Understanding nitrogen in soils (2024)

Environmental and economic issues have increased the need to better understand the role and fate of nitrogen (N) in crop production systems. Nitrogen is the nutrient most often deficient for crop production in Minnesota, and its use can result in substantial economic return for farmers.

However, when N inputs to the soil system exceed crop needs, there’s a possibility that excessive amounts of nitrate (NO3--N) may enter either ground or surface water.

Managing N inputs to achieve a balance between profitable crop production and environmentally tolerable levels of NO3--N in water supplies should be every grower’s goal. Nitrogen’s behavior in the soil system is complex, yet understanding these basic processes is essential for a more efficient N management program.

Nitrogen exists in the soil system in many forms, and changes (transforms) very easily from one form to another. The route N follows in and out of the soil system is collectively called the nitrogen cycle (Figure 1).

The nitrogen cycle is biologically influenced. Biological processes, in turn, are influenced by prevailing climatic conditions along with a particular soil’s physical and chemical properties. Both climate and soils vary greatly across Minnesota and affect N transformations for the different areas.

Atmospheric N is the major reservoir for N in the N cycle (air is 79 percent N2 gas).

Although unavailable to most plants, leguminous plants can use large amounts of N2 via biological N fixation. In this biological process, nodule-forming Rhizobium bacteria inhabit the roots of leguminous plants and, through a symbiotic relationship, convert atmospheric N2 to a form the plant can use.

Legumes can fix substantial amounts of N2 into usable N. An alfalfa crop, for example, has the potential to fix several hundred pounds of N per acre per year. Any legume crop that’s left after harvest, including roots and nodules, can supply N to the soil system when the plant material is decomposed.

Several nonsymbiotic organisms fix N, but N additions from these organisms are quite low (1 to 5 pounds per acre per year). In addition, precipitation adds small amounts of N to the soil. In Minnesota, precipitation supplies an average of 5 to 10 pounds of N per acre per year.

Commercial N fertilizers are also derived from the atmospheric N pool. The major step is to combine N2 with hydrogen (H2) to form ammonia (NH3). Anhydrous ammonia is then used as a starting point in the manufacture of other nitrogen fertilizers.

Anhydrous ammonia or other N products derived from NH3 can then supplement other N sources for crop nutrition.

Nitrogen can also become available for plant use from organic N sources. Before these organic sources are available to plants, they must be converted to inorganic forms. Nitrogen is available to plants as either ammonium (NH4+-N) or nitrate (NO3--N).

Manures

Animal manures and other organic wastes can be important sources of N for plant growth. The amount of N supplied by manure will vary with the type of livestock, handling, rate applied and method of application. Because the N form and content of manures widely varies, a manure analysis is recommended to improve N management.

Crop residues

Crop residues from non-leguminous plants also contain N, but in relatively small amounts compared to legumes. Nitrogen exists in crop residues in complex organic forms and the residue must decay – a process that can take several years – before N becomes available for plant use.

Soil organic matter

Soil organic matter is also a major source of N used by crops. Organic matter is primarily composed of rather stable material called humus that has collected over a long period of time.

Easily decomposed portions of organic material disappear relatively quickly, leaving behind residues more resistant to decay. Soils contain approximately 2,000 pounds of N in organic forms for each percent of organic matter. This portion of organic matter decomposes at a rather slow rate and releases about 20 pounds of N per acre per year for each percent of organic matter.

Organic N that’s present in soil organic matter, crop residues and manure is converted to inorganic N through the mineralization process. In this process, bacteria digest organic material and release NH₄⁺-N.

Formation of NH₄⁺-N increases as microbial activity increases. Bacterial growth is directly related to soil temperature and water content. The NH₄⁺-N supplied from fertilizer is the same as the NH₄⁺-N supplied from organic matter.

Ammonium-N has properties of practical importance for N management. Plants can absorb NH₄⁺-N. Also, because ammonium has a positive charge, it’s attracted or held by negatively charged soil and soil organic matter. This means that NH₄⁺-N doesn’t move downward in soils.

Nitrogen in the NH₄⁺-N form that isn’t taken up by plants is subject to other changes in the soil system. Nitrification is the conversion of NH₄⁺-N to NO₃^--N.

Nitrification is a biological process. It rapidly proceeds in warm, moist, well-aerated soils, and slows at soil temperatures below 50 degrees Fahrenheit.

Nitrate-N is a negatively charged ion and isn’t attracted to soil particles or soil organic matter like NH₄⁺-N. Nitrate-N is water-soluble and can move below the crop rooting zone under certain conditions.

In denitrification, bacteria convert NO₃^--N to N gases that are lost to the atmosphere. Denitrifying bacteria use NO₃^--N instead of oxygen in the metabolic processes. The process takes place in waterlogged soil and with ample organic matter to provide energy for bacteria.

For these reasons, denitrification is generally limited to topsoil. Denitrification can rapidly proceed when soils are warm and become saturated for two or three days.

Immobilization, or the tie up of soil N, can temporarily reduce the amount of plant-available N. Bacteria that decompose high-carbon, low-N residues, such as corn stalks or small grain straw, need more N to digest the material than is present in the residue.

Immobilization occurs when the growing microbes use NO₃^--N and/or NH₄⁺-N present in the soil to build proteins. The actively growing bacteria that immobilize some soil N also break down soil organic matter to release available N during the growing season.

There’s often a net gain of N during the growing season because the additional N in the residue will be the net gain after immobilization-mineralization processes.

Unlike the previously described biological transformations, loss of nitrate by leaching is a physical event.

Leaching is the loss of soluble NO₃^--N as it moves with soil water, generally excess water, below the root zone. Nitrate-N that moves below the root zone has the potential to enter groundwater or surface water through tile drainage systems.

Coarse-textured soils have a lower water-holding capacity and, therefore, more potential to lose nitrate from leaching compared to fine-textured soils. Some sandy soils, for instance, may retain only a half inch of water per foot of soil while some silt loam or clay loam soils may retain up to 2 inches of water per foot.

Nitrate-N can be leached from any soil if rainfall or irrigation moves water through the root zone.

Significant losses from some surface-applied N sources can occur through the process of volatilization. In this process, N is lost as ammonia (NH₃) gas.

Manure and fertilizer products containing urea can cause nitrogen to be lost this way. Ammonia is an intermediate form of N during the process that transforms urea to NH₄⁺-N. Incorporating these N sources will virtually eliminate volatilization losses.

Nitrogen loss from volatilization is greater when:

• Soil pH is higher than 7.3.
• Air temperature is high.
• The soil surface is moist.
• There’s a lot of residue on the soil.

Nitrogen can be lost from agricultural lands through soil erosion and runoff. Losses through these events normally don’t account for a large portion of the soil N budget, but should be considered for surface water quality issues.

Incorporating or injecting manure and fertilizer can help protect against N loss through erosion or runoff. Where soils are highly erodible, conservation tillage can reduce soil erosion and runoff, resulting in less surface loss of N.