Nutrient management is an essential part of sustainable agriculture. Twenty nutrients are recognized as needed for good crop growth. Nitrogen, phosphorus, potassium, along with sulfur, calcium, and magnesium, are called macronutrients because plants need these in relatively large amounts. Plants also need micronutrients such as boron, copper, manganese, molybdenum and zinc to remain healthy. A soil with a pH of 6.0 to 7.0 with adequate soil organic matter (about 2 to 4% in Georgia) will help supply many of these nutrients; however, additional nitrogen, phosphorus and potassium are usually needed for good yields.
Although nitrogen, phosphorus, potassium and other nutrients are essential for good crop growth and yields, too much of a good thing can have negative impacts. For example, excess nitrogen can move as nitrate into groundwater and create drinking water problems. Excess phosphorus can move into surface water and cause algae blooms. Too much copper or zinc can cause poor crop growth. Consequently, nutrient applications should be managed to maintain adequate but not excessive concentrations.
Nutrient management consists of several steps:
- Testing the soil to determine the nutrients needed: view publication from UGA Extension
- Determining the recommended amounts of fertilizer needed to produce the desired yields,
- Accounting for nutrient inputs from other sources such as legumes, manures, or composts, and
- Then applying the additional nutrients needed.
Nutrient management for inorganic fertilizers is relatively easy, because nutrients can be purchased in the concentrations needed. Nutrient management can be more difficult when organic fertilizers such as manures or composts are used. This is because crops typically require three to four times more nitrogen than phosphorus while the phosphorus content of manures tends to be close to that of nitrogen. Consequently, when manures are regularly applied to meet the nitrogen needs of the crop, phosphorus is overapplied. Composts can create similar conditions if manures are used as a feedstock. Composting causes nitrogen concentrations to decrease because some of the original nitrogen is lost as a gas, while phosphorus is concentrated because it does not have a gaseous form.
The process of nutrient management outlined above will help avoid overapplication of phosphorus and other nutrients with manures and composts. A couple of strategies can be used to prevent a buildup of phosphorus in the soil. Organic fertilizers can be tested for nutrient content and applied based on the phosphorus need of the crop with the additional nitrogen needed applied as inorganic fertilizer or another low phosphorus nitrogen source. Legume cover crops such as crimson clover can be used to help supply the nitrogen for your cash crop. Legumes can also be interseeded in a pasture to reduce nitrogen needs.
If you are looking for additional information on nutrient management, please see our Resources page to find other publications and useful tools.