Discussion
If LR is Leaching requirement, ECw is salinity of irrigation water (ds/m) and ECe is the average soil salinity tolerated by specific crop (ds/m), then-
Answer: C
The leaching requirement is \(LR = EC_{w}/(5\,EC_{e} - EC_{w})\).
For surface irrigation, the FAO (Rhoades) relation gives the fraction of applied water that must drain below the root zone to keep salts at a level the crop tolerates:
\[ LR = \frac{EC_{w}}{5\,EC_{e} - EC_{w}} \]
For example, with \(EC_{w} = 1.2\) dS/m and a crop tolerating \(EC_{e} = 2.0\) dS/m, \(LR = 1.2/(10 - 1.2) \approx 0.14\), so about 14 per cent extra water is needed. The other options rearrange the same terms wrongly; \(5\,EC_{e} - EC_{w}\) alone is not even a fraction.
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