Part 1, Section 1: Soil Management
Soil Health
Soil health is defined as “the capacity of a soil to function within ecosystem boundaries to sustain biological productivity, maintain environmental health, and promote plant and animal health.” In some cases, the term “soil quality” may be used; the two terms have the same meaning. Soil properties that determine soil health include soil texture, depth of soil, infiltration, bulk density, water-holding capcity, soil organic matter, pH, electrical conductivity, microbial biomass, carbon and nitrogen, potentially mineralizable nitrogen, and soil respiration. In this section we will focus on the physical and biological aspects of soil health (Box 1.1-1); chemical aspects are discussed in the soil fertility section.
Measuring soil health with scientific methods takes major investments. Therefore, Penn State developed the Pennsylvania Soil Quality Assessment Scorecard, available from your Penn State Cooperative Extension county office or the Internet at http://pubs.cas.psu.edu/freepubs/pdfs/uc170.pdf. It is useful to compare agricultural soil with nearby soils under permanent vegetation such as trees or sod. Soils with poor health often have inferior tilth, lower organic matter contents, few living organisms, and show signs of soil erosion, crusting, and soil compaction. Eventually, poor soil health results in problems with crop establishment, root growth, and crop yields. Increasing amounts of fertilizers, pesticides, and tillage are needed to maintain yields on poor-quality soil. We will now discuss some important soil properties that determine soil health.
Box 1.1-1. Key soil physical and biological properties.
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Soil texture affects almost all other soil health indicators. The size distribution of primary soil particles is smaller than 2 mm (0.79 inches). Gravel is larger than 2 mm. Sand (2- to 0.05-mm diameter), silt (0.05 to 0.002 mm), and clay (smaller than 0.002mm) 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, and so forth. Local information about soils can be gleaned from the county soil survey, available from your local USDA Natural Resources Conservation Service (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 much soil plant roots can occupy.
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.
The CEC (cation exchange capacity) of a soil is determined by the soil’s clay and organic matter content. 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 texture | Ideal bulk densities | Bulk densities that may affect root growth | Bulk densities that may restrict root growth |
|---|---|---|---|
| g/cm3 | |||
| Source: generalized from USDA-NRCS soil quality test kit guide. | |||
| Sand, loamy sand | <1.60 | 1.70 | >1.80 |
| Sandy loam, loam, sandy clay loam, clay loam, silt, silt loam, silty clay loam | <1.40 | 1.60 | >1.75 |
| Sandy clay, silty clay, clay | <1.10 | 1.50 | >1.60 |
The plastic and liquid limits of a soil are two measures 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. A simple method to determine soil readiness for tillage and traffic is the “ball test.” Take a handful of soil and squeeze it into a ball. If the soil molds together, it is in the plastic state and too wet for tillage or field traffic.
Soil structure and soil tilth are very important but still elusive concepts. Soil tilth refers to the state of aggregation of a soil. Aggregates are conglomerates of clay, silt, and sand particles that are held together by physical and chemical forces. A common method of determining aggregate stability is to place aggregates on a sieve and move the sieve up and down in water bath. If a lot of soil passes through the sieve, the aggregate stability is low; if most of the soil remains on top of the sieve, the aggregate stability is high. Soils with stable aggregation tend to have better soil tilth, greater water infiltration, and better aeration for crop growth.
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.
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. 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 since 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.