index needs to be adaptable to local or regional conditions. For example, the parameters needed to determine changes in soil quality may differ between a semi-arid wheat field and a rice paddy. Suitable reference points and optimum ranges are needed for soil quality attributes. The present challenge is to integrate a suite of soiltests into a meaningful index that correlates with productivity, environmental, and health goals.

The problem of land degradation is not new, but never before has it been so widespread or affected the lives of so many people. The need for a global assessment of land degradation was highlighted by the United Nations Conference on the Human Environment in 1972. Since then, several attempts have been made to develop methodologies that will allow for systematic assessment and monitoring of global land degradation. Systematic work started in 1975, when FAO and United Nations Environment Programme (UNEP) collaborated in a study to develop a provisional method for soil degradation. This method was tested, and maps on a scale of 1:5,000,000 were produced for North Africa and the Near East. However, the method proved difficult to apply accurately, so this project was followed by another where the method was revised and refined in an effort to assess and map desertification. Again, problems were encountered because of the complexity of the subject and the lack of usable reliable data. More recently, ISRIC, using a simplified methodology and the assistance of some 250 scientists from around the world, has produced a world map of human induced soil degradation at a scale of approximately 1:10,000,000. Meanwhile, interest has been growing in the problem of nutrient depletion in developing countries and a study has been completed of N, P and K balances in the soils of 38 African countries. Although we still do not have an accurate assessment of global land degradation and no systematic monitoring is yet in place, these innovative studies have provided us with a sound basis for future work and have identified many of the practical problems of measuring land degradation, data collection and analysis that will have to be overcome before more accurate assessment and monitoring can be achieved.

The impact of soil degradation on human welfare and the global environment presents a major challenge. A significant decline in soil quality has occurred worldwide through adverse changes in its physical, chemical and biological attributes and contamination by inorganic and organic chemicals. There is a need to develop criteria to evaluate soil quality so that the progress of any corrective action required by the international community can be monitored.

There currently are no generally accepted criteria to evaluate changes in soil quality. This lack impedes the design and evaluation of meaningful soil management programs. This paper examines the principal physical and chemical attributes that can serve as indicators of a change in soil quality under particular agroclimatic conditions. Proposed indicators include soil depth to a root restricting layer, available water-holding capacity, bulk density/penetration resistance, hydraulic conductivity, aggregate stability, organic matter, nutrient availability/retention capacity, pH, and where appropriate, electrical conductivity and exchangeable sodium. We also discuss the justification for selecting these key attributes, their measurement, critical limits for monitoring changes in soil productivity, and future research needs in soil quality.

Diverse soil microbiological studies have attempted to assess deterioration or improvement in soil quality. These studies have been done on three levels: population level studies of the dynamics of species that are presumed to be important or sensitive; community level studies of microbial community structure, such as species diversity and frequency of occurence of species; and ecosystem level studies of a range of soil processes. We suggest that ecosystem level approaches offer the best possibilities for rapidly assessing changes in soil quality. Data from such studies will allow researchers to decide whether to proceed with population or community level studies.

Invertebrates are an integral part of soils and are important in determining the suitability of soils for the sustainable production of healthy crops or trees. We discuss the importance of the soil invertebrate fauna in relation to terrestrial habitats and global biodiversity as we understand it. We describe the role of the main invertebrate groups in soils, including earthworms, termites, springtails, and nematodes, and how they determine soil quality. Practical problems in dealing with the invertebrate fauna include sampling, taxonomy and availability of biological information on species. Various measures are available that use invertebrates to assess soil quality, each with its advantages and disadvantages. They include abundance, biomass, density, species richness, trophic/guild structure, food web structure, keystone species and ecosystem engineers. We propose the three most useful and practical of these as suitable to be combined with other biological (microbial) and non-biological (hydrological, physical, chemical) criteria into a single index of soil quality that might be used on a regional, if not international basis.

People are becoming more aware that our soil resources are as vulnerable to degradation as air or water, but criteria are needed to learn how soil quality is changing. Our objectives in this review are: (1) to illustrate that interactions between human and natural factors determine soil quality; (2) to identify indicators that can be used to evaluate human-induced effects on soil quality; and (3) to suggest soil and crop management strategies that will sustain or improve soil quality. The physical, chemical, and biological processes and interactions within the soil are critical factors affecting all indicators of soil quality. The biological processes are especially important because they provide much of the resiliency or buffering capacity to ameliorate stress. Presumably, no single soil or crop management practice will guarantee improved soil quality, but conservation tillage, cover crops, and crop rotations are practices that may be effective. Alley or narrow-strip cropping may facilitate adoption of several of those agronomic practices and increase temporal and spatial diversity across the landscape. To maintain or possibly improve soil quality and simultaneously address a growing waste disposal problem, we suggest that urban lawn and newspaper waste be evaluated as carbon sources. We conclude that the most critical factor, regardless of the soil and crop management strategy, is to recognize that carbon is an essential element for improving soil quality in the U.S. and around the world.

The formation of crusts or seals at the soil surface is exceedingly important in determining how much rainfall infiltrates into the soil and how much runs off, causing soil erosion. This paper explores the processes involved in the formation of crusts, such as raindrop impact and clay dispersion, to formulate a picture of the mechanisms involved. We discuss the major consequences of crusting, namely, runoff, erosion, and impaired seedling emergence, and present strategies to reduce soil degradation. Examples are offered from many parts of the world.

Several factors can directly or indirectly affect the nutritional quality of crops. Among these are soil factors, such as pH, available nutrients, texture, organic matter content and soil-water relationships; weather and climatic factors, including temperature, rainfall and light intensity; the crop and cultivar; postharvest handling and storage; and fertilizer applications and cultural practices. This paper deals primarily with fertilizer and cultural management practices, and on certain environmental factors that affect the nutritional quality of field crops and of fruits and vegetables. Earlier research that has investigated the nutritional status of crops grown with either chemical fertilizers or organic fertilizers is discussed. These studies often have given contradictory results on crop yields and on crops' mineral and vitamin contents. Other factors, such as maturity at harvest, postharvest handling and storage, anti-nutritive components, and residues of chemical fertilizers and pesticides are reviewed with respect to food safety and quality, and their implications for human and animal health. Future research needs are identified so that comparable results and valid comparisons can be obtained to identify the best management practices to ensure that food is safe and nutritious for the consumer.

Conversions from conventional to sustainable farming systems could affect local rural economies either positively or negatively, by several means: changes in income of agricultural households; backward linkages to input supply firms; forward linkages to transportation, processing, and marketing firms; and changes in consumer expenditures by agricultural and other households. We estimated these effects for five local rural economies in South Dakota, representing different agroclimatic and population settlement patterns. Whole-farm economic models of case study conventional and sustainable farms in each area were used to estimate differences in input purchase and marketing patterns. We found declines in on-farm personal income (returns to farm labor and management) in four of the five case comparisons under a conversion to sustainable farming systems if organic price premiums are ignored; we found increases in three of five cases if applicable organic premiums are included. In all five study areas, total off-farm personal income drops in the portions of local economies connected to farming through backward linkages. It also drops in four of the five study areas in the portions of local economies connected through forward linkages if organic premiums are ignored (all five if organic premiums are included). However, net forward linkage effects are usually much smaller than net backward linkage effects, and on-farm personal income effects are substantially greater than either backward or forward off-farm linkage effects in most cases. The total (on-and off-farm) personal income effect of converting to sustainable systems is positive in only one area without organic premiums, and in one additional area with premiums. The ratio of total to on-farm personal income effects within local economies, considering only first round effects on backward- and forward-linked firms, averages 1.2 without organic premiums, or 0.6 with them. Taking account of more complete multiplier effects, which also consider changes in consumer expenditures and changes in expenditures for supplies by backward- and forward-linked firms, the average is 1.8 without organic premiums, or 0.8 with them. With more time for changes in sustainable production techniques and in the structure of farms and the rural economy, the overall economic effects of conversions to sustainable farming are likely to be more positive than our estimates.

Adoption of agricultural practices that reduce or eliminate chemical use creates tradeoffs among yields and output, production costs and net income at the farm level. If widespread adoption is to occur, potential improvements in human health and the environment from reduced chemical use must be weighed against effects on aggregate farm income and consumer food prices. At present, these tradeoffs are subject to considerable debate.

I review available farm level and aggregate economic studies of reductions in total agricultural chemical use. At the farm level, available studies suggest that organic and low-input farming systems result in declines in per acre yields and in lower net returns than most conventional farming systems, although the studies do indicate that net returns for the two systems can be comparable under some conditions. Several studies suggest that low-input systems perform well with respect to risk criteria. I also discuss the potential influence of government programs on the profitability of some low-input farming systems.

All aggregate economic models conclude that widespread adoption of organic farming would result in decreased yields, decreased aggregate output, increased farm income, and large increases in consumer prices. These models have limitations, however. Recommendations are made for further economic research of the impacts of reducing agricultural chemical use at both the farm and aggregate levels.