Overview

The SOIL section of MatchClover provides information on the important aspects of soil which affect selecting and managing 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.

Soil Classification Systems

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Slope

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 area in favor of more easily accessed areas. Machinery operations are also affected, with moderate and steep-sloped areas requiring modifications to those on slight to moderate slopes.

Drainage, Flooding, and Ponding

Drainage class along with 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 soil stored water. Warm-season species are more efficient water users than cool-season species. In addition, various genera within each species type also vary significantly. This can be accommodated by matching precipitation characteristics of the region and irrigation availability.

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.

Available Water Holding Capacity

“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.” (http://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb1043060.pdf; page 88)

Soil Reaction (pH)

Soil reaction [the balance of exchangeable hydroxyaluminum ions, hydrogen ions (H+), carbonate ions, and hydroxyl (OH–) ions in the soil solution] is measured in pH units. 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 at absolute neutral, pH = 7.0, an equal number of positively and negatively charged ions are in the soil solution.

Very 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 these alkaline soils though is their high salt content.

The 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. The management needed to address acid soils and alkaline soils is so different that it is best to split soil reaction into two categories: acid soils and alkaline soils.

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.

Some forage crops are tolerant of acid soil conditions. They out-compete forages better suited to alkaline or neutral soils for nutrients. Similarly, other forages are better able to grow under alkaline or near-neutral soil pH conditions. If the soil reaction is not going to be altered by soil amendments, 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.

Salinity

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 need leaching to lower their salt concentrations to levels that the forage crop to be grown will tolerate. This is accomplished best by applying excess irrigation water low in sodium and dissolved salts to cause downward percolation of water through the soil profile. Then, underlying tile drains convey the resultant leachate to an outlet.

When growing forage crops, selecting salt tolerant species is useful to protect from crop failures 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 a gradual increase in soil salinity before treatment is initiated again.

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 cause foliar injury to at least alfalfa, barley, corn, and sorghum. The amount of damage also varies with the weather conditions, spray droplet size, and crop growth stage as well as from the salt concentrations in the irrigation water.

Soil information slightly revised from: USDA NRCS Range and Pasture Handbook. Chapter 3. Ecological Sites and Forage Suitability Groups, Section 2: Forage Suitability Groups.

Clover Species Soil Tolerance Table

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