Fertilization of Pasture and Hay Fields The soils in the southeast US are highly weathered and typically require addition of nutrients to support optimum plant growth. Soil testing should be the initial step in developing a fertilization program. Soil pH has a direct effect on availability of several nutrients so correcting any pH problems should be a priority in a fertilization program. Phosphorous reacts with aluminum, iron and manganese in acid soils and calcium or magnesium in alkaline soils to form insoluble compounds. So, to maximize phosphorous solubility, and thus plant availability, it is optimum to maintain soil pH between 6.0 and 7.5. Soil pH, especially below 5.5, affects availability of several other plant essential nutrients. Application of lime or potassium and phosphorous fertilizers should be based on soil analyses to avoid unnecessary application. Nitrogen (N) is the nutrient most commonly deficient for plant growth, although adequate levels of other essential nutrients must be available. Nitrogen application rates should be determined based on the amount of forage needed. Nitrogen is readily taken up by plants and facilitates a relatively quick growth response but it can easily be lost from the system by leaching so it should be applied in split applications through the growing season. Urea is the predominant N fertilizer used in agricultural production although ammonium sulfate (21% N) and diammonium phosphate (DAP) (18% N) also contain N. There are some fertilizer formulations combining nitrogen sources. The liquid N fertilizers are mixtures of urea with ammonium sulfate (30% N) or ammonium nitrate (UAN) (32% N). There is a granular N fertilizer that is a mixture of urea and ammonium sulfate with a resulting N concentration of 33%. The efficient use of urea as a N fertilizer requires some understanding of the risk of volatilization and loss of N. Urea has to be hydrolyzed before the nitrogen is in a form for plants to absorb the nutrient. Urea is hydrolyzed by the enzyme urease, and this process results in the formation of ammonia which can volatilize and be lost into the air. The urease enzyme is present in plant tissue and some microorganisms so urea can be hydrolyzed at the soil surface, which increases the risk of volatilization and loss. The hydrolyzing process does require water and the rate of reaction is greater as the temperature increases. The loss of N through volatilization of ammonia is minimized if urea is incorporated into the soil or if rain moves the urea into the soil. So if urea fertilizer is to be surface applied as in pasture or hay fields, it is best to apply it when the soil surface is dry and rainfall of at least 1/3 to ½ inch occurs within 1-2 days after application. Technologies have been developed to reduce the risk of urea being volatilized at the soil surface. A urease inhibitor (Agrotain) can be applied, which reduces the risk of urea hydrolysis occurring at the soil surface by delaying the action of the urease enzyme. This compound will be depleted over time so it only delays the hydrolysis reaction but this does provide a 1-2 week window of time before a rain is needed to carry the urea into the soil. Another technology is to coat each urea granule with a polymer which acts a physical barrier. The polymer coating has micropores and allows moisture into the granule more slowly and thus reduces the rate of diffusion of the urea. The thickness of the coating regulates the flow of moisture and thus the rate urea is solubilized and moves into the soil solution. This allows for N release over time so not all the N is readily available soon after application and studies have indicated the need to include a reduced rate of a more soluble form to get a rapid early response. — Dr. Wink Alison, LSU AgCenter Scott Research & Extension Center