Synchrony and Contribution of Legume Nitrogen for Grain Production Under Different Tillage Systems
Successful low-input sustainable agriculture (LISA) systems will depend on sufficient quantities
of available nitrogen (N) from legume crops preceding non-legume crops. Ideally, the period of
maximum N mineralization rate should overlap or coincide with the period of maximum rate of
N uptake for the non-legume crop. Synchronization of N mineralization and crop rotations needs
must be quantified to improve the scientific basis for recommendations to farmers.
1) Quantify nitrogen (N) mineralization form forage and grain legumes and subsequent N
availability to non-legume grain crops.
2) Evaluate the influence of tillage on the quantity of N mineralization and availability to grain
sorghum and winter wheat.
3) Describe and quantify the synchrony of soil and legume N mineralization and N uptake by
non-legume grain crops.
4) Evaluate the potential use and N balance of mono- and poly-culture production of perennial
Research was initiated on a Reading silt loam at two sites to determine the contribution and
synchrony of legume N from soybeans, hairy vetch, and red clover for grain sorghum, and from
hairy vetch, sweet clover, and alfalfa for winter wheat, under conventional and no-tillage residue
management. Nitrogen rates of 0, 60, and 120 kg N/ha were applied to sorghum following
legumes. Nitrogen rates of 0, 30, and 60 kg N/ha were applied to wheat in 1991, and 0, 50, and
100 kg N/ha to wheat in 1992 following legumes.
The rotational benefit gained from legumes in these rotations were highest following hairy vetch
(wheat and sorghum), red clover (sorghum), and sweet clover (wheat). Residual rotational
benefit was also greatest from hairy vetch. At site one in 1991 sorghum yields were highest
following continuous sorghum. Due to the advanced maturity of sorghum following hairy vetch
and red clover in 1991, more of the grain filling period of sorghum following these crops was
subjected to drought conditions which reduced yields. Sorghum yields were highest following
hairy vetch and red clover at site two in 1992. Wheat yields in 1991 following hairy vetch were
lower than following the other previous crops because the high N availability following hairy
vetch contributed to extensive powdery mildew infection. Tillage did not significantly influence
grain yield. Nitrogen uptake patterns were similar for both tillage systems and both years.
Generally, N uptake increased with N rates. At the beginning of the growing season, soil nitrate
levels were highest following hairy vetch, sweet clover, and red clover in both years. Increased
soil nitrate levels during much of the maximum sorghum N uptake period in 1992 suggested that
N synchrony was better than in 1991 when soil nitrate decreased after the initial sampling. The
reverse was true for winter wheat.
The critical component of these rotations was legume establishment. Under the proper growing
conditions, hairy vetch, sweet clover and red clover have the ability to fix large quantities of N
and supply most or all the N required by a wheat or sorghum crop. In order to maximize the
efficiency of the rotation, synchronization of N mineralization with wheat and sorghum N uptake
is important, but will vary between years.
The results of these studies showed that sufficient plant available N was mineralized following
hairy vetch and red or sweet clover to meet the N requirements of wheat and grain sorghum.
With average fertilizer N rates of 80 and 100 kg N/ha for wheat and sorghum, respectively, $35
to $45/ha reduced N costs were realized ($0.44/kg N). The costs of established hairy vetch and
clover are greater than the fertilizer costs; however, the added value of feeding the legume
forage to livestock would offset these costs.
Farmer Adoption and Direct Impact:
Four farmers participated in the on-farm component in the studies. These farmers still plant
legumes in rotation with wheat and sorghum, although the legumes are grown for at least three
years to offset establishment costs.