Fertiliser Calculator
N-P-K are percentages. That is the whole problem.
Work out Fertiliser. N-P-K are percentages. That is the whole problem. Names the misconception directly.
Fertiliser to spread
30 kg
Delivers 6 kg of nitrogen across 200 m²
The N-P-K numbers are percentages by weight of the bag, which is the point everyone misses. A 20-5-10 fertiliser is 20% nitrogen, so delivering 6 kg of nitrogen takes 30 kg of product — not 6. Applying by the nitrogen figure instead of the product figure under-feeds by a factor of five. Nitrogen drives leaf growth, phosphorus roots and establishment, potassium hardiness and disease resistance. Most established lawns need nitrogen and very little phosphorus, which is why high-P lawn feeds are largely wasted and restricted in some jurisdictions for runoff reasons. Water in after applying granular feed unless rain is due. Fertiliser left dry on the leaf scorches, and the burn pattern follows the spreader exactly.
How the Fertiliser Calculator works
How much product to spread to deliver a target nitrogen rate, and what phosphorus and potassium come with it. The N-P-K numbers are percentages by weight, so a 20-5-10 fertiliser needs five times its nitrogen figure in product.
Also known as: how much fertilizer for my lawn · what does 20-5-10 mean · npk to kilograms · nitrogen per square metre lawn
Frequently asked questions
What do the numbers on fertiliser mean?
Percentages by weight of nitrogen, phosphorus and potassium. A 20-5-10 bag is 20% N, 5% P and 10% K — so a 25 kg bag contains 5 kg of actual nitrogen, not 20.
How much fertiliser do I need for my lawn?
Work backwards from the nitrogen rate. For 3 kg of nitrogen per 100 m² using a 20% N product, you need 15 kg of product per 100 m². Applying 3 kg of product instead under-feeds by a factor of five.
How much nitrogen is too much?
Above about 5 kg per 100 m² in a single application risks scorching and runoff. A full season's programme of 15 to 25 kg per 100 m² should be split across three or four applications rather than delivered at once.
Do I need phosphorus in lawn feed?
Rarely. Established lawns need nitrogen and very little phosphorus, and high-P feeds are restricted in some jurisdictions because the excess washes into waterways. Phosphorus matters at establishment and seldom after.
Should I water fertiliser in?
Yes, unless rain is forecast. Granular feed left dry on the leaf scorches, and the burn pattern follows the spreader path exactly — a permanent record of the mistake for the rest of the season.
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