Soil, Growing Media and Plant Nutrition
Soil is a living system of mineral particles, organic matter, air, water and organisms. Candidates must understand texture (sand, silt, clay) and structure (aggregates, pores), because these control drainage, aeration and root growth. Learn how to assess soil: hand texturing, ribbon test, jar sedimentation, pH testing, and EC (salinity) readings. Know cation exchange capacity (CEC) and how clay and organic matter hold nutrients against leaching. Understand pH effects on nutrient availability - iron and manganese lock up at high pH, phosphorus at low pH - and the difference between soil pH and lime requirements.
Nutrition: macro- and micronutrients, mobile versus immobile deficiencies, and the difference between deficiency and toxicity. Learn symptom patterns: nitrogen (older leaves pale), iron (young leaves interveinal yellow), magnesium (older leaves interveinal), calcium (new growth distorted). Growing media differ from soil: potting mixes rely on controlled release fertiliser, wetting agents and pH buffers, and they are free-draining and light.
Traps: assuming more fertiliser fixes every yellow leaf; confusing pH with nutrient content; and treating potting mix like garden soil. Always check pH and EC before adding nutrients.
Worked rule of thumb: for a yellowing plant, check the youngest versus oldest leaves first. If young leaves are affected, suspect immobile nutrients (iron, calcium); if old leaves, suspect mobile nutrients (nitrogen, magnesium). Confirm with a pH test before acting.