Soil Management on Organic Farms
Measuring soil quality is an exercise in identifying soil properties that are responsive to management, affect or correlate with environmental outcomes, and are capable of being precisely measured within certain technical and economic constraints. There are three main categories of soil indicators: physical, chemical, and biological. Soil quality attempts to integrate all three types of indicators. The categories do not neatly align with the various soil functions, so integration is necessary.
Physical properties (mechanical behavior) of a soil greatly influence its use and behavior towards plant growth. The plant support, root penetration, drainage, aeration, retention of moisture, and plant nutrients are linked with the physical condition of the soil. Physical properties also influence the chemical and biological behavior of soil. The physical properties of a soil depend on the amount, size, shape, arrangement and mineral composition of its particles. These properties also depend on organic matter content and pore spaces. Growers often describe soil’s physical conditions using terms such as “softness,” “mellowness,” “workability,” or “tilth.”
Soil structure refers to the arrangement of soil particles (sand, silt, and clay) into stable units called aggregates (known to soil scientists as peds). Aggregation is important for increasing stability against erosion, for maintaining porosity and soil water movement, and for improving fertility and carbon sequestration in the soil.
The particles that make up soil are categorized into three groups by size – sand, silt, and clay. Sand particles are the largest and clay particles the smallest. Most soils are a combination of the three.
The space between soil particles is the pore space. This pore space contains varying amounts of water and air. Soil porosity depends on soil texture and structure. Soils with lesser bulk densities have greater porosities. Good porosity is essential to adequate soil aeration, water drainage and root penetration.
Soil color is influenced primarily by soil mineralogy—telling us what is in a specific soil. Soils high in iron are deep orange-brown to yellowish-brown. Those soils that are high in organic matter are dark brown or black. Color can also tell us how a soil “behaves.”
Soil permeability is a measure of the ease with which air and water move through the soil. A consistent and moderate supply of water, along with deep and spreading root growth are some of the benefits of good drainage or permeability. Plants need good internal soil drainage to grow.
Bulk density of a soil is defined as the weight per unit volume of soil. A unit volume of soil includes both the solids and the pore space. Bulk density is important because it reflects the porosity of a soil. Loose, porous soils have lesser bulk densities than tight, compacted soil.
Chemical properties reflect the influence between soil solution (soil water and nutrients) and exchange sites (clay particles, organic matter); plant health; the nutritional requirements of plant; and levels of soil contaminants and their availability for uptake by plants.
Soil pH is the foundation of essentially all soil chemistry and nutrient reaction and should be the first consideration when evaluating a soil test. Soil pH refers to the acidity or alkalinity of the soil. It is a measure of the concentration of free hydrogen ions (H?) and hydroxide ions (OH¯) that are in the soil. The total range of the pH scale is from 0 to 14. Soil pH is neutral when it is 7 and acid when the pH is less than 7 and alkaline when it is greater than 7. A neutral pH occurs where the hydrogen (H?) and hydroxide (OH¯) concentrations are equal (H?= OH¯). Soil pH is directly related to base saturation; as base saturation increases, so dose pH.
In addition to soil pH, many soil tests provide a reading called buffer pH (sometimes called lime index). Soil pH is a measure of hydrogen ion (H?) concentration in the soil solution, which is called active acidity—an indicator of current soil conditions. However, there are hydrogen ions, referred to a reserve acidity that are released into the soil solution to replace those neutralized by the lime.
Cation Exchange Capacity
Cation exchange capacity (CEC) is a measure of a soil’s capacity to hold (or adsorb) positively charged (cations) nutrients. The major soil cations include: calcium (Ca2?), magnesium (Mg2?), potassium (K?), sodium (Na?), hydrogen (H?), ammonium (NH4?), and aluminum (Al3?). The unit of measurement commonly used to express CEC is centimoles of positive charge per kilogram of soil (cmol/kg) and is equivalent to the units formerly used to express CEC—milliequivalents per 100 grams of soil (meq/100g). Soils with a greater clay or organic matter content will have a higher CEC. Examples of CEC values for different soil textures are as follows in table 5.1.
Electrical conductivity (ECe) is a measure of the total soluble salt concentration in a soil (i.e., salinity). Sodium chloride is the most common salt and others include bicarbonates, sulfates, and carbonates of calcium, potassium, and magnesium. A high ECe value corresponds with high amounts of soluble salts, and vice versa. ECe values can be expressed in micromhos/cm (μmhos/cm), millimhos/ centimeter (mmhos/cm), or decisiemens/meter (dS/m).
Base saturation refers to the proportion of cation exchange sites in the soil that are occupied by the various base cations—potassium (K?), calcium (Ca2?), magnesium (Mg2?), and sodium, (Na?). The percent base saturation is calculated as follows:
Sodium Adsorption Ratio
Soil sodium adsorption ratio (SAR) is expresses the proportion of sodium (Na?) relative to the proportions of calcium (Ca2?) and magnesium (Mg2?). Soil SAR is calculated from soil-test extractable levels of sodium, calcium, and magnesium (expressed in milliequivalents/liter, meq/L). The formula for calculating sodium adsorption ratio is:
Exchangeable Sodium Percentage
Sodium levels are evaluated based on exchangeable sodium percentage (ESP). ESP is the percentage of soil exchange sites occupied by sodium (Na?) and is calculated by dividing the concentration of sodium cations by the total cation exchange capacity—ESP = (exchangeable sodium/CEC) x 100. Units of concentration for ESP are milliequivalents per 100g (meq/100 g).
Soil Organic Matter
Soil organic matter is a measurement of the amount of plant and animal residue in the soil. It has several important implications for soil fertility. Organic matter acts as a revolving nutrient bank account, which releases vine available nutrients over an extended period to the vines. Soil organic matter is a source of both macronutrients like nitrogen and phosphorus, as well as micronutrients including iron, copper, and zinc.
Macronutrients that may be tested in your soil include nitrogen (N), phosphorus (P), potassium (K), sulfur (S), calcium (Ca), and magnesium (Mg). Nitrogen, phosphorous and potassium are considered “primary” macronutrients, because they are required in higher quantities than sulfur, calcium and magnesium (“secondary” macronutrients), and because vines develop nitrogen, phosphorous, and potassium deficiencies more often.
Iron (Fe), manganese (Mn), copper (Cu), boron (B), zinc (Zn), nickel (Ni), chlorine (Cl), and molybdenum (Mo) may also be listed on a soil test report. Micronutrients are required by plants in small quantities with the availability directly dependent on the soil pH. Where the pH is high, manganese and zinc are inaccessible to plants since these elements do not remain in solution. In soils with low pH, boron and zinc shortages may also be expected.
An incredible diversity of organism’s make-up the soil food web ranging in size from the tiniest one-celled bacteria, algae, fungi, and protozoa, to the more complex nematodes and micro-arthropods, to the visible earthworms, insects, and small vertebrates. While some soil fauna can cause diseases in plants, the vast majority of soil fauna and flora are critical to soil quality. They affect soil structure and, therefore, soil erosion and water availability. They can protect plants from pests and diseases and are central to decomposition and nutrient cycling. The maintenance of this living aspect of the soil is essential to the maintenance of a healthy field.
Bacteria are the most numerous type of soil organism: every gram of soil contains at least a million of these tiny one-celled organisms. One of the major benefits bacteria provide for plants is in making nutrients available to them.
Fungi come in many different species, sizes, and shapes in soil. Some species appear as threadlike colonies, while others are one-celled yeasts. Many fungi aid plants by breaking down organic matter or by releasing nutrients from soil minerals. Fungi are generally quick to colonize larger pieces of organic matter and begin the decomposition process. Arbuscular mycorrhizal (my-cor-ry¢-zal) fungi are beneficial soil organisms that contribute to many aspects of soil health. Mycorrhizal fungi form a symbiotic association with plant roots. Symbiosis is a close association between different species. Mycorrhizal fungi are especially effective in helping plants acquire phosphorus, a nutrient that is highly immobile in the soil.
Nematodes are abundant in most soils, and only a few species are harmful to plants. The harmless species eat decaying plant litter, bacteria, fungi, algae, protozoa, and other nematodes. Like other soil predators, nematodes speed the rate of nutrient cycling.
Arthropods are species of soil organisms that can be seen by the naked eye. Among them are sowbugs, millipedes, centipedes, slugs, snails, and springtails. These are the primary decomposers. Their role is to eat and shred the large particles of plant and animal residues. Some bury residue, bringing it into contact with other soil organisms that further decompose it.
Earthworm burrows enhance water infiltration and soil aeration. Fields that are “tilled” by earthworm tunneling can absorb water at a rate four to 10 times that of vineyards lacking worm tunnels. This reduces water runoff, recharges groundwater, and helps store more soil water for dry periods.
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