Nitrate leaching in No-Till

Nice to hear from you Noah!  
	Increased nitrate leaching in no-till is a widely-observed result of
increased infiltration.  The rain water has to go in some combination of
three paths: leaching (sub-surface runoff to groundwater), surface runoff,
or evapotranspiration.  Put in this perspective it seems that the answer to
the problem is clearly not to use tillage and increase runoff (a more direct
path to water pollution and one that does not provide water to crops) but to
use cropping systems and cover crops that increase evapotranspiration (and
nitrate use) and provide active root systems for N uptake during periods of
high soil nitrate.  No-till without a winter cover crop is not good no-till
for this and other reasons.  
	We  have observed higher nitrate in groundwater under hairy vetch than
under fertilizer N plots in some instances, because of poorer control over
the timing of N release from the vetch.  N left over in Fall from either
source is susceptible to leaching...or to being intercepted by a good cover
crop root system. Earthworm channels are likely to conduct nitrate to depth
only when the nitrate is soluble on the soil surface at the time of a heavy
rain.  This situation may occur a week or so after killing a no-till vetch
cover, or between a broadcast N fertilizer application and the first heavy
rain (a light rain would dissolve the N and move it into the upper few cm of
soil, preventing subsequent saturated flow down worm channels).  Delayed
application of N (say to lay-by in corn) should pretty much eliminate the
problem since the corn roots would soon catch the N in worm burrows.
Controlling cover rate of N release by using mixtures of legumes and grasses
with various c/n ratios and decomposition rates is another possible
strategy. Noah, I know you've researched this. What have you found?

Ray R. Weil
Professor of Soil Science
Dept. of Natural Resource Sciences & LA
1103 H.J. Patterson Hall
University of Maryland
College Park, MD 20742

telephone: 301 405 1314
FAX:            301 314 9041
e-mail:  rw17@umail.umd.edu