Development of Organic N Availability Functions for a Nitrogen Management Model
To summarize field results on response of corn and other cereal grains to legume N sources and
to incorporate appropriate organic N response functions into a Nitrogen Management Model for
production agriculture in Wisconsin.
The legume N contributions to succeeding cereal crops were evaluated in this study using crop
yield data from a long-term crop sequence experiment (1967-1991) conducted at the University
of Wisconsin agricultural research station. Three cropping systems having a combination of
legume and cereal crops in a five-year cycle were used in this report. The crop species included
alfalfa (M), corn (c), oat (O), and soybeans (S) in the sequences: CSCOM, CCOMM, and
CCCOM. The last crop sequence was modified in 1977 to replace oat with alfalfa (CCCMM).
Fertilizer N was applied to corn before spring tillage as ammonium nitrate at four N rates. The N
rates were 0, 84, 168, and 224 kg N/ha from 1966 to 1976, and 0, 56, 112, and 224 kg N/ha from
1977 to 1990. No N was applied to oats or any crops other than corn during the sequence.
Yield data of the cereals from the three crop sequences were pooled within a year according to
previous legume species and position in the rotation, which produced five groups representing
distinct levels of N availability: 1) M C; 2) MCC; 3) MCCC or MCCO; 4) SC; and 5) SCO.
Nitrogen availability for the first and second year crop succeeding the legume was evaluated
using group #3 as the reference crop, following the assumption that by the third phase of cereal
the legume N effect is nearly zero, and because current Wisconsin N recommendations for corn
were calibrated against similar data.