Sign up for email alerts of new Fluid Journal issues!
Fluid Journal : Fluid Journal 1993-1995
(Figure 1), which contains nearly an unlimited supply. About 80 percent of the air we breathe is nitrogen (N2). Each acre of the earth's surface is covered by about 37,000 tons of N, but this N2 is an inert gas. It must be combined with other elements before plants can use it. N occurs in the soil in three major forms: Organic N is part of soil organic matter and is unavailable to growing plant. Organic N may represent up to 98 percent of the total N in the soil. Ammonium N (inorganic N) is often fixed by clay minerals and is slowly available to plants. It is held by soil particles and does not leach readily. Ammonium and nitrate ions (inorganic N) or soluble compounds constitute the N form plants use. Nitrate ions may leach and move with soil water. Conversion Two biologically mediated processes affect the amount of plant-available N. Mineralization. The process by which unavailable organic forms are converted to available forms is defined as mineralization. It occurs as microorganisms decompose organic materials for their energy supply. As the organic matter is decomposed, the organisms use some of the energy released, plus part of the essential nutrients in the organic matter. When the organisms have used all the nutrients they need, the excess (such as N) is released into the soil for plant growth. Immobilization. Nitrogen can also be converted from inorganic to organic forms. The process is called immobilization and is the reverse of mineralization. It occurs when crop residues high in carbon and low in N are incorporated into the soil. Mineralization and immobilization occur simultaneously in soils. Whether the soil shifts toward an organic or inorganic N pool depends largely on the C/N ratio of the decomposing organic materials. When immobilization of soil N exceeds mineraliza- tion, there may be practically no N available for growing crops unless N fertilizers have been applied in a band near the roots. As microorganisms vigorously decom- pose the new energy supply in these crop residues, they need N to build protein for their body tissues. Unless residues are relatively high in N, organisms take up inorganic N from the soil to get needed N. So the inorganic N in the soil is converted into organic N in microbial proteins, unavailable for plant growth. But much of this N is gradually returned to the available form as bacterial bodies decompose. Destabilization Under conditions favoring plant growth, much of the NH4+-N in soils will be con- verted to NO3- -N by certain nitrifying bacteria. This process is called nitrification (Figure 2). The process is important be- cause: 1) nitrate is readily available for use by crops and microorganisms, 2) nitrate is highly mobile in the soil and may leach, 3) nitrate can be lost through denitrification. Denitrification (Figure 3) usually occurs in soils high in organic matter, under extended periods of waterlogged conditions (absence of oxygen) and as temperature rises. Soil conditions that have the greatest influence on nitrification or denitrifica-tion are: pH, moisture, temperature, aeration, and plant residues. Stabilization An important part of fertilizer N management is to apply proper rates and sources, place the N for best use efficiency, and time applications when crop needs are greatest. Nitrification inhibitors and slow-release forms of N also help. Nitrification inhibitors block the conver- sion of NH4+ to NO3- by deactivating nitrifying bacteria for varying periods of time, sometimes up to three months. Ample data are available showing that inhibitors increase yields, especially in sandy soils, under wet conditions, or where N has been applied in the fall or early spring. Slow-release N is made by reacting urea with formaldehyde to form com-pounds only slightly water soluble. Its cost often prohibits use on field crops. ! Figure 2. The process of nitrification Figure 3. The process of denitrification Spring 1994 Nitrification Urea Manure Rotting Plant Residues Anhydrous Ammonia nitrite and nitrate forming bacteria add oxygen These steps do require oxygen These steps do not require oxygen ammonia ammonium nitrite oxygen nitrate NH3 NH- 4 + NO- 2 NO- 3 O Denitrification part escapes as gases Caused by: Soil organisms that live without air in a wet soil and get their oxygen from nitrates. nitrite nitrous nitric nitrogen nitrate NO- 2 NO 2 N2 NO NO- 3
Fluid Journal 1996-1998