Nitrogen status of corn after alfalfa in 29 Iowa fields

ABSTRACT:

Excessive applications of nitrogen (N) fertilizer can occur when farmers do not give adequate credits for N supplied by legumes. Newly developed soil and tissue tests, were used to evaluate the N status of 29 fields of first-year corn (Zea mays L.) afer alfalfa (Medicago sativa L.) in northeastern Iowa in 1982 Unlike other tests that have been used, these tests have the ability to characterize degrees of N excess. The tests showed that most farmers had applied more N fertilizer than needed to attain maximum yields. The soil test showed that 25 of the 29 fields had nitrate Concentrations greater than optimal concentrations. Seventeen of the 29 fields had at least twice the critical concentration of soil nitrate, and 6 of the 29 fields had at least three times this concentration. The tissue test supported these findings. Most farmers could have increased their profits and decreased the potential for groundwater contamination if they had given more appropriate credits for N supplied by the alfalfa. Soil and tissue tests that can characterize degrees of N excess during corn production can be used to demonstrate the economic and environmental benejts that can be obtained by giving these credits.