Nutrient Management and Fertilization
Domain 5 tests soil-based nutrient decisions and accurate application. Expect questions on soil tests, N-P-K labelling, release patterns, application rates, timing and nutrient loss to water.
Reading the numbers
Fertilizer grades are percent by weight: N total, P expressed as P₂O₅, K as K₂O. A 20-5-10 product applied at 5 g/m² delivers 1 g N/m². Always calculate from the nitrogen figure when matching a recommended N rate. Get a laboratory soil test before fertilizing - pH, phosphorus, potassium, organic matter, and micronutrients where relevant. pH controls availability: iron and manganese lock up above pH 7, phosphorus above about 7.5, and micronutrients can become toxic at very low pH. Lime raises pH, elemental sulphur lowers it, both slowly. The essential elements include macronutrients N, P, K, Ca, Mg, S plus micronutrients such as Fe, Mn, Zn, B, Cu, Mo, Cl and Ni. Nitrogen is the most mobile and most easily lost through leaching, volatilization and denitrification; split applications reduce loss.
Timing and products
Apply when plants are actively growing and roots can take up the nutrient. Slow- and controlled-release products reduce burn and leaching; soluble products need small, frequent doses or fertigation. Match compost applications to a carbon-to-nitrogen ratio near 24–30:1 to avoid nitrogen tie-up. Keep fertilizer off impervious surfaces and away from water bodies; sweep spills back onto the bed.
Worked example
A recommendation of 5 g N/m² for a 150 m² bed using 16-4-8: 150 × 5 = 750 g N; 750 ÷ 0.16 = 4.7 kg of product, delivering about 188 g P₂O₅ and 375 g K₂O. Split into two applications six weeks apart.
Traps that cost marks
Confusing P₂O₅ with elemental P. Adding phosphorus when the soil test is already high. Fertilizing to compensate for poor drainage or compaction.