Pollution of Water Resources from Agricultural Fields and Its Control - Practices of Irrigation and On-farm Water Management - page 245

through the soil profile because advective transport of pollutants may also increase.
It has been found that in irrigated agriculture, the timing, and the method of irriga-
tion and fertilizer application are the key factors affecting the leaching of chemical
fertilizers (Sarkar and Ali, 2010 ) . In addition to fertilizer rate, factors such as tillage
system, cropping system, soil type, and environmental conditions can play important
roles in determining the amount of solute/contaminants leached into groundwater.
The groundwater contamination is more likely where soils are well-drained, and
irrigation is necessary for crop growth and where nitrogen is applied above recom-
mended rates. There is a direct relationship between large NO 3 -N losses, excess
nitrogen inputs and inefficient irrigation management.
The major factors affecting nitrate and/or other solute leaching are as follows:
(a) Excess use of fertilizer : Use of chemical fertilizer in excess of crop require-
ment results in disproportionate leaching. The use of N-fertilizer at rates higher
than the rate of uptake by the plant increases the potential for increased nitrate
leaching. If nitrogen fertilizer is applied when temperature is not high enough
to support rapid plant growth, is at risk of leaching. Applying excess nutrients
fertilizers will directly affect subsurface water quality especially for NO 3 -N,
which is highly mobile.
(b) Inconsistence with irrigation water : If nitrogen fertilizer is applied without con-
sideration of the water regulation, a high concentration of nitrate appears in drain
outlet water. Downward movement of ammonia and nitrate from fertilizers in
rain and irrigation water are observed.
(c) Drier condition & residue of fertilizer at crop harvest : Under dryland culture
or under limited irrigation, more solute/nutrient retains in the soil (Fig. 7.1 ) .
Nitrate leaching strongly depends on the amount of N remaining in the soil at
the harvest of the crop. Nitrogen uptake by plants and nitrogen mineralization
both increase under irrigation and could mitigate the increased nitrate leaching.
(d) Irrigation schedule and fertilizer type : Nitrate leaching could be attributed to
both irrigation schedule and fertilizer type. Slow-releasing fertilizer has low risk
of leaching that of fast releasing fertilizer. The effect of advection is higher than
that of diffusion in the nitrate transfer phenomena when the rate of irrigation is
increased.
(e) Crop root length & density : Absorption of nutrient or solute largely depends on
the extent and length of root zone. If the root length is higher, there is greater
opportunity time to absorb the solute. Inclusion of deep-rooted crops, such as
alfalfa, in the rotation could reduce the amount of residual N capable of leaching
below the root zone.
(f) Crop type & growth rate : The uptake rate of water and solute depends on the
vegetative growth pattern of the crop grown, which is a function of crop type
and growth rate.
(g) Soil type and organic matter content : Decomposition and mineralization of
organic residues and other nitrogenous fertilizers depend on the organic mat-
ter content, microorganisms, and physical and bio-physical properties of the soil
concerned. A decrease in organic matter lower total mineralization. Sandy soil