The formula
nutrient = area × rate × the analysis on the bag
Where it comes from
The three big numbers on a bag — 15-5-10, say — are the percentages of nitrogen, phosphorus and potassium, in that order. A 15-5-10 bag is 15 % nitrogen, and the rest up to 100 is other elements and carrier. That is why comparing bags by weight tells you nothing: what you are buying is units of nutrient, and an expensive bag with a high analysis can work out cheaper per kilo of nitrogen than a cheap weak one.
How to work it out by hand
- Multiply the area by the rate in grams per m²
- Divide by a thousand: those are the kilos of product
- Apply each nutrient percentage to see what you actually put down
- Divide by the area for the figure per square metre
What is worth knowing
The three nutrients do different things, which is why the proportions change with the season. Nitrogen drives growth and green colour, and dominates in spring. Phosphorus goes to the roots, which is why starter feeds carry more of it. Potassium hardens the tissue and improves resistance to cold and drought, so autumn feeds raise it. Too much nitrogen in high summer makes for growth of the wrong kind: soft leaf, more watering and more fungal disease. And it is worth watering after feeding, because the granule has to dissolve to reach the root and because, left dry on the leaf, it can scorch it.