Soil Science and Soil Fertility Management: Acid Soils and Fertilizer Arithmetic
Soil groups of Assam
Alluvial soils of the Brahmaputra and Barak valleys are deep and fertile but commonly acidic; laterite soils of Karbi Anglong and the hill districts are shallow, low in bases and erosion-prone; red soils occupy the southern hill fringes. Heavy rainfall leaches bases, so pH often falls between 4.5 and 6.0, available phosphorus is low because P is fixed by iron and aluminium oxides, and boron and molybdenum deficiencies appear on acid soils. Nitrogen is the first limiting nutrient, followed by phosphorus and potassium.
Fertility rules
- Lime is applied on the soil-test lime requirement, not on pH alone; over-liming induces micronutrient deficiency.
- Grade conversions: urea 46% N, DAP 18% N + 46% P2O5, MOP 60% K2O, SSP 16% P2O5 plus gypsum.
- Split nitrogen into basal, tillering and panicle-initiation doses to cut losses.
- Green manuring with dhaincha before transplanting adds organic matter and nitrogen.
- A recommendation without a soil test is an assumption, not a prescription.
Worked example
Target 40 kg N, 20 kg P2O5, 20 kg K2O per hectare; supply phosphorus through DAP. DAP = 20 ÷ 0.46 = 43.5 kg/ha, which also supplies 43.5 × 0.18 = 7.8 kg N. Balance N = 40 − 7.8 = 32.2 kg → urea = 32.2 ÷ 0.46 = 70 kg/ha. MOP = 20 ÷ 0.60 = 33.3 kg/ha. Final mix: 43.5 kg DAP + 70 kg urea + 33.3 kg MOP per hectare.
Traps that cost marks
- Forgetting the nitrogen credit from DAP and double-dosing N.
- Treating SSP as a nitrogen fertilizer.
- Confusing lime requirement with pH value.
- Assuming all applied potassium remains available in sandy alluvial soil.