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Soil Functions: Services Provided by Soil Resources
We depend on soil to perform many functions. Healthy soil gives us clean air and water, bountiful crops and forests, productive rangeland, diverse wildlife, and beautiful landscapes. Soil does all this by performing five essential functions.
1.Nutrient Cycling
Soil stores, moderates the release of, and cycles nutrients and other elements. During these biogeochemical processes, analogous to the water cycle, nutrients can be transformed into plant available forms, held in the soil, or even lost to air or water.
Nutrient cycling can be assessed by measuring the following indicators:
Fertility Indicators including mineral nitrogen, potentially mineralizable nitrogen, soil nitrate, soil test phosphorus, potassium, sulfur, calcium, magnesium, boron, and zinc
Organic Matter Indicators including C:N ratio, decomposition, microbial biomass carbon, particulate organic matter, soil enzymes, soil organic matter, total organic carbon and total organic matter
Soil Reaction Indicators including soil pH
Soil is the major "switching yard" for the global cycles of carbon, water, and nutrients. Carbon, nitrogen, phosphorus, and many other nutrients are stored, transformed, and cycled through soil.
Decomposition by soil organisms is at the center of the transformation and cycling of nutrients through the environment. Decomposition liberates carbon and nutrients from the complex material making up life forms-putting them back into biological circulation so they are available to plants and other organisms. Decomposition also degrades compounds in soil that would be pollutants if they entered ground or surface water.
Decomposition is a stepwise process involving virtually all soil organisms. Arthropods and earthworms chew the material and mix it with soil. A few fungi may break apart one complex compound into simpler components, then bacteria can attack the newly created compounds, and so on. Each organism gets energy or nutrients from the process. Usually, but not always, compounds become simpler after each step. The portion of plant and animal residue that is not broken down plays a crucial role in soil. It is transformed into the highly complex organic compounds called humic substances that can persist in soil for centuries and are important to soil structure and nutrient storage.
Carbon Dioxide and Soil
The carbon cycle illustrates the role of soil in cycling nutrients through the environment. More carbon is stored in soil than in the atmosphere and above-ground biomass combined. Soil carbon is in the form of organic compounds originally created through photosynthesis in which plants convert atmospheric carbon dioxide (CO2) into plant matter made of organic carbon compounds, such as carbohydrates, proteins, oils, and fibers. The organic compounds enter the soil system when plants and animals die and leave their residue in or on the soil. Immediately, soil organisms begin consuming the organic matter, extracting energy and nutrients and releasing water, heat, and CO2 back to the atmosphere. Thus, if no new plant residue is added to the soil, soil organic matter will gradually disappear. If plant residue is added to the soil at a faster rate than soil organisms convert it to CO2, carbon will gradually be removed from the atmosphere and stored (sequestered) in the soil. Cultivation aerates the soil, triggering increased biological activity, and therefore rapid decomposition, loss of soil organic matter, and release of CO2 into the atmosphere. Most soil carbon losses occur in the first several years after cultivation begins, as took place in many U.S. soils in the 1800's. Farmers and other conservationists are interested in reversing that effect and increasing the amount of carbon stored in the soil. In general, reducing tillage can increase the extent of carbon sequestration and the amount of organic matter retained in the soil.
2. Water Relations
Soil can regulate the drainage, flow and storage of water and solutes, which includes nitrogen, phosphorus, pesticides, and other nutrients and compounds dissolved in the water. With proper functioning, soil partitions water for groundwater recharge and use by plants and animals.Soil water relations can be assessed by measuring or observing the following indicators:
Physical Stability Indicators including aggregate stability, erosion patterns, slaking, soil loss, and soil depth
Water Availability Indicators including available water capacity, hydraulic conductivity, infiltration, ponding patterns, soil moisture, water filled pore space, and water holding capacity
Salinity and Sodicity Indicators including electrical conductivity, exchangeable sodium percentage, sodium, and sodium absorption ratio
When rain or irrigation water falls to earth, some of the water will infiltrate into the soil and some will flow over the surface. If the soil is loose, porous, and has a stable structure, a drop of water will be likely to infiltrate. If the soil has few openings and unstable structure so that a crust forms and seals the soil surface, a drop of water will be more likely to run over the surface. Plants are also important in determining the fate of water. Leaves intercept water so some evaporates before it ever reaches the soil, and leaves and plant residue protect the soil so rain hits more gently. Roots and residue slow down the flow of water over land so water has more time to soak in.
If the soil becomes saturated, some water will drain down to groundwater. The remainder will be held in the soil until it evaporates or is drawn into plant roots, eventually transpiring from leaves. At all these stages water is carrying sediment, organic matter, plant nutrients such as nitrogen and phosphorus, pesticides, and other dissolved or suspended compounds. Water flowing over the surface may carry sediment and nutrients into lakes. Water draining into groundwater may contain nitrate or pesticides. Where does rainwater go after it falls on your property? During a downpour, watch where it flows and where it ponds. After the rain, notice how the soil surface dries more slowly under residue or mulch compared to bare soil.
3. Biodiversity and Habitat
Soil supports the growth of a variety of unstressed plants, animals, and soil microorganisms, usually by providing a diverse physical, chemical, and biological habitat.
The ability of soil to support plant and animal life can be assessed by measuring the following indicators:
Biological Activity Indicators including active fungi, earthworms, microbial biomass, potentially mineralizable nitrogen, respiration, soil enzymes.
Biological Diversity Indicators including habitat diversity and diversity indices for organisms such as bacteria, macro and microarthropods, nematodes, and plants.
What do plants, animals, and microbes need from soil?
Microbes need soil for:
- Food. Most microbes need regular inputs of organic matter (e.g. plant residue) into the soil.
- Space. Larger soil organisms such as nematodes and insects need enough space to move through soil.
- Air. Most soil organisms require air, though some require a lack of oxygen. They live in low-oxygen micro-sites such as within soil aggregates. Generally, soil biological activity is enhanced by an increase in soil aeration.
Plants need soil for:
- Support of the microbiological activity necessary for plant growth.
- Support for, and minimum resistance to, root penetration.
- Intake and retention of water in soil, while maintaining adequate aeration.
- Exchange of soil air with the atmosphere.
- Resistance to erosion.
- Mineral and organic sources of nutrients.
- In addition, farmers need adequate traction for farm implements to grow crops.
Animals and people need soil for:
- Healthy plant growth.
- Availability of nutrients essential for animal health. These are absorbed by plants, but are not necessarily essential for plant health.
All organisms need:
- Low levels of toxic compounds.
- Filtering of water and air.
At a landscape scale, a variety of soil environments are needed to support a variety of plants, animals, and microorganisms. (Lists adapted from Yoder, 1937, and Cihacek, 1996.)
Diversity of soil and soil organisms
Each animal, plant, and microbe species requires a slightly different habitat. Thus, a wide variety of habitats are required to support the tremendous biodiversity on earth. At the microbial level, diversity is beneficial for several reasons. Many different organisms are required in the multi-step process of decomposition and nutrient cycling. A complex set of soil organisms can compete with disease-causing organisms, and prevent a problem-causing species from becoming dominant. Many types of organisms are involved in creating and maintaining the soil structure that is important to water dynamics in soil. Many antibiotics and other drugs and compounds used by humans come from soil organisms. Most soil organisms cannot grow outside of soil, so it is necessary to preserve healthy and diverse soil ecosystems if we want to preserve beneficial microorganisms. Estimated numbers of soil species include 30,000 bacteria; 1,500,000 fungi; 60,000 algae; 10,000 protozoa; 500,000 nematodes; and 3,000 earthworms (Pankhurst, 1997).
4. Filtering and Buffering
Soil acts as a filter to protect the quality of water, air, and other resources. Toxic compounds or excess nutrients can be degraded or otherwise made unavailable to plants and animals.
The filtering function of soil can be assessed by measuring or observing the following indicators:
Toxicity Indicators including arsenic, copper, pesticides, and zinc
Organic Matter Indicators including C:N ratio, decomposition, microbial biomass carbon, particulate organic matter, soil organic matter, total organic carbon, and total organic matter
Soil Reaction Indicators including soil pH
Salinity and Sodicity Indicators including electrical conductivity, exchangeable sodium percentage, sodium, and sodium adsorption ratio
Biological Activity and Diversity Indicators including active fungi, earthworms, potentially mineralizable nitrogen, respiration, soil enzymes, and diversity indices for organisms such as bacteria, macro and microarthropods, nematodes, and plants.
The minerals and microbes in soil are responsible for filtering, buffering, degrading, immobilizing, and detoxifying organic and inorganic materials, including industrial and municipal by-products and atmospheric deposits. Soil absorbs contaminants from both water and air. Some of these compounds are degraded by microorganisms in the soil. Others are held safely in place in the soil, preventing contamination of air and water. When the soil system is overloaded, such as with the excess application of fertilizer or manure, or when the soil is unstable, some contaminants will be released back to the air and water through erosion or leaching.
Wetlands function as nature's filters. Many human-made wetlands are created for this purpose. What happens to water that flows over pavement or through drain tiles?
5. Physical Stability and Support
Soil has the ability to maintain its porous structure to allow passage of air and water, withstand erosive forces, and provide a medium for plant roots. Soils also provide anchoring support for human structures and protect archeological treasures.
The stability and support function of soil can be assessed by measuring the following indicators:
Soil Stability, Aggregate Size and Stability Indicators including erosion patterns, soil depth, soil loss, mean weight diameter of water stable aggregates, aggregate stability, and soil slaking
Soil Structure Indicators including bulk density, penetration resistance, porosity, or root growth pattern
Organic Matter Indicators including soil organic matter or total organic carbon
Also, inherent soil properties, like soil texture and particle size distribution, play a major role in physical stability.
Soil support is necessary to anchor plants and buildings. Both flexible (it can be dug) and stable (it can withstand wind and water erosion), soil also provides valuable long-term storage options including protecting archeological treasures and land-filling human garbage. The need for structural support can conflict with other soil uses. For example, soil compaction may be desirable under roads and houses, but can be devastating for the plants growing nearby.