Pollution of Water Resources from Agricultural Fields and Its Control - Practices of Irrigation and On-farm Water Management - page 265

7.7 Control of Solute Leaching from Agricultural
and Other Sources
The mass of solute leaching is directly related to the percolation volume and solute
concentration of the leachate. Thus, both water and nitrogen fertilizer (nitrogen)
management is important in controlling nitrate leaching.
The application of the following guiding principles minimizes nitrate leaching:
(1) Nitrogen use in excess of crop requirement results in disproportionate leaching
(2) Nitrogen application when temperature is not high enough to support rapid plant
growth is at risk from leaching
(3) Irrigation should be used to enhance nitrogen recovery by crops, and
(4) Accumulation of nitrate in soil by mineralization after harvest should be
minimized
7.7.1 Irrigation Management
Poor irrigation management aggravates the pollution effects of fertilizers. Over
irrigation should be avoided. Traditional irrigation applications can be altered to
enhance nutrient and water uptake while minimizing leaching below the root zone.
Increasing irrigation efficiency could reasonably be accomplished by changing
from a furrow or basin to a drip or sprinkler system which has potentially high
irrigation efficiency. Nitrogen leaching is less for the alternate-furrow irrigation.
Contamination of groundwater with nitrate is attributed to deep percolation of
water containing the chemical. Thus proper irrigation design, scheduling irrigation
according to available soil-water depletion (implementing zero runoff irrigation),
and improving efficiency of water application and irrigation uniformity can reduce
deep percolation, resulting in reduced nitrate losses to a certain extent. A combina-
tion of sprinkler irrigation and N fertigation significantly reduces N leaching with
only minor reductions in crop yield. Matching irrigation amount to crop ET also
will reduce N loading to the groundwater.
7.7.2 Nitrogen Management
Nitrogen (N) management practices for reduction of nitrate losses include the
following:
(i) Correct nitrogen application (applying fertilizer based on plant species need
to meet yield goals) : Proper N application for realistic yield goals is probably
the best method for controlling nitrate leaching
(ii) Split application of N (reduce the availability and thus loss) : Applying small
amount of N fertilizer early in the growing season, use tissue analysis to
schedule additional fertilizer N as the season progresses.