|
John Lloyd recently completed his Ph.D. degree
in entomology at The Ohio State University,
Ohio Agricultural Research and Development Center,
in Wooster, and is currently an assistant professor
in the Department of Plant, Soil, and
Entomological Sciences at the University of Idaho in
Moscow, Idaho;
Dan Herms, Ben Stinner, Harry Hoitink, |
The amount of nitrogen available for plants is determined by the net balance between the rate of nitrogen mineralized from decomposing organic matter and the rate that nitrogen is immobilized by growth of soil microbes. Microbes are considered to be stronger competitors than plants for nitrogen (Kaye and Hart, 1997). In soils where nitrogen is limiting, microbes generally out-compete plants for nitrogen, resulting in plant nutrient deficiencies and decreased plant growth. In fertile soils, there may be enough nitrogen to adequately support both microbial and plant growth.
The balance between nitrogen mineralization and immobilization is strongly influenced by the C:N ratio of the decaying organic matter (Facelli and Pickett, 1991; Kaye and Hart, 1997; Mary etal., 1996). Since soil microbes are generally carbon-limited, the addition of organic matter to the soil stimulates microbial growth. Organic matter with a high C:N ratio (greater than 30:1) does not contain enough nitrogen to fully support microbial growth. Therefore, microbes must scavenge additional nitrogen from the soil as they decompose high C:N organic matter, which decreases the amount of nitrogen available to plants.
Addition of nitrogen fertilizer to high C:N mulch (1 to 2 lbs N / 1,000 ft2 is often recommended) can relax nitrogen competition between plants and microbes and stimulate plant growth. Conversely, decomposition of organic matter with a C:N ratio less than 30:1, which contains more nitrogen than required to support microbial growth, increases the availability of nitrogen for plants.