Soil Science and Fertility: Nutrients, Fertilizer Grades and Problem Soils
Soil Science and Fertility covers soil physical behaviour, the seventeen essential nutrients, fertilizer grade calculations, and reclamation of acid and sodic soils.
Soil properties that drive management
- Texture controls water holding and aeration. Clay holds more water and nutrients but drains slowly; sand drains fast and leaches nutrients.
- Soil pH governs availability. Most nutrients are readiest between pH 6.0 and 7.5. Acid soils are corrected with agricultural lime; sodic soils (pH above 8.5, ESP above 15) are reclaimed with gypsum.
- Residues with a C:N ratio above roughly 30:1, such as cereal straw, temporarily tie up nitrogen. Material near 10:1 decomposes quickly and releases nutrients.
Essential nutrients and grades
Seventeen elements are essential: carbon, hydrogen and oxygen from air and water; nitrogen, phosphorus, potassium, calcium, magnesium and sulphur as macro-nutrients; iron, manganese, zinc, copper, boron, molybdenum, chlorine and nickel in micro amounts. Grades are written as N–P2O5–K2O: urea 46-0-0, DAP 18-46-0, single superphosphate 0-16-0 with 11% sulphur, muriate of potash 0-0-60.
Worked example
A soil test recommends 60 kg N and 30 kg P2O5 per hectare. If all the phosphate is supplied through DAP: DAP needed = 30 ÷ 0.46 = 65.2 kg/ha, which also carries 65.2 × 0.18 = 11.7 kg N. Urea then supplies the balance of 60 − 11.7 = 48.3 kg N, so urea = 48.3 ÷ 0.46 = 105 kg/ha.
Traps
- Stating phosphorus and potassium as the element instead of P2O5 and K2O.
- Forgetting the nitrogen hidden inside DAP.
- Reversing amendments: gypsum for sodic soil, lime for acid soil.