Nitrogen is often the most common element limiting plant growth, making its availability in the soil a primary concern for gardeners and farmers. This element is a fundamental building block for life, performing many functions within plant cells. Specifically, nitrogen is a core component of chlorophyll, the pigment that captures sunlight to power photosynthesis. Nitrogen is also an inseparable part of all amino acids, which create the proteins and enzymes necessary for growth, cell division, and all metabolic processes. Without sufficient nitrogen, plants display stunted growth and a distinctive yellowing of older leaves, a condition called chlorosis. Maintaining adequate soil nitrogen levels is a constant challenge because the element is highly mobile and easily lost from the soil system through several distinct pathways.
Removal Through Plant Uptake and Harvesting
Plants naturally require a large supply of nitrogen to fuel their growth and structure, absorbing it primarily in the form of nitrate (NO3-) and ammonium (NH4+). When any plant material is taken away from the growing site, such as vegetables from a garden or hay from a field, the nitrogen contained within those tissues is permanently removed from the soil’s nutrient pool. This removal is a continuous drain on the soil’s nitrogen capital in any managed system. Annual crops, like corn or wheat, tend to remove a greater total mass of nitrogen because they are often fertilized heavily to achieve high yields, and the entire above-ground portion is removed at harvest. Even in lawns, the practice of removing grass clippings after mowing constitutes a significant, regular export of nitrogen from the soil. The removal of all harvested biomass prevents the natural decomposition and recycling of that nitrogen back into the soil organic matter.
Physical Loss via Water Movement
One of the most significant ways nitrogen is physically lost is through its movement with water, a process called leaching, which carries the nutrient below the root zone. The nitrate form of nitrogen (NO3-) is highly susceptible to this loss because it is both water-soluble and carries a negative charge. Unlike the positively charged ammonium (NH4+), nitrate is not attracted to or held by the negatively charged clay and organic matter particles in the soil. Because of its negative charge, nitrate moves freely with the soil water, especially when the soil becomes saturated from heavy rainfall or excessive irrigation. This downward movement is particularly severe in sandy or coarse-textured soils, which have larger pore spaces. Surface runoff is another physical loss pathway, where heavy rains carry away soil particles and dissolved nitrate before the water can soak into the ground. When nitrate is washed below the depth of a plant’s roots, it is essentially lost to the crop.
Microbial Transformations and Gaseous Loss
Some of the most complex causes of low soil nitrogen involve microbial activity that converts plant-available forms of nitrogen into atmospheric gases. This loss occurs primarily through two distinct microbial processes: denitrification and ammonia volatilization. Denitrification is carried out by specialized bacteria that use nitrate (NO3-) instead of oxygen for respiration when the soil is waterlogged or compacted, creating anaerobic conditions. In these oxygen-deprived environments, the bacteria convert the nitrate first into nitrous oxide (N2O) and then into dinitrogen gas (N2), which are both gases that escape into the atmosphere. Ammonia volatilization is a separate gaseous loss that happens when ammonium (NH4+) on the soil surface, particularly from certain fertilizers or manures, is converted into ammonia gas (NH3). This conversion is accelerated by high soil pH levels (alkaline conditions) and warm temperatures, especially when the nitrogen source is left unincorporated on the surface.
Insufficient Replenishment and Slow Cycling
A low soil nitrogen condition is often a result of the rate of loss exceeding the rate of natural replenishment from the soil’s internal cycle. The largest reservoir of soil nitrogen is locked up within the soil organic matter, which includes decomposing plant and animal residues. Before this organic nitrogen can be used by plants, it must undergo mineralization, a microbial process that converts it into the inorganic forms of ammonium and nitrate. If the soil organic matter content is low, the amount of nitrogen available for this conversion is reduced, leading to an inherently low supply. Furthermore, the rate of mineralization slows down in conditions that inhibit microbial activity, such as cold or very dry soil temperatures. This slowdown means that even if a large amount of organic nitrogen is present, it is not being converted quickly enough to meet the demands of a rapidly growing plant, causing a temporary or chronic nitrogen deficiency.