Soil Science and Plant Nutrition
Soil Science and Plant Nutrition tests soil physical properties, acidity and salinity, the seventeen essential nutrients, and fertilizer calculations. Maharashtra papers commonly use the red laterite soils of Konkan, the black cotton soils of Vidarbha and Marathwada, and the fertilizer schedules of state agricultural universities.
Core soil rules
- Texture (sand, silt, clay) decides water holding, drainage and aeration; loam suits most horticultural crops.
- Cation exchange capacity is high in clay and organic-matter-rich soils, so they hold potassium, calcium and magnesium better.
- Soil pH controls nutrient availability: most crops prefer 6.0-7.0; tea and pineapple tolerate acidity; potato prefers slightly acid soil that reduces scab.
- Saline soils need leaching with drainage; alkali (sodic) soils need gypsum, followed by salt-tolerant crops such as coconut and sugarbeet.
Essential nutrients
Carbon, hydrogen and oxygen come from air and water; the six macronutrients are N, P, K, Ca, Mg and S; the eight micronutrients are Fe, Mn, Zn, Cu, B, Mo, Cl and Ni. Nitrogen drives vegetative growth, phosphorus roots and flowering, potassium fruit quality and disease resistance, calcium cell strength, boron flowering and fruit set. Organic sources, FYM, compost, vermicompost and green manures (sunhemp, dhaincha), improve structure and supply micronutrients.
Worked example. A crop needs 120 kg N and 60 kg P2O5 per hectare. Urea carries 46% N, so urea = 120 / 0.46 = 261 kg/ha. Single superphosphate carries 16% P2O5, so SSP = 60 / 0.16 = 375 kg/ha. Reasoning: divide the nutrient dose by the nutrient fraction in the fertilizer, and always write the unit.
Traps that cost marks
- Confusing P with P2O5, or K with K2O.
- Forgetting to divide by the fertilizer percentage.
- Recommending lime for saline soil instead of gypsum for alkali soil.
- Assuming that more fertilizer always means more yield.