Soil Productivity · Agriculture

Soil fertility and productivity

Soil fertility is the ability of a soil to supply nutrients to crops. Soil productivity is the ability to produce crop yields and depends on fertility, water, depth, aeration, management and the climate. Plants need 16 essential elements. The macronutrients needed in large amounts are nitrogen (N), phosphorus (P), potassium (K), calcium, magnesium and sulphur. Micronutrients, needed in small amounts, are iron, zinc, copper, boron, manganese and molybdenum.

Nutrient	Use in the plant	Deficiency symptoms
Nitrogen	Leaf growth, protein, green colour	Pale yellow older leaves, stunting
Phosphorus	Roots, flowering, seed	Purple leaves, poor roots
Potassium	Water balance, disease resistance, fruit quality	Yellow and scorched leaf edges
Calcium	Cell walls	Poor root tips, blossom-end rot
Magnesium	Part of chlorophyll	Yellowing between veins

Maintaining fertility

Method	How it helps
Fertilisers	Supply nutrients quickly (e.g. urea, DAP, NPK)
Manure and compost	Supply nutrients slowly and add humus
Green manure	A legume is ploughed in, adds N and organic matter
Crop rotation	Different crops use different nutrients; legumes add nitrogen
Liming	Corrects acidity
Mulching and cover crops	Prevent erosion and add humus
Fallow and agroforestry	Let the soil recover

Fertiliser calculations

A bag of fertiliser labelled NPK 20-10-10 contains 20% N, 10% P₂O₅ and 10% K₂O by mass.

First find how much of the nutrient is in each bag.

Worked exampleHow many bags?

A crop needs 60 kg of N per hectare. How many 50 kg bags of urea (46% N) are needed?

Nitrogen per bag.
Number of bags.

Answer: About 2.6 bags per hectare.

Yield rises with nitrogen but with diminishing returns.

Try itFertiliser rate

Choose the nutrient needed, the percentage of the nutrient in the fertiliser and the size of the bag.

Check yourself Which nutrient makes leaves green?

Check yourself A legume ploughed into the soil as green manure adds mainly

Check yourself Purple leaves with poor roots suggest a shortage of

Key terms

Soil fertility: The ability of soil to supply nutrients.
Macronutrient: A nutrient needed in large amounts.
Green manure: A crop ploughed in to improve the soil.
Fertiliser: A material added to supply nutrients.

Summary

Productivity depends on nutrients, water, depth, aeration and management.
N, P and K are the main nutrients.
Maintain fertility by manure, rotation, liming, mulching and cover crops.
Bags per hectare = nutrient needed ÷ nutrient per bag.