Agricultural Science (AS)·Soil · NSSCAS 2.1b
Managing fertility: fertilisers, N:P:K:S calculations & pH
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A farmer near Grootfontein reads that her maize needs 60 kilograms of nitrogen for every hectare. At the co-op she finds bags of LAN and urea, each printed with a percentage — but not one bag tells her how many kilograms to spread. How does she turn a nutrient recommendation into an actual weight of fertiliser to buy, and how does she make sure the crop can actually take it up? Today we learn the two fertiliser formulas, the factors that lock or free the nutrients, and how to steer the soil pH.
What you'll learn in this lesson
By the end you should be able to (NSSCAS Agricultural Science (AS) 2.1b):
- Explain how organic and inorganic fertilisers improve and maintain soil fertility
- Calculate the amount of fertiliser to apply from a nutrient recommendation using: fertiliser (kg/ha) = kg/ha nutrient ÷ % nutrient × 100
- Calculate the amount of nutrient delivered by a fertiliser using: nutrient (kg/ha) = kg/ha fertiliser × % nutrient ÷ 100
- Work out N:P:K:S requirements using Namibian fertilisers such as LAN (28% N), urea (46% N) and superphosphate
- Evaluate the factors affecting nutrient availability: soil pH, moisture, temperature, organic matter, leaching, nutrient interactions and cation exchange capacity
- Discuss how to raise soil pH by liming
- Discuss how to lower soil pH using sulphur, acidifying fertilisers or organic matter
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