by Don Ball
General climatic conditions vary throughout the South. Knowledge of these conditions, together with knowledge of the requirements of various forage varieties or species, allows us to make general statements about their geographic adaptation. Yet, there can be specific location influences that mitigate or intensify general climatic influences and create "microclimates" within a farm or field.
Some location-related climate factors can be influenced by a livestock producer’s management while others cannot. However, a producer needs to at least be aware of the existence of various location influences because they may explain or help explain the dominance or demise of particular plants in particular areas.
- Two important reasons why soil type is important are that it affects the moisture-holding capacity and the nutrient-holding capacity of the soil. In addition, infiltration of water, crusting, and the extent of "heaving" of plants during freezing and thawing are different in various soils. Finally, some soils naturally contain and release higher levels of certain nutrients than others.
Soil pH and Fertility
- The acidity or alkalinity of a soil is measured on a 14 point scale with 7.0 being neutral and numbers above or below this level being basic or acid, respectively. This is a logarithmic scale. Thus, a pH of 5.0 is ten times more acid than one of 6.0. Most soils within the South tend to be acid, but a few are near neutral or alkaline. Soil pH has an important influence on the survival and competitiveness of plants. In general, grasses are more tolerant of soil acidity than are legumes. Some forage crops are tolerant of a wide range of soil pH, while others are much more limited. For example, bermudagrass can tolerate highly acid or quite alkaline conditions, while alfalfa requires a pH of 6.5 to around 8.0. Some weeds are also quite sensitive to soil pH.
Fertility also has a major impact on plant growth and survival, and some plants require much a much higher fertility level than others. Fortunately, inherent fertility limitations that some soils have can be overcome through soil testing and the application of needed nutrients.
- Both the internal and external drainage of a field affect plant growth and survival. Alfalfa, orchardgrass, and arrowleaf clover are examples of forage crops that require good drainage, while reed canarygrass, white clover, and tall fescue are much more tolerant of moist soil.
Obviously, there is more runoff and less opportunity for water infiltration on a hillside than on a relatively flat area. This accounts for the often-observed situation in the mid- and upper South in which hillsides in a pasture may contain substantial amounts of sericea lespedeza and common bermudagrass, both of which require good drainage, while bottoms or areas between hills may consist primarily of tall fescue, white clover, and/or dallisgrass, which require better moisture availability. Various rushes and sedges are undesirable plants that are frequently found in wet areas such as along the edges of waterways or in low-lying portions of fields.
- The highest point in a field can be the area in which plants are severely stressed by wind. High areas also tend to be the most eroded parts of fields, and therefore the soil may be less productive. However, the lowest point in the field is likely to be the coldest area where winterkill is most likely.
- The direction a slope faces can have an important influence on plant competition, especially for plants that are only marginally adapted in an area. Southern slopes receive more sunlight and are warmer than northern slopes, thus reducing the likelihood of winterkill. The opposite situation can occur with cool season plants that are sensitive to heat stress. For example, timothy and Kentucky bluegrass are only marginally adapted in the mid-South. Therefore, other things being equal, they would be more likely to persist on northern, or to a lesser extent, eastern slopes.
Color of Soil and Ground Cover
- A higher percentage of the sunlight reaching a field is absorbed if the field is dark, rather than light, in color. Therefore, factors such as soil type and pasture height can also affect the extent of reflectance of sunlight.
- The height to which a pasture is grazed has an extremely important influence on shading of individual leaves within a pasture sward. In addition, trees, buildings and other
objects that interfere with sunlight interception by plants obviously can have an important influence on plant growth.
- Forage crops can’t escape their location, and location influences definitely affect pasture composition and growth. Selection of adapted species and varieties, together with management to favor persistence in a given environment, are essential in order to obtain good forage production.