The availability of nitrogen, a major nutrient required in large quantities by plants, depends on several factors. This article examines the factors affecting the soil’s ability to supply nitrogen for crop production.
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Symbiotic Nitrogen Fixation
The symbiosis between legumes and bacteria of the genera Rhizobium and Bradyrhizobium provides a biological source of fixed nitrogen in agricultural soils. These bacteria infect root hairs and cortical cells, inducing the formation of root nodules, which serve as the primary sites for nitrogen fixation.
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Microbial and Plant C:N Ratio
Soil microbes require a balanced nutrient supply to build cells and extract energy. Carbon is needed to construct essential organic compounds and provide energy for life processes, while nitrogen is essential for synthesizing nitrogen-containing cellular components, such as amino acids, enzymes, and DNA.
On average, soil microbes incorporate approximately 8 parts of carbon for every 1 part of nitrogen (a C:N ratio of 8:1). However, only one-third of the carbon metabolized by microbes is incorporated into their cells; the excess is respired and lost as CO2.
Consequently, microbes require about 24 parts of carbon for every 1 part of nitrogen assimilated (a C:N ratio of 24:1). When organic material added to the soil has a C:N ratio exceeding 25:1, microbes scavenge nitrogen from the soil solution to meet their needs, depleting soluble nitrogen and potentially causing deficiency.
Additionally, the decomposition of organic materials can be delayed if insufficient nitrogen is present in the material or available in the soil solution to support microbial growth.
Nitrogen availability in the soil is determined by the quality of organic material added and microbial activities. As a critical nutrient, nitrogen plays a major role in plant nutrition, and understanding its availability is essential for optimizing crop production.
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