Problem Soils and Reclamation Questions and Answers

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.

Answer: A

The definition is that of the Leaching requirement.

Irrigation water always carries some salt. To stop it building up, part of the applied water must drain below the root zone and carry salts away. The leaching requirement is that fraction, usually estimated as

\[ LR = \frac{EC_{iw}}{EC_{dw}} \]

where \(EC_{iw}\) is the electrical conductivity of the irrigation water and \(EC_{dw}\) that of the drainage water allowed for the crop.

Irrigation potential is the area that can be irrigated, an irrigation cycle is the interval between irrigations, and perennial irrigation is year-round supply. None of them concerns salinity control.

Answer: A

A safe SAR for irrigation water is Less than 10.

The sodium adsorption ratio compares sodium with calcium and magnesium in water:

\[ SAR = \frac{Na^{+}}{\sqrt{(Ca^{2+} + Mg^{2+})/2}} \]

with concentrations in milliequivalents per litre. In the US Salinity Laboratory classification, water with SAR up to 10 is low in sodium hazard (class S1) and can be used on almost all soils. As SAR rises above 10, sodium increasingly disperses clay, destroys soil structure and lowers permeability, so the higher ranges in the other options are progressively more hazardous.

Answer: A

The most effective amendment is Gypsum (calcium sulphate).

Sodic soils have too much exchangeable sodium, which disperses the clay, destroys structure and makes the soil sticky and impermeable. Gypsum dissolves to supply calcium directly; the calcium replaces sodium on the exchange sites, and the released sodium forms sodium sulphate, which is leached out with irrigation water. The soil then flocculates and its structure and infiltration improve. Gypsum is also cheap and widely available.

Elemental sulphur works only indirectly, by oxidising to sulphuric acid that dissolves native lime, and so is slow and needs calcium carbonate in the soil. Lime is hardly soluble at the high pH of sodic soils, so it supplies little calcium. Organic compost helps structure but does not by itself displace sodium.

Answer: D

The answer is All of the above.

Intensive Green Revolution farming in Punjab, Haryana and western Uttar Pradesh has caused each of these problems.

• Loss of soil fertility: continuous rice-wheat cropping and heavy use of urea deplete organic matter and micronutrients such as zinc.
• Alkalinity and salinity: over-irrigation and waterlogging bring salts to the surface.
• Chemical poisoning: heavy use of pesticides and fertilisers contaminates soil and groundwater.

Since all three are real, choosing any single one is incomplete.