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Management-Intensive Grazing: Management of Forage Supply

Soil Fertility

Maintaining soil fertility enables the forage to reach its yield potential, improves stand persistence, boosts forage quality, allows for faster re growth, enhances disease resistance, and decreases the competitiveness of weeds. Fertility requirements vary with crop species, intensity of management, and native soil fertility.

Soil pH
Soil pH is a measure of soil acidity. When soils are too acid (pH is too low) root development is hindered and crop growth will be reduced. Most crops grow best when the soil pH is 6.5 - 7.0, but some species may not exhibit enough of a response to lime addition when the soil pH is greater than 6.0 to justify the additional lime.  

Most Georgia soils are acidic and require liming to increase soil pH and to supply calcium and magnesium. Common liming materials, calcitic and dolomitic limestone, contain calcium. Dolomitic also contains magnesium and should be used when the soil magnesium level is low.
When ammonium forms of nitrogen are applied, the soil can be made more acidic. As a result, heavily-fertilized hay or silage fields require more frequent applications of lime than grazed pastures.

Elements
Forage crops, like other crops, need at least 13 mineral elements for normal growth and reproduction. These elements are grouped into two basic categories. The macronutrients nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sulfur (S) are required in greater quantities than micro-nutrients, such as boron (B) , copper (Cu), manganese (Mn), zinc (Zn), iron (Fe), molybdenum (Mo) and chlorine (Cl). Plants get these elements from the soil. Georgia soils do not contain sufficient concentrations of some of these elements to meet the plant's need. As a result, supplemental fertilizer (via inorganic or organic sources) must be applied.

Nitrogen is necessary for rapid growth and influences plant protein content. The amount of fertilizer needed and the correct timing of applications varies with crop species and how the crops are used (for grazing or hay). Less N is needed on pastures that are grazed.

Phosphorus is an essential plant element that plays a key role in many vital plant processes such as root development. Low soil levels of P can cause difficulties in establishing new pastures. This element does not leach from the soil (except in very rare cases) and one application per year is sufficient.

Potassium is essential for producing economical yields. Potassium is second only to N in the concentration found in plants. It affects plant vigor, disease resistance, forage quality, winter survival and stand life. Multiple applications are more efficiently used by plants and ensure an adequate supply of K throughout the growing season. This is particularly important with alfalfa and bermudagrass harvested for hay. Two to three applications of K per growing season are advisable for these crops. Potassium is a mobile element and some reports indicate it may leach from some sandy Coastal Plain soils when rainfall or irrigation applications are excessive.

Calcium deficiencies are rare if the soil pH is above 6.0. However, many soils in Georgia are low in Ca and the addition of lime or other Ca-containing amendments may be necessary to meet the Ca needs of some forage species. This is especially true for legumes, which accumulate higher levels of Ca than grasses.

Magnesium deficiency causes yellowing between the leaves and is especially prevalent on acid sandy soils and soils low in Mg. Use of dolomitic limestone increases soil Mg and reduces deficiencies. Additional Mg applications might be required for alfalfa and irrigated corn.

Sulfur is needed by plants that accumulate high levels of N in their tissues. In Georgia, yields have increased in some cases where S has been applied. Deficiencies have occurred recently because of a shift to the use of high analysis phosphate fertilizers which contain only small quantities of S. Sandy soils are more likely to be deficient in S than heavier soils. Sulfur can be deficient on soils containing little organic matter or where such factors as drought or low pH cause decomposition of the organic matter to be slow.

Boron is needed in very small quantities by plants. In Georgia, it is necessary for alfalfa and clovers. Boron may be mixed with other fertilizer materials and applied in the spring.

Molybdenum is required by legumes for nodulation and nitrogen fixation. Molybdenum deficient legumes are stunted, the plants may become yellow, and the lower leaves can die and drop prematurely. Supplement alfalfa with Mo.

Manganese deficiencies (rare in forage crops in Georgia) can be caused by poor drainage or high soil pH. Deficiencies may occur in depressions or poorly-drained areas where the soil pH is higher than the surrounding areas. Manganese problems have been observed on small grains and soybeans grown on poorly-drained soils in southeast Georgia when the soil pH was over 7.0. Leaves of deficient plants show a mottled yellowing between the veins, similar to Fe or Mg deficiencies, but the veins never turn yellow.

Zinc deficiencies are more likely to occur on coarsely-textured soils in the Coastal Plain than on the heavier soils in the Piedmont and Mountain regions. Zinc has not been a significant problem with forage crops.

Soil Testing
To determine what your soil needs, sampling and testing are key to fertility management. The soil test evaluates the pH of the soil and the availability of nutrients. Based on the results of the soil test, application rates can be recommended for corrective soil amendments such as liming to correct low soil pH and fertilizer (inorganic or organic sources) addition. Your county Extension office provides this testing service in your area and your county Extension agent can make recommendations that are tailor-made to locally available sources of lime and fertilizer.

More information on soil testing in Georgia.

Lime and fertilizer recommendation guidelines are available on the "Fertilization Guidelines" page at www.georgiaforages.com.

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