Soil: The Foundation of Agriculture
Soil Health
Healthy soil is the foundation of sustainable agriculture systems. In an agricultural context, soil health most often refers to the ability of the soil to sustain agricultural productivity and protect environmental resources. Managing for soil health allows producers to work with the land to reduce erosion, maximize water infiltration, improve nutrient cycling, and ultimately improve the resiliency of the working land to sustain crop production. Soil health is a reflection of its inherent and dynamic properties. Inherent soil properties change very little or not at all with management. Inherent soil properties form over thousands of years and result primarily from the soil-forming factors: climate, topography, parent material, biota, and time. Examples of inherent properties are soil texture, type of clay, depth to bedrock, and drainage class. In contrast, dynamic soil properties are affected by management practices (e.g., crop and livestock production) and natural disturbances.
Benefits of a Healthy Soil
A healthy soil gives us clean air and water, bountiful crops and forests, productive rangeland, diverse wildlife, and beautiful landscapes. A healthy soil does all this by performing eight essential functions.
Nutrient Cycling
Nutrient cycling refers to the many pathways through which nutrients are added to, removed from, and changed within the soil. This cycling helps to maintain the essential nutrients required for plant growth in the soil. Nutrients are found in two basic forms in the soil: organic and inorganic (sometimes called “mineral”). Organic forms of nutrients contain carbon in the structure of the molecule, while inorganic forms do not. Nutrients are stored in several pools within the soil: as inorganic forms in soil particles, as organic forms in soil organic matter, as inorganic forms on cation exchange sites, and as organic and inorganic forms dissolved in the water surrounding soil particles, known as the soil solution.
Regulation of Soil, Air, and Water
Plants require both oxygen and water in the root zone for optimum growth. In soil, water, and air are held in the pore space between soil particles and soil aggregates. The sizes of the pores that occur between and within soil aggregates determine how water and gases move in and are held by the soil. Larger pores, known as macropores, are important to promote good aeration and rapid infiltration of rainfall. Smaller pores, known as micropores, are important for absorbing and holding water. Macropores are often visible to the naked eye, while micropores between and within microaggregates are not.
Maintaining Soil Organic Matter
Soil organic matter is important for all aspects of soil health: chemical, physical, and biological. Organic matter is a valuable nutrient source for plants and living organisms. As microorganisms increase their activity during warmer weather that occurs predominantly in the spring and summer, greater amounts of nutrients are cycled from organic forms into those that are inorganic and plant available. Organic matter causes soil particles to bind and form stable soil aggregates, which improves soil structure. With better soil structure, water infiltration through the soil increases and improves the soil’s ability to absorb and hold water as well as reduces the potential for surface runoff. As organic matter increases, microbial activity tends to increase.
Biodiversity and Habitat
Soil biodiversity reflects the variability among living organisms including a myriad of organisms not visible with the naked eye, such as micro-organisms (e.g., bacteria, fungi, protozoa and nematodes) and meso-fauna (e.g., springtails), as well as the more familiar macro-fauna (e.g., earthworms and termites). Plant roots can also be considered as soil organisms in view of their symbiotic relationships and interactions with other soil components. These diverse organisms interact with one another and with the various plants and animals in the ecosystem forming a complex food web of biological activity.
Maintaining Soil pH
Soil pH is a key aspect of long-term soil sustainability (Section 5.6). Often referred to as the master variable of soil, pH controls a wide range of physical, chemical, and biological processes and properties that affect soil fertility and plant growth. Soil pH, which reflects the acidity level in soil, significantly influences the availability of plant nutrients, microbial activity, and even the stability of soil aggregates.
Filtering and Buffering
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. The ability of soil to provide this filtration is through one or more physical, chemical, or biological processes that remove or degrade various constituents in water as it passes through on its way to groundwater. Physical filtration is directly analogous to passing water through a screen. The soil acts as a sieve and holds back particles that are too large to pass through. Unlike a simple screen, however, this characteristic is enhanced by the tortuous path that water takes through the soil filter, which provides multiple opportunities to capture constituents.
Temperature Moderation
The soil also moderates temperature fluctuations. The insulating properties of soil protect the deeper portion of the root system from extremes of hot and cold that often occur at the soil surface. For example, it is not unusual for the mid-afternoon temperature at the surface of bare soil to reach 104 degrees F (40°C), a condition lethal to most plant roots. Just a few centimeters deeper, however, the temperature may be 50 degrees F (10°C) cooler, allowing roots to function normally.
Carbon Sequestration
Perhaps the most notable and pervasive role of soils in global warming is the regulation of the carbon dioxide budget. Soil carbon sequestration result from the interactions of several ecosystem processes, of which photosynthesis, respiration, and decomposition are key. The primary way that carbon is stored in the soil is as soil organic matter. Soil organic matter is a complex mixture of carbon compounds, consisting of decomposing plant and animal tissue, microbes (protozoa, nematodes, fungi, and bacteria), and carbon associated with soil minerals. Soil organic carbon levels are directly related to the amount of organic matter contained in soil and soil organic carbon is often how organic matter is measured in soils.
Soil Health Indicators
Since soil quality cannot be measured directly, it must be inferred from measuring changes in its attributes or attributes of the ecosystem, referred to as indicators. Soil indicators are often divided into physical, chemical, and biological categories depending on how they affect soil function. Indicators of soil quality should give some measure of the capacity of the soil to function with respect to plant and biological productivity, environmental quality, and human and animal health. They should also be used to assess the change in soil function within land use or ecosystem boundaries.
Physical Indicators
Physical indicators for example include bulk density, infiltration, soil structure, soil depth, and water-holding capacity. These characteristics relate to retention and transport of water and nutrients, habitat for soil microbes, crop productivity potential, compaction, plow pan, and water movement.
Chemical Indicators
Chemical indicators for example include electrical conductivity, reactive carbon, soil nitrate, soil pH, and extractable phosphorus and potassium. All of these relate to biological and chemical activity, plant and microbial activity, and plant-available nutrients, as well as the potential for nitrogen and phosphorus loss.
Biological Indicators
Biological indicators for example are earthworms and other macro-organisms, microbial biomass carbon and nitrogen, particulate organic matter (small pieces of decomposing active organic matter), potentially mineralizable nitrogen, soil enzymes, soil respiration, and total organic carbon. These relate to microbial potential as a repository for carbon and nitrogen and soil productivity and nitrogen-supplying potential.
Descriptive Indicators
There are also descriptive indicators which are inherently qualitative can be used in assessing soil quality. Some of the descriptive indicators include soil crusting/surface sealing, rills, gullies, ripple marks, sand dunes salt crusting and standing or ponding water. Crop yield (grain or biomass production), plant vigor, rooting patterns, and other aspects of the crop have been used as indicators of soil quality. Crop yield is an important indicator because it gives information about the interacting soil properties of the system as whole as in a bioassay.
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