Part1, Section 1 Soil Management
Soil Health
Soil health1 is now receiving more attention because of increasing recognition that many agricultural soils are "sick." Soil health is the ability of a soil to perform a desired function. In general, six functions of soils are distinguished. Soil is a:
- Medium for plant growth
- Recycling system for nutrients and organic wastes
- Filtering and buffering system for water
- Habitat for soil organisms
- Store of carbon
- Engineering medium for roads and buildings.
In agriculture, we are concerned with all but the last of these functions. The health of an agricultural soil is a composite result of the chemical, physical, and biological properties of the soil. These properties are partly a given—as discussed in the general soil regions in the first section of this chapter —whereas most properties are also influenced by soil management.
Box 1.1-1. Key soil physical and biological properties.
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One symptom of poor health of agricultural soils is their inferior tilth compared with that of neighboring virgin soils. Organic matter contents of agricultural soils are generally less than half that of soils under natural vegetation. Some soils seem to be “dead,” having few if any visible living organisms in them. Soil erosion creates gullies in fields and may be carrying the most fertile portion of the soil, the topsoil, to streams. Sometimes, a crust develops after a heavy thunderstorm, leading to water runoff and poor crop establishment. Soil compaction also takes its toll, creating a dense layer that is impermeable to air and water and inhibiting root growth.
In some extreme cases, poor soil health has led farmers to abandon land. In other cases, crop yields have decreased. It is also possible that increasing amounts of fertilizers, pesticides, and tillage are needed to maintain yields. Therefore, an urgent need exists to improve soil health through changes in soil management.
In this section, we will discuss some key physical and biological properties of soil health (see Box 1.1-1). The chemical aspects of soil management are dealt with in the chapter on soil fertility.
Soil texture affects almost all other soil health
indicators. It is the size distribution of primary soil particles that are
smaller than 2 mm (0.79 inches). Sand (2-0.05 mm diameter), silt (0.05-0.002
mm) and clay (smaller than 0.002 mm) contents are used to determine the
textural class from the textural triangle (Figure 1.1-2). Most soils in
Pennsylvania are "silt loams." This classification refers to the
surface soil and does not take into account differences in clay content in the
subsoil, impermeable layers near the surface, rock fragments, etc. Soil texture
is changed by tillage and soil erosion. Soil-inverting tillage (moldboard or
disk plowing) mixes subsoil with topsoil. This can lead to an increase in clay
content in the surface of soils that have a clayey subsoil. Soil erosion by
water selectively removes fine silt-sized particles. Erosion by tillage moves
topsoil downslope and is a major reason for the formation of clay knobs. Local
information about soils can be gleaned from the county soil survey, available
from your local USDA-NRCS office.
Soil depth is the depth of soil to bedrock or to an impermeable layer. Soil depth determines how deep roots, water, and air can penetrate into a soil. This, in turn, influences how much water can infiltrate the soil, how much water can be held by the soil, and how large a volume of soil can be occupied by plant roots. Soil tillage and erosion can lead to the loss of soil depth. Soil depth can be increased when soil is deposited in depressions or lower parts of the field, or if high quantities of compost, manure, or sludge are applied to the landscape.
Soil organic matter consists of living, partially to fully decomposed organic materials. Soil organic matter is typically 1 to 5 percent of the total dry weight of topsoil, with lower amounts in the subsoil. Different types of organic matter play unique roles in soil. Highly decomposed organic matter (also called humified organic matter) typically makes up 95% of the total soil organic matter, and contributes to the cation exchange capacity, the water holding capacity, and stability of small aggregates. Other, less highly decomposed types of organic matter such as polysaccharides are produced by bacteria and determine the stability of larger aggregates. Living organic matter such as fungal hairs and plant roots are also important for the stability of large aggregates. Soil organic matter is an important indicator of soil health. Its content can be increased by growing high-residue-producing crops; growing crops that produce large amounts of fine roots (species such as corn, small grains, grasses); growing cover crops; and by adding compost and manure (especially bedded manure). If crop residue is removed, for example, as hay or silage, care should be taken to supply residue in other forms such as a cover crop, compost, or manure. These steps will help maintain organic matter contents. Soil tillage increases losses of organic matter. Besides increasing the amount of residue added to soil, it is important to provide soil with a "diverse diet" of different types of organic materials.
The CEC (cation exchange capacity) of a soil is determined by the clay and organic matter content of a soil. These particles carry a negative charge that enables a soil to hold on to positively charged molecules called "cations." Potassium, calcium, and magnesium are nutrient cations that dissolve in water and would wash out of the soil if they were not held by the CEC. The CEC of your soil is reported on soil test reports.
Bulk density is a measure of the mass of particles that are packed into a volume (e.g., a cubic foot) of soil. If bulk density goes up, porosity goes down. It is favorable to have a low bulk density so that water and air can move through the soil. The optimal bulk density depends on soil texture. Ideal and problem bulk densities of different soils are given in Table 1.1-2. Soil compaction causes the bulk density to increase, whereas any practice that improves soil tilth decreases bulk density. Soil tillage temporarily decreases bulk density, after which the soil recompacts to similar (or greater) densities as a no-till soil at the end of the growing season. The moldboard plow and the offset disk compact the soil below the tillage tool.
Porosity is the total volume of pore space in a volume of soil. Some pores in a soil are filled with water, whereas others are filled with air (the "air-filled porosity"). A general rule of thumb is that roots need at least 10% air-filled porosity to be able to grow. The size distribution of the pores is also important because large pores act as conduits for water to move into the soil, whereas small pores hold water for plants to use when they need it. Large pores also allow oxygen to move into the soil and allow carbon dioxide to escape.
The plastic and liquid limit of a soil are two measures that are used to characterize the ease with which a soil can be compacted. The plastic limit is the moisture content at which it is possible to make a wire of approximately one-quarter inch in diameter by rolling the soil between two hands. The liquid limit is the moisture content at which soil starts to flow and act as a liquid. The difference between the plastic and liquid limit is the plasticity index. As a rule of thumb, the soil is most easily compacted when it is at the plastic limit, because it is at this moisture content that soil particles start to slide over each other and pack into greater densities. Traffic should therefore be avoided whenever the soil is at or wetter than the plastic limit. Soils with good tilth have a higher plastic limit than soils with poor tilth. This means that traffic is possible at higher moisture contents on soils with good tilth than on those with poor tilth.
Aggregate stability is a measure of the stability of soil structure and soil tilth. Aggregates are conglomerates of clay, silt, and sand particles that are held together by physical and chemical forces. The bonds that hold these particles together can be broken by applying energy to the soil, for example, by shaking aggregates in water. A common method of determining aggregate stability is to place aggregates on a sieve with uniform openings and move the sieve up and down in a water bath. If a lot of soil passes through the sieve, the aggregate stability is low, while it is high if most soil remains on top of the sieve. Tillage destroys aggregates. Increasing soil organic matter content is the best method to increase aggregate stability. Crop rotations and crop mixtures can help to improve the aggregation of soils. Crops with extensive, fine root systems such as grasses and cereals stimulate aggregate stability in the long term. Crops with easily decomposed residue stimulate aggregate stability in the short term, because bacteria that feast on the residue produce polysaccharides that act as glue holding aggregates together. Amendments (such as manure or sewage sludge) that stimulate biological activity will help improve aggregate stability.
Water content is the mass of water divided by the dry mass of the soil. Water content changes all the time. Water-holding capacity, on the other hand, hardly changes during a year. Water-holding capacity is a measure of the amount of water available for plants to take up. The upper limit of water-holding capacity is called "field capacity." As a rule of thumb, the soil is at field capacity 24 hours after it has been soaked by rain. The lower limit of waterholding capacity is the "wilting point," at which crops completely wilt. Usually, crops start to suffer from drought long before they reach the wilting point. Water-holding capacity is determined by the texture, organic matter content, and structure (tilth) of the soil. Sandy soils have a low water-holding capacity because water content at both field capacity and wilting point is low (Figure 1.1-3). Much of the water in sandy soils is gravitational water that drains out quickly after a rainstorm. Clay soils usually have higher water-holding capacity than sandy soils. Clay soils contain low amounts of gravitational water, but they contain high amounts of water at wilting point. Well-aggregated loam soils, intermediate in texture between sand and clay soils, have the highest water-holding capacity. Any method that improves the tilth of soil, especially an increase in soil organic matter content, helps to increase the water-holding capacity.
Hydraulic conductivity (permeability) and infiltration rate are two closely related properties. Hydraulic conductivity is the rate of water movement in the soil, whereas infiltration is the rate at which water enters into the soil from the surface. Hydraulic conductivity and infiltration are determined by soil texture, changes in soil texture between surface and subsurface, impermeable layers, and depth to bedrock, as well as by soil management. Soil management that improves soil tilth also will help to increase water infiltration. Nightcrawler burrows, easily more than 3 feet deep, are very important for water infiltration. Soil tillage creates an immediate increase in infiltration, but as the growing season progresses, the infiltration rate decreases. In a no-tilled soil, infiltration rates are determined by macropores such as old root channels and earthworm burrows. On an annual basis, infiltration in long-term no-till soils often exceeds that in tilled soils because of the abundance of macropores, but it takes time to build up this pore system. Sealing and crusting also reduce water movement in bare soils with poor soil tilth.
Respiration measures the carbon dioxide produced by microbes that decompose organic materials. A high carbon dioxide flux (or respiration) indicates high levels of microbial activity. High respiration rates are indicators of a healthy soil if the soil is undisturbed. It is common to measure extremely high respiration immediately after tillage. Soon after tillage, respiration rates will drop back to the same or a lower level compared with a soil that is not tilled. The rapid release of carbon dioxide immediately after tillage may be a result of the escape of carbon dioxide that was locked up in the soil, or the breaking up of aggregates that are brought into close contact with microorganisms, and is not considered an indicator of good soil health. A steady, high respiration rate throughout the year is desirable because it indicates that soil organisms are continuously restoring the soil. High soil porosity, optimum water contents, the application of a variety of organic materials by the use of crop rotations, cover crops, manure, and compost will feed microorganisms in the soil and increase respiration levels.
Earthworms generally increase microbial activity, increase the availability of nutrients, and enhance soil physical properties. They also accelerate the decomposition of crop residue by incorporating litter into the soil and activating mineralization and humification processes. Earthworms improve aggregation and porosity, suppress certain pests or disease organisms, and enhance beneficial microorganisms. There are different types of earthworms: some live in the surface of the soil and make horizontal burrows, while others live in the vertical burrows that can be more than 3 feet deep. Some earthworms make permanent burrows, while other earthworms fill their burrows with excretions.
Nightcrawlers are among the important earthworm species in agricultural soils. They make permanent, vertical burrows that provide channels for infiltration in no-till soils. They need surface residue, which they gather and deposit on top of their burrows. Tillage is detrimental to many, but not all, earthworm species. The species that make vertical burrows and need surface residue as a food source (such as nightcrawlers) are negatively affected by tillage. Earthworms do better in silt loams than in sandy or clayey soils. Earthworms flourish in well-drained soils, but not in poorly drained or droughty soils. The optimum pH range for earthworms is 5.0-7.4; they are scarce in soils with pH less than 4.5. This explains why some acidifying fertilizers, such as ammonium sulfate or urea, can have a negative impact on earthworms.
The quantity and quality of food available to earthworms often determines their density per acre. In many instances, crop residue contains too little nitrogen to be digested by earthworms. Manure applications, therefore, have been found to favor earthworm populations, because they help make the crop residue more palatable. Many fungicides and insecticides are toxic to earthworms. Most herbicides are not toxic to earthworms, with the exception of triazine herbicides, which are slightly toxic. A good method to monitor earthworm populations is to excavate one cubic foot (1 x 1 x 1 ft) of soil and count the earthworms in and beneath it. A good time to do this is after a rainstorm or early in the morning when the soil is moist, because earthworms tend to hide deeper when the soil is dry. Earthworms that reside below the foot-deep hole will come to the surface if you pour some mustard powder dissolved in water in the hole. Ten earthworms per square foot of soil surface is generally considered a good population in agricultural systems.
1 Some people (especially scientists) prefer the term "soil quality"
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