Nitrogen Leaching from Corn-Cover Crop Systems

 

Application of nitrogen (N) fertilizers to already nitrogen-rich corn fields could lead to nitrate pollution of groundwater.  Planting cover crops such as rye (Secale cereale) can improve overall N management on corn fields and also prevent soil erosion. 

Winter rye is the predominant cover crop species seeded in the northeastern U.S. because it is inexpensive, can survive the harsh winters, and protects the soil from wind and water erosion.  A winter cover crop reduces the potential for nitrate leaching by extracting water and absorbing nitrates that remain in the soil after the main cash crop is harvested.

Field experiments were initiated at the University of Massachusetts Research Farm to determine the impact of rye and hairy vetch (a legume) cover crops, and their mixtures, on nitrate leaching, when rotated with sweet corn.  Cover crop-sweet corn rotations have been maintained on the same plots with the same treatments in this experiment since the 1990-91 season.  The soil is a low organic matter fine sandy loam (coarse-silty, mixed, nonacid, mesic Typic Udifluvent). 

The experiment is a comparison of N contribution from hairy vetch to contributions from rye and no cover, and the assessment of nitrate-N leaching with varying N fertilizer rates on sweet corn.  Cover crop treatments were check (no cover crop), rye (90 lb/ac), and hairy vetch + rye (40 + 56 lb/ac) in combination with four nitrogen rates applied to the sweet corn crop - 0, 60, 120, and 180 lb/ac.  Treatment were replicated in 4 randomized blocks.

Four years of research have demonstrated that significant reductions in nitrogen fertilizer use can be achieved with the use of hairy vetch cover crops in vegetable crop rotations.  For example hairy vetch cover crops supplied sufficient N to sweet corn so that N fertilizer could be reduced below 60 lb/ac. or even eliminated. 

Fig. 1.             Mean nitrate-N content of water samples in summer beneath sweet corn in 1994 following spring incorporation of cover crops, in fall after seeding cover crops following the sweet corn crop, and in spring (1995) before incorporating cover crops.

Research has shown that vetch should be seeded no later than Sept. 15 to ensure adequate establishment before winter.  Seeding this cover crop after this date could be a waste of expensive seed.  For this reason it is unlikely to fit most dairy farm corn silage situations.  Furthermore, soil incorporation of hairy vetch in spring should be delayed until mid to late May i.e. long enough to allow adequate time for vetch growth and N accumulation.  Most field corn fields should be planted by early May, thus N contribution might be less than optimal.

Soil nitrogen status has been assessed by sampling plots at varying soil depths and times during the growing season.  Porous suction cup water samplers were installed at depths of 2 and 4 feet for collection of water samples for nitrate-N analysis.  Results of the analyses for the varying seasons are presented in Figure 1.  Data illustrates what can occur on high N status soils analogous to heavily manure corn fields on dairy farms.

Nitrate-N in soil water was low in all treatments for spring and summer water samples.  However in the fall, nitrate-N in soil water rose above the clean water standard (10 ppm) in the hairy vetch and no cover crop systems when fertilizer added to the sweet corn exceeded 60 lb/ac. Thus, for vegetable farmers to take advantage of hairy vetch as an alternative N-fixing cover crop, or for dairy farmers to benefit from the N supplying capacity of manure they must be prepared to cut back on or eliminate the use of applied synthetic N fertilizer.  This provides both economic and environmental benefits.

 

Stephen J. Herbert

Dept. of Plant and Soil Sciences

University of Massachusetts Amherst

Crops, Dairy, Livestock News. Vol. 1:2, Summer 1996




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