6.3 - Soil Quality
Soil Quality, as a general concept, can be thought of as the ability of a soil to function, in either natural or managed ecosystems, to sustain plant and animal life, and maintain or enhance air and water quality. For agricultural ecosystems, we may consider Soil Quality as the ability of a soil to produce safe and nutritious crops in a sustained manner over the long-term, without impairing the resource base or harming the environment.
Soil Quality has the potential for many different interpretations, dependent upon factors such as land use, soil management practices, ecosystem and environmental interactions, and the priorities of human societies. When considering Soil Quality in any specific case, it is necessary to identify the major issues of concern with respect to that soil’s function. Whatever definition of the term Soil Quality is deemed appropriate for a specific instance, it should relate to the capacity of the soil to function effectively with regard to productivity; environmental quality; and plant, animal, and human health now and in the future. Since the majority of food and fiber needs of the human population are met by crops grown in managed agricultural ecosystems, we will focus on those systems here. However, the basic principles presented should be applicable to soils in other ecosystems, both natural and managed.
Some soil properties can be relatively easy to observe, measure, and monitor over time:
| Table 1: Soil properties used as indicators of soil quality. | ||
|---|---|---|
| Physical | Chemical | Biological |
| Topsoil depth | Organic matter content | Soil Respiration (CO2) |
| Texture and aggregation | Salinity-electrical conductivity | Microbial activity/biomass |
| Aeration and infiltration | Acidity - alkalinity (pH) | Earthworm counts |
| Surface cover | Nitrate nitrogen | Plant vigor |
| Compaction |
Major factors which lead to reductions in soil quality, land degradation, and soil erosion:
- Mismanagement: Lands that are improperly managed (e.g., improper tillage) lose their topsoil. Either in large chunks during extreme erosive events, or little by little over an extended period of time, the soil disappears from the land resulting in reduced productivity and a degraded condition.
- Salinization: Results from the accumulation of salts in improperly irrigated soils, most frequently in arid regions.
- Overharvesting: Occurs on cultivated soils when repeated harvests are made from land without returning organic residues and mineral nutrients to the soil.
- Contamination: Exposure of soil to toxic substances, as a result of industrial processes or chemical spills, can severely damage the ability of a soil to perform its ecosystem function.
Figure 7. Inadequate soil cover on sloping surfaces can result in topsoil removal by erosive forces of water and wind. Here, rills and gullies are visible on a hillside where little fertile topsoil remains. Image courtesy of Tim Kettler
| Table 1 - Cultural and environmental factors which enhance or degrade soil quality | |
|---|---|
| Soil Quality Enhancing | Soil Quality Degrading |
| organic material additions | overharvesting |
| plant growth | bare fallow |
| fibrous root systems of plants | fire |
| cool, humid climate | hot, arid climate |
| vegetative cover | exposed soil |
| minimal tillage operations | intense tillage |
| wildlife | wildlife |
| erosion |
For plant growth, the topsoil is the richest and most valuable part of the soil. Topsoil formation is a very slow process, which makes it a non-renewable (but re-usable) resource in terms of human lifespans. Keeping the soil in place while it is used for construction or crops is one of the greatest challenges faced by engineers and land managers.
Figure 8. Soil erosion losses are greatest when the soil surface is exposed to intense rainfall, resulting in gulley formation as shown above. Image courtesy of John Doran
Irrigation water quality is of great concern in arid climates. Water high in dissolved salts can lead to salt accumulation in soils.
Figure 9. Soil erosion losses are greatest when the soil surface is exposed to intense rainfall, resulting in gulley formation as shown above. Image courtesy of John Doran
Natural soil fertility is largely contained in the remains of formerly living things, also known as organic matter. Continuous removal of plant material for food or forage leads to gradual depletion of natural soil fertility.
Figure 10. Cattle grazing in the Nebraska Sandhills. Grazing by livestock must be carefully managed to maintain grassland productivity and protect soil from degradation. Image courtesy of Walter Schacht.