Nitrogen (N) is a fundamental macronutrient required by all plants for growth and development. It is a core component of chlorophyll, essential for photosynthesis, and is required to synthesize amino acids, the building blocks of proteins and enzymes. Despite the atmosphere containing an enormous reservoir of dinitrogen gas (\(\text{N}_2\)), this form is unavailable for direct plant use, making available soil nitrogen a frequent limiting factor for crop productivity. A shortage occurs because the usable forms of nitrogen in the soil are highly dynamic and easily lost through several distinct physical and biological processes.
Physical Removal Through Harvest and Erosion
The physical export of the nutrient from the field is a direct cause of nitrogen shortage. When crops are harvested, a substantial amount of nitrogen incorporated into plant tissues is removed along with the grain, fruit, or forage. This process effectively takes the nitrogen out of the soil nutrient pool, representing a permanent removal unless fertilizer or organic matter is added to replace it.
Erosion is another physical process that removes nitrogen by washing or blowing away the topsoil. Nitrogen is highly concentrated in soil organic matter, which typically resides in the surface layer. When rainfall or wind causes this organic-rich topsoil to leave the field, the nitrogen within it is lost. The ammonium form of nitrogen (\(\text{NH}_4^+\)) often adheres to negatively charged clay particles, which are easily detached and carried away by erosion, compounding the loss.
Loss Through Gaseous Conversion
Nitrogen shortage is significantly caused by the conversion of plant-available forms into atmospheric gases, accomplished through biological and chemical pathways. Denitrification is mediated by soil bacteria and occurs when the soil becomes waterlogged and oxygen-depleted. Under these anaerobic conditions, microorganisms utilize nitrate (\(\text{NO}_3^-\)) as an alternative to oxygen for respiration.
This biological conversion reduces the plant-available nitrate into gaseous compounds such as nitrous oxide (\(\text{N}_2\text{O}\)) and dinitrogen gas (\(\text{N}_2\)). These gases escape from the soil into the atmosphere, making the nitrogen permanently unavailable for uptake. Denitrification losses can be high in warm, saturated soils and account for a percentage of applied nitrogen.
Another major gaseous loss pathway is ammonia volatilization, where ammonium-based nitrogen converts to ammonia gas (\(\text{NH}_3\)). This happens when nitrogen sources like urea fertilizer or manure are left on the soil surface, especially under warm, moist conditions. The conversion is accelerated in alkaline soils (pH greater than 7.3). The resulting ammonia gas escapes into the air, substantially reducing the amount of usable nitrogen in the soil.
Movement of Soluble Nitrogen Compounds
Nitrogen can also be lost through the physical downward transport of dissolved compounds in a process called leaching. The nitrate form of nitrogen (\(\text{NO}_3^-\)) is an anion, carrying a negative electrical charge. Since most soil particles, such as clay, also carry a negative charge, the nitrate ion is repelled and does not bind to the soil structure.
This lack of attraction means nitrate is highly soluble and mobile, allowing it to move freely with water. When heavy rainfall or excessive irrigation occurs, the water percolates down through the soil profile, carrying the dissolved nitrate. Once the nitrate is pushed below the plant root zone, it becomes inaccessible to the crop and is considered a loss from the system, often contaminating groundwater. Sandy soils are prone to this loss because they have a lower water-holding capacity, allowing water and dissolved nitrate to move through them more quickly.
Temporary Biological Tie-Up
A different type of shortage occurs when nitrogen is temporarily rendered unavailable to plants through immobilization. This happens when soil microorganisms require nitrogen to decompose organic materials that have a high Carbon-to-Nitrogen (\(\text{C:N}\)) ratio, such as straw or crop residues. The microbes consume available mineral nitrogen from the surrounding soil to support their population growth, as their food source may not contain enough nitrogen.
This action effectively locks the nitrogen inside the microbial biomass, making it temporarily inaccessible to plant roots. The shortage is temporary because the nitrogen will eventually be released back into the soil as plant-available forms when the microorganisms die and decompose, a process known as mineralization.