Soil Science and Soil Fertility Management
Soil Science and Soil Fertility Management
This domain tests soil physical, chemical and biological properties, essential plant nutrients, fertilizer grades and soil conservation. Numerical questions usually require converting a nutrient recommendation into fertilizer material or interpreting soil test values.
Core rules
- Soil texture refers to sand, silt and clay proportions; structure refers to aggregation. Clay and organic matter raise cation exchange capacity (CEC), usually expressed in cmol/kg.
- Soil pH controls nutrient availability. Acid soils may need lime; sodic soils need gypsum, not lime. Base saturation is the percentage of CEC occupied by basic cations.
- Fertilizer grades are expressed as N-P2O5-K2O, not elemental P and K. Urea is about 46% N, DAP about 18% N and 46% P2O5, and MOP about 60% K2O.
- Conservation practices include contour bunding, terracing, strip cropping, mulching and vegetative barriers.
Worked example
A recommendation is 120 kg N, 60 kg P2O5 and 40 kg K2O per ha. DAP needed for 60 kg P2O5 = 60/0.46 = 130.4 kg. This also supplies 130.4 x 0.18 = 23.5 kg N. Remaining N = 120 - 23.5 = 96.5 kg; urea = 96.5/0.46 = 209.8 kg. MOP for 40 kg K2O = 40/0.60 = 66.7 kg. The reasoning is to credit the N supplied by DAP before calculating urea.
Traps
Do not divide by elemental P or K when the recommendation is in P2O5 or K2O. DAP is not a straight P fertilizer because it carries N. Lime raises pH; gypsum is for sodic soils. CEC is not the same as organic carbon, and a high soil test value does not always mean no response.