Soil, Growing Media, and Plant Nutrition
The Soil, Growing Media, and Plant Nutrition domain tests how you judge a growing medium, interpret a soil test, and feed plants without creating new problems. Expect questions on texture and structure, compaction, pH and cation exchange capacity, organic matter and topsoil specification, mulches and composts, and fertilizer calculations and timing.
Texture, structure and the root environment
- Texture is the proportion of sand, silt and clay, judged by ribbon or jar test and confirmed by laboratory particle-size analysis. Structure is how those particles aggregate, and it is destroyed by compaction and by tilling wet soil.
- A good landscape loam is roughly 50% solids, 25% water and 25% air by volume, with organic matter commonly around 4-6% in quality topsoil.
- Compaction raises bulk density and collapses macropores; roots need oxygen, so never work soil when it is wet.
pH, CEC and nutrient availability
- pH 6.0-7.0 suits most landscape plants. Above pH 7.0 iron and manganese tie up; above about 7.5 phosphorus locks up; below 5.5 micronutrients leach and aluminium can become toxic.
- Cation exchange capacity is the medium's ability to hold Ca, Mg, K and ammonium; clay and organic matter raise it, sand lowers it, so sandy soils need smaller, more frequent feeds.
- Always test before fertilizing; a lab report gives extractable ppm plus a recommendation for the crop.
Fertilizers
- N-P-K on the label is percent nitrogen, available P2O5 and soluble K2O by weight, not elemental P and K.
- Slow-release coated and organic products feed steadily and burn less; quick-release products act fast and leach or burn if misapplied. Follow the label rate per 100 m² and water in.
Worked example
A soil test recommends 2 kg of actual nitrogen per 100 m². With a 20-5-10 product: product needed = 2 ÷ 0.20 = 10 kg per 100 m². That same application supplies 10 × 0.05 = 0.5 kg P2O5 and 10 × 0.10 = 1.0 kg K2O, which is why you check all three numbers before buying.
Traps that cost marks
Treating P2O5 as elemental phosphorus; fertilizing to cure a drainage or pH problem; over-amending a planting hole; using fresh manure or uncomposted material.