The SOIL section of MatchClover provides information on the important aspects of soil that affect selection and management clovers and other forage species.
Landscape and soil properties have a significant and direct influence on forage plant production and management. Factors include slope, drainage, flooding and ponding, available water holding capacity, soil reaction (acid and alkaline soils), and salinity.
For soil resources, a natural system approach to classification, i.e. grouping soils by their intrinsic property (soil morphology), behavior, or genesis, results in classes that can be interpreted for many diverse uses. Differing concepts of pedogenesis, and differences in the significance of morphological features to various land uses can affect the classification approach. Despite these differences, classification criteria group similar concepts so that interpretations do not vary widely.
The USDA soil taxonomy system uses taxonomic criteria involving soil morphology and laboratory tests to inform and refine hierarchical classes. In soil survey, soil classification means criteria based on soil morphology in addition to characteristics developed during soil formation. Criteria are designed to guide choices in land use and soil management. This is a hierarchical system that is a hybrid of both natural and objective criteria. USDA soil taxonomy provides the core criteria for differentiating soil map units. Soil taxonomy-based soil map units are additionally sorted into classes based on technical classification systems; e.g. Land Capability Classes, hydric soil, and prime farmland.
Soil texture triangle showing the USDA classification system based on grain size.
Slope categories (slight, moderate, and severe) are important since livestock decrease their movement as slope increases. Grazing pressure on hilly ground becomes uneven as livestock ignore steeper areas in favor of more easily accessed areas. Machinery operations on moderate and steep-sloped areas require modifications to those on slight to moderate slopes.
Drainage class, available water capacity, flooding, and ponding deal with water supply issues that affect forage production and management. Because water use efficiency varies greatly among forage species, selection can be based on the availability of water stored in the soil. Warm-season species are more efficient water users than cool-season species. In addition, various genera within each species type also vary significantly. Matching precipitation characteristics of the region and irrigation availability to the variation in water use efficiency among and between forages is an important factor in successful forage establishment.
Some species have a broad spectrum of adaptation to soil drainage conditions. Others have a narrow band of adaptation. These tolerance characteristics are included in the species database.
Soil Drainage Class Map of Conterminous USA:
“Available water capacity (AWC) differs from drainage class in that it deals only with plant available water on a site. AWC is a function of soil texture, organic matter content, salinity, clay type, and rooting depth. Available water capacity is the inches of plant available water held by the soil profile to the depth indicated for the soil moisture regime in which the soil map unit component belongs. Or, it is to the depth the first root restrictive layer is encountered, if less. AWC values should be zero for dense layers from which roots are excluded and zero for all soil layers below them. In some cases where soil internal drainage is poor, the root-restrictive layer very well could be water saturated soil.” (National Range and Pasture Handbook: http://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb1043060.pdf; page 88)
Soil reaction is measured in pH units. pH is a means of determining whether soils are acidic or basic. It is a measure of the balance of exchangeable hydroxyaluminum ions, hydrogen ions (H+), carbonate ions, and hydroxyl (OH–) ions in the soil solution. The pH of a soil solution is the negative logarithm of the concentration of H+ ion activity in the soil solution. When the soil pH is said to be neutral, pH = 7.0 and an equal number of positively and negatively charged ions are in the soil solution.
The management for acid soils and alkaline soils is very different. Extremely acid soils decrease the solubility of most major plant nutrients as well as some micronutrients, such as molybdenum. Nutrients must be soluble in water to be adsorbed by plant roots. At the same time, very acid soils may release toxic amounts of aluminum, iron, and manganese.
Alkaline soils can also decrease plant nutrient solubility, principally phosphorus, boron, copper, iron, manganese, and zinc. Often, the largest problem with alkaline soils is a high salt content.
High salt content interferes with water uptake by many forage species and their photosynthetic rate. For instance, sodic soils, soils with a pH greater than 8.5, are generally unproductive for culturally managed forages because of excess sodium and OH– ions that cause poor soil aggregation and plant root desiccation.
Saline and saline-sodic soils are other alkaline soils. They have a pH less than 8.5, but have high amounts of soluble salts that interfere with plant growth.
Critical breakpoints on the pH scale have been identified in relation to forage plant growth. Many agronomically managed forages have a wide range of pH suitability. Most prosper in the pH range from 5.6 to 7.3, moderately acid to neutral. As the pH drops below 5.5, to strongly acid, increasingly more exchangeable aluminum is released. At pH 4.0, exchangeable aluminum has saturated soil cation exchange sites. Few forage plants survive, and none thrive. At pH 8.5 or greater, strongly alkaline, sodium carbonate is present in the soil in amounts that interfere with forage growth.
If soil amendments will not be applied to alter soil pH, select forage plants based on their ability to prosper under the pH conditions at the site. pH tolerance characteristics for each species are included in the species database.
Soil pH Map of Conterminous USA:
Soil salinity is of great importance in the Western United States where culturally managed forages are grown. It may be a general condition of a particular soil series, or it may occur as a saline seep area.
Saline soils may require leaching to lower their salt concentrations to levels that the forage crop to be grown will tolerate. Leaching is accomplished by applying excess irrigation water that is low in sodium and dissolved salts. The downward percolation of water through the soil profile moves salts below the rooting area. This is most successful when underlying tile drains convey the salt-laden leachate to an outlet.
Salt tolerant species provide protection from crop failure even when saline soils have been leached. These soils tend to become salty again over time, especially if irrigated with water high in soluble salts. Therefore, planting salt-tolerant species provides a safeguard against the inevitable, gradual increase in soil salinity before treatment until leaching is once again required.
Salt tolerance information in the species database applies to surface-irrigated plants and conventional irrigation management. Sprinkler-irrigated crops may suffer leaf burn from salt in the spray water contacting leaves and foliar salt uptake. The available data for predicting yield losses from foliar spray effects is limited. Sodium and chloride concentrations of 10 to 20 millimoles per liter in sprinkler irrigation water can, at a minimum, cause foliar injury to alfalfa, barley, corn, and sorghum. The amount of damage also varies with the weather conditions, spray droplet size, crop growth stage, and salt concentrations in the irrigation water.
Soil Salinity Map of Conterminous USA:
Soil information slightly revised from: USDA NRCS Range and Pasture Handbook. Chapter 3. Ecological Sites and Forage Suitability Groups, Section 2: Forage Suitability Groups.