Nitrogen is a primary plant nutrient that plays a major role in achieving the maximum economic yields from your production acres. Management of Nitrogen, and other essential nutrients, is part of a balanced fertility program. This can lead to increased efficiency and profitability for the grower.
Plants absorb Nitrogen in some of the greatest amounts of any essential nutrient. In order for a crop to achieve its maximum yield potential, Nitrogen is needed in large quantities and must be in balance with other nutrients. Please reference the table below for specified crop Nitrogen requirements.
Nitrogen Requirements of Several Crops
N lbs/A Required
N lbs/A Required
N lbs/A Required
N lbs/A Required
*Approximations (Corn, Soybeans, and Wheat requirements are based on grain and stover Nitrogen requirements. Soybeans and Alfalfa Nitrogen requirements will mainly be met by Nitrogen fixation by Rhizobia and other symbiotic bacteria.)
In order to manage your soil’s Nitrogen, you must understand what happens to Nitrogen in the soil. Most of the Nitrogen (97 – 98%) in the soil is tied up in the organic matter and unavailable to plants. Only 2 – 3% is in the inorganic form of nitrate (NO3–) and the ammonium (NH4+) forms that is available to plants. The organic matter (at proper moisture, temperature, and oxygen content) is continuously being broken down by microorganisms and released as inorganic Nitrogen into the soil. This process is called mineralization. An opposite process also occurs where microorganisms feed on inorganic Nitrogen. This process is called immobilization.
In healthy soil both processes are taking place at the same time. When large amounts of stover (straw or corn stalks) which have a high carbon and low Nitrogen content is incorporated into the soil, immobilization can take place at such a rapid rate and deplete the soil of all or most of its available Nitrogen. Later the microorganisms bodies will breakdown and decay going through the process of mineralization. This Nitrogen is again released into the soil and becomes available for plant growth.
During the process of mineralization, most of the organic matter is first converted to ammonium (NH4+). The process that breaks down the ammonium (NH4+) to
nitrate (NO3–) by nitrifying bacteria is called nitrification. This process is very important because nitrate is readily available for use by crops and microorganisms. Nitrates are very mobile in the soil.
Nitrogen is lost from the soil in several ways: plant uptake, microorganisms, nitrates that move out with drainage water, and the loss of nitrates by denitrification. Denitrification occurs in flooded or saturated soils during periods of warm temperatures. In this state of depleted oxygen, microorganisms take oxygen from the nitrate (NO3–). Then the Nitrogen escapes into the air as gas. Denitrification is commonly observed in wet spots in corn fields where the plants are yellow and stunted.
Applied Nitrogen can also be lost in several ways: urea applied to the surface converts rapidly to NH3 and escapes into the air as ammonia gas when adequate moisture, temperature, and the enzyme urease is present. To avoid this loss, incorporate the urea, or irrigate immediately. A urease inhibitor can also be utilized to reduce loss. Anhydrous ammonia can also be lost rapidly if the injection slot is not sealed.
FORMS OF NITROGEN TAKEN UP BY THE PLANT
Most plants absorb a majority of their Nitrogen in the nitrate (NO3–) form and to a lesser extent the ammonium (NH4+) form. Rice utilizes the ammonium (NH4+) as its primary source of Nitrogen. Plant growth seems to improve when a combination of ammonium and nitrate Nitrogen is taken up by the plant. Once inside the plant, the nitrate is transforms to NH4-N through energy provided by photosynthesis.
In alfalfa and soybeans, most of the Nitrogen is supplied by Nitrogen fixation by rhizobia and other symbiotic bacteria. This Nitrogen is fixed in the nodules on the roots of the plants. Presence of nodules does not mean that Nitrogen will be fixed. Healthy nodules are usually located on the primary root and are larger in size, elongated, form clusters, and have pink-to-red centers. The red color represents the presence of leghemoglobin, which means that the rhizobia are fixing Nitrogen in the nodules.
FUNCTIONS IN THE PLANT
Inside the plant, Nitrogen converts to amino acids, the building blocks for proteins. These amino acids are then used in forming protoplasm, which is used in cell division. These amino acids are also utilized in producing necessary enzymes and structural parts of the plant and can become part of the stored proteins in the grain.
Nitrogen serves as the source for the dark green color in the leaves of various crops. This is a result of a high concentration of chlorophyll. Nitrogen combined with high concentrations of chlorophyll utilizes the sunlight as an energy source to carryout essential plant functions including nutrient uptake.
Chlorophyll is associated with the production of simple sugars from carbon, hydrogen, and oxygen. These sugars along with their conversion products play a role in stimulating plant growth and development along with higher protein content in the grain.
Nitrogen deficiency shows up in the yellowing or chlorosis of the plant leaves. The yellowing will start in the oldest leaves, and then will proceed to develop on younger leaves if the deficiency continues.
Plants will typically be shorter or stunted and grow slower than plants with sufficient Nitrogen. Nitrogen stress also reduces the amount of protein in the seed and plant. Tillering can also be reduced in small grains.
A Nitrogen deficiency can also affect the standibility of crops as grain fill occurs. If a plant is deficient in Nitrogen, it will draw Nitrogen out of the leaves and stalk for grain fill. This will weaken the stalk or stem causing standability problems.
Nitrogen is in high demand throughout the growing season. In the first 25 days of a corn plants growth, it will absorb 8% of its total Nitrogen. This is critical because the corn plant determines the number of kernel rows in the first 25 days of growth. The number of kernel rows can dramatically impact yield, so placement in the root zone is extremely important. Placement on the seed is recommended for optimum results. This provides the Nitrogen right where the growing seedling can utilize it to promote early season plant growth. Be sure that the starter fertilizer you are using is a low salt fertilizer to avoid potential injury to the seedling.
Nitrogen can also be applied throughout the growing season to meet the high demands of the crop. Careful management should be given to the Nitrogen applied in season because of the volatility. Foliar feeding of Nitrogen is practical and efficient during the growing season. Nitrogen (as Urea) can be absorbed into the plant in ½ -2 hours and then translocate through out the plant. Foliar feeding Nitrogen is 4 times more efficient than soil applied Nitrogen.
If applying Nitrogen in the fall as anhydrous ammonia, we recommend the use of a Nitrogen stabilizer such as N-Serve. If you are broadcasting Urea in the spring, we recommend the use of a urease inhibitor such as Agrotain. In both cases, the specific products can be an asset in Nitrogen management and reduce the loss of Nitrogen.