Introduction to Agricultural Chemistry and Soil Fertility
Scope of agricultural chemistry
Agricultural chemistry applies chemical principles to soils, plants, fertilizers, pesticides and the environment. The Intermediate syllabus tests whether you can connect chemistry (ions, pH, solubility, oxidation–reduction) with field decisions such as how much urea to apply or why a soil is unproductive.
Soil fertility is not soil productivity
Soil fertility is the inherent capacity of a soil to supply all essential nutrients in available form and in balanced proportion. Productivity is the actual yield, which also depends on climate, water, variety, seed rate, pests and management. A fertile soil can be unproductive if rainfall fails; a less fertile soil can out-yield it under better management.
- Liebig's law of the minimum: growth is governed by the nutrient in shortest supply relative to need; a surplus of other nutrients cannot compensate.
- Arnon and Stout (1939) essentiality criteria: the element is indispensable for completing the life cycle, its function is specific and not replaceable by another element, and it acts directly in plant metabolism.
- Fertility is influenced by texture, organic matter, pH, CEC, moisture, temperature, erosion and cropping intensity.
Worked example
A trial on wheat gives: control 2.0 t/ha; +N 3.2; +P 2.1; +K 2.0; +NP 3.4; +NPK 3.45. Nitrogen alone raises yield by 1.2 t, phosphorus by 0.1 t, potassium by 0. Adding P to N adds 0.2 t; K adds almost nothing. Reasoning: N is the primary limiting nutrient, P secondary, K not limiting, so fertilizer money should go to N first, then P.
Traps that cost marks
Treating fertility and productivity as synonyms; assuming a "balanced" fertilizer means equal N-P-K; ignoring the law of the minimum; assuming high organic matter always means high available nutrients.