Agriculture Reference
In-Depth Information
Considerations in Leaching Method
(a) In situations where a grower has multiple water sources of varying quality,
consider planned leaching events at key salinity stress periods for a given crop.
Most crops are highly sensitive to salinity stress in the germination and
seedling stages. Once the crop grows past these stages, it can often tolerate
and grow well in higher salinity conditions. Planned periodic leaching events
might include a post-harvest irrigation to push salts below the root zone to pre-
pare the soil (especially the seedbed/surface zone) for the following spring. Fall
is the best time for a large, planned leaching event because nutrients have been
drawn down. However, since each case is site-specific, examine the condition
of the soil, groundwater, drainage, and irrigation system for a given field before
developing a sound leaching plan.
(b) Apply the leaching fraction to coincide with periods of low soil N and residual
pesticide.
(c) Surface irrigators should compare leaching requirement values to measurements
of irrigation efficiency to determine if additional irrigation is needed. Adding
more water to satisfy a leaching requirement reduces irrigation efficiency and
may result in the loss of nutrients or pesticides and further dissolution of salts
from the soil profile.
Other Aspects/Options in Leaching Calculation
There are several methods available for estimating leaching requirement (LR)
based on different perspectives on how to estimate the average root zone salinity.
Differences among methods can be significant, particularly if the root zone salin-
ity is weighted for the amount of water uptake as Rhoades and Merrill ( 1976 )
proposed for high frequency irrigation. Leaching requirement obtained using this
approach are considerably smaller than for the other methods, indicating that
increasing irrigation frequency should be beneficial when irrigating with saline
water.
According to Rhoades and Merrill ( 1976 ) and Ayers and Westcot ( 1985 ) , the LR
is based on a water uptake distribution of 40:30:20:10% for the first through fourth
quarters of the root zone. LRs obtained using equation of Rhoades tends to be low
for crops with low threshold salinities. However, the equation is quite useful for
quick estimates of LR.
Different Perspectives on Calculating LF
Case 1: No drainage limitation
The depth of infiltrated water can be expressed as the product of average
infiltration rate ( I f , cm/day), and irrigation time ( t i ), as
D i =
I f ×
t i
(8.13)
 
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