Soil & Irrigation — PPSC previous year questions

Moong is a leguminous pulse; Rhizobium bacteria in its root nodules fix atmospheric nitrogen, so rotating it with cereals such as wheat restores the nitrogen content of the soil. Wheat, carrots and tea all draw nitrogen from the soil without replacing it.

Blue-green algae are cyanobacteria such as Anabaena, Nostoc and Aulosira. They fix atmospheric nitrogen in the waterlogged conditions of paddy fields, adding 20–30 kg N per hectare. The Azolla–Anabaena pair is used the same way. For wheat, maize and gram the usual biofertilisers are Azotobacter, Azospirillum and Rhizobium.

All three statements are correct. Chemical fertilisers are water-soluble, so their nutrients (N, P, K) are available to plants almost immediately, which is why they gave quick yield gains during the Green Revolution. Their continuous use, however, acidifies and salinises soil and kills beneficial bacteria and other micro-organisms, reducing soil fertility. As soils degrade, farmers need ever larger doses, so the cost of production rises in the long run.

Watershed development works on the ridge-to-valley principle. It uses contour bunding (earthen ridges built along a slope to hold water), check dams, trenches and afforestation. So it prevents soil runoff and erosion. It harvests rainwater and recharges groundwater. It also regenerates natural vegetation on treated catchments. That covers statements 1, 3 and 4. Statement 2 is wrong. Interlinking perennial and seasonal rivers belongs to the National River Linking Project. It has nothing to do with watershed treatment of a micro-catchment.

Soil structure is the arrangement of individual soil particles into aggregates (peds) such as granular, blocky, platy or prismatic units. Soil texture, by contrast, refers to the relative proportions of sand, silt and clay, while porosity and consistency are properties that depend on structure.

Nitrogen (N), phosphorus (P) and potassium (K) are the three primary macronutrients that plants need in the largest amounts; NPK fertilisers such as urea, DAP and muriate of potash supply them. Carbon, hydrogen and oxygen are taken from air and water, and sulphur is only a secondary nutrient.