Soil
Senior 2 · Agriculture
Soil
Lesson 5 of 5
22 min
Let's begin
Under Rwanda's Crop Intensification Programme, farmers in consolidated maize, wheat, bean and potato fields have been supplied with improved seed and mineral fertilisers such as urea, DAP and NPK 17-17-17. Used correctly, a few bags of fertiliser can double a harvest. Used wrongly, the same bags waste money, burn seedlings or wash into streams.
To use fertiliser well you must be able to read a bag, choose the right product and work out how much to apply.
An inorganic (mineral or artificial) fertiliser is a manufactured or mined material that supplies plant nutrients in a concentrated, mostly water-soluble form. Unlike manure, a small mass carries a lot of nutrient, and the nutrient is available to plants almost at once.
Straight fertilisers supply one main nutrient:
| Nutrient | Fertiliser | Typical content |
|---|---|---|
| Nitrogen | Urea, CO(NH₂)₂ | 46% N |
| Nitrogen | Calcium ammonium nitrate (CAN) | about 26% N |
| Nitrogen | Ammonium sulphate (SA) | 21% N, plus sulphur |
| Phosphorus | Single superphosphate (SSP) | about 18% P₂O₅ |
| Phosphorus | Triple superphosphate (TSP) | about 46% P₂O₅ |
| Potassium | Muriate of potash (MOP), KCl | 60% K₂O |
Compound (multinutrient) fertilisers supply two or more nutrients, for example DAP (diammonium phosphate, 18-46-0) and NPK 17-17-17.
The three numbers on a bag are always in the order N – P₂O₅ – K₂O, as percentages by mass. By convention phosphorus is reported as P₂O₅ and potassium as K₂O.
Worked example 1
How much of each nutrient is in a 50 kg bag of NPK 17-17-17?
N = 17% × 50 kg = 0.17 × 50 = 8.5 kg; P₂O₅ = 8.5 kg; K₂O = 8.5 kg. The remaining 24.5 kg is other elements in the compounds and filler.
Worked example 2
A recommendation says maize needs 60 kg N per hectare. Mutesi applies DAP at planting at 100 kg/ha. How much urea must she top-dress to make up the nitrogen?
N from DAP = 0.18 × 100 = 18 kg N.
Still needed = 60 − 18 = 42 kg N.
Urea needed = 42 ÷ 0.46 ≈ 91 kg of urea per hectare.
(If she used CAN instead: 42 ÷ 0.26 ≈ 162 kg per hectare. Lower-concentration fertiliser means more bags to carry.)
Worked example 3
Her field is 0.2 ha. How much DAP does she need at 100 kg/ha?
0.2 ha × 100 kg/ha = 20 kg.
| Advantages | Disadvantages |
|---|---|
| High nutrient content, so less to carry | Costly to buy |
| Quick action | Adds no organic matter; overuse harms soil life |
| Exact amounts can be applied | Nitrogen fertilisers acidify soil over time |
| Clean and easy to handle | Excess washes into lakes and rivers, causing algal blooms |
Tip
Memory hook: the label order is "N-P-K, always that way", and the first number is always nitrogen.
Exam technique: in fertiliser calculations, write the formula first: mass of fertiliser = nutrient needed ÷ (percentage ÷ 100). Show units at every step and round only at the end.
Distinguish between a straight and a compound fertiliser, with one example of each.
Solution: A straight fertiliser supplies one main nutrient (urea: N). A compound fertiliser supplies two or more (DAP: N and P; NPK 17-17-17: N, P and K).
How many kilograms of nitrogen are in a 25 kg bag of urea?
Solution: 0.46 × 25 = 11.5 kg N.
A crop needs 30 kg N per hectare from urea. How much urea is needed for a 0.1 ha plot?
Solution: Urea per hectare = 30 ÷ 0.46 ≈ 65.2 kg. For 0.1 ha: 0.1 × 65.2 ≈ 6.5 kg of urea.
Why is DAP applied at planting while urea is usually top-dressed later?
Solution: DAP supplies phosphorus, which moves little in the soil and is needed early for root growth, so it must be placed near the seed at planting. Urea supplies nitrogen, which leaches easily, so it is given when the growing crop can use it quickly.
Give two dangers of applying too much fertiliser.
Solution: Any two: scorching of seeds or seedlings; wasted money; soil acidification; excess nitrate or phosphate washing into water, causing algal blooms.
Summary
Sources and credits
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
