Soil Science and Soil Fertility: Reading a Soil Test and Converting Fertilizer Doses
This domain asks you to interpret a soil test report, identify the deficiency and convert a recommendation into bags of urea, DAP or MOP.
Physical and chemical properties
- Texture is the proportion of sand, silt and clay and governs water holding, drainage and workability.
- Soil pH below 6.5 is acidic, 6.5-7.5 neutral and above 8.5 alkaline. EC in dS/m measures salinity, with 4 dS/m as the usual saline threshold. ESP above 15% indicates a sodic or alkali soil.
- Organic carbon is the index of fertility; Rajasthan's desert soils are mostly low, below 0.5%.
Soils of Rajasthan
- Arid and desert soils (Entisols) in Jaisalmer, Bikaner, Barmer and Jodhpur.
- Alluvial soils in the IGNP command of Ganganagar and Hanumangarh.
- Grey-brown and red-black soils in the south-east, including the Kota, Jhalawar and Udaipur belt.
- Saline-sodic reh and usar patches in Ganganagar, Alwar, Jaipur and Jodhpur; gypsum is the standard amendment for sodic soils.
Essential nutrients
- Seventeen elements are essential: macro N, P, K; secondary Ca, Mg, S; micro Fe, Mn, Zn, Cu, B, Mo, Cl and Ni.
- Zinc deficiency is widespread in bajra, wheat and citrus in Rajasthan, showing as white bud and interveinal chlorosis.
Worked example
Wheat needs 120 kg N and 60 kg P2O5 per hectare. Urea at 46% N gives 120 divided by 0.46 = 261 kg per hectare. DAP (18-46-0) for 60 kg P2O5 = 60 divided by 0.46 = 130 kg, which also supplies 130 multiplied by 0.18, about 23.5 kg N. Balance N through urea = (120 minus 23.5) divided by 0.46, about 210 kg per hectare.
Traps
- Forgetting that grades are N-P2O5-K2O, not elemental P and K; multiply P2O5 by 0.436 to get elemental P.
- Confusing saline soil (high EC, white crust) with sodic soil (high ESP, black crust).
- Top-dressing urea on dry soil with no moisture, wasting nitrogen through volatilisation.