Articles on forages, animals, and grazing systems
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Extension Forage Specialist
University of Kentucky
Phone: (859) 257-3358
Fax: (859) 323-1952
Plants require several minerals and nutrients for growth and production. The three primary nutrients required for plant growth are nitrogen, phosphorous, and potassium. Nitrogen (N) is an essential nutrient necessary for photosynthesis, enzymatic reactions, and creating amino acids, the building blocks of protein. Increasing nitrogen in the soil has been proven to greatly increase pasture production.
Nitrogen can be increased in pastures by incorporating nitrogen-fixing legumes (i.e. clover) into grass stands or by applying nitrogen fertilizer. If applying a chemical form of N, it is important to be aware of the various forms and the benefits and possible disadvantages of each. When deciding which form to utilize, consider costs and potential for loss. Nitrogen can be lost from the soil three different ways. Denitrification occurs when there is not enough oxygen to supply the needs of the soil bacteria and microorganisms. This is common when soils have been saturated for 2 to 3 days. This causes oxygen to be stripped from the nitrates and produces N gas or nitrous oxide which volatilizes from the soil and is lost. Leaching occurs when there is more precipitation than the soil can hold. Nitrates are moved through the soil with the excess water and lost. Soils with low water holding capacities, such as sandy soils, have higher potential for N loss due to leaching. Last, N is commonly lost to surface volatilization which is caused from N breaking down and forming ammonia gasses. The type of N fertilizer utilized affects the probability for loss due to denitrification, leaching, or volatilization.
Nitrogen fertilizer should be applied when pastures will have the best opportunity to have a yield response. For cool season pastures, research shows that split N applications generally maximize yields when N is applied in the late summer, early spring and for added growth again in the late spring. Mid-summer N applications are not recommended for cool-season grasses since they typically do not respond and summer weeds are stimulated. Therefore, to fully benefit from the cost of N fertilizer, apply when plants will have the best response and are able to uptake the most N. Ample precipitation and/or soil moisture along with cooler temperatures provide plants the best chance to respond. Timely application will result in less loss due to leaching or denitrification. A split application can maximize overall forage production. Ask your local county extension agent for more information. See http://www.ca.uky.edu/agc/pubs/agr/agr5/agr5.htm for more information on when to apply lime and fertilizer. See http://www.ca.uky.edu/agc/pubs/agr/agr1/AGR1.PDF for lime and fertilizer recommendations in Kentucky.
The various forms of N fertilizer all have benefits and disadvantages that need to be understood and considered for best management practices. Common forms of N are anhydrous ammonia, urea, ammonium sulfate, ammonium nitrate, and urea ammonium nitrate solution (UAN).
Urea (46-0-0) is often the most commercially available form and is commonly used by forage producers. It is generally safe to handle, easy to store, and the high analysis of N may reduce handling, storage, and transport costs in comparison to some other forms of N fertilizer. If not placed in direct contact with the soil, volatilization losses can be significant. Volatilization is more severe when urea is spread on mild to heavy residues or on moist soil surfaces. Also, losses are greater when applied at temperatures greater than 50⁰F or on soils with a pH greater than 6.5. It is suggested that urea be applied in cooler temperatures and not on soils which have been limed in the past 3 months. Incorporate urea into the soil for best use or apply when rain is expected shortly after application. The use of a urease inhibitor is suggested to decrease loss potential. This granular N form is generally broadcasted onto fields.
Ammonium Sulfate (21-0-0) is a granular form commonly broadcasted on alkaline soils. This form generally experiences little or no volatilization when applied to wet soils and is a good source of sulfur when needed. This form is the most acidifying and may require a larger lime application to balance soil pH. It is the lowest in N concentration which may increase costs.
Ammonium Nitrate (34-0-0) is a non-urea form with little potential for volatilization. This form may be expensive and difficult to obtain in some areas due to regulatory issues and storage problems due to increased potential for water uptake. Ammonium nitrate is usually applied by a broadcast application.
Urea Ammonium Nitrate Solution (UAN) (32-0-0) gets its N from approximately half ammonium nitrate and half from urea which generally lowers the potential for volatilization. Denitrification is possible with this form of N. This liquid form can be broadcasted by spraying or can be banded by dribbling. The use of a urease inhibitor prior to application is suggested when applying to the soil surface.
Maximizing uptake and minimizing loss is important for the best use of any fertilizer. When deciding which form of N fertilizer to utilize, consider overall cost, application method, potential for loss, location and climate, weather forecast, soil type and condition, and current pasture conditions.