Fertiliser Calculator

Reading the label: what N–P–K means

Every fertiliser bag or bottle carries three numbers, such as 10-10-10 or 5-10-5. They are the percentages by weight of nitrogen (N), phosphate (P) and potassium (K) — the three nutrients plants need in the largest amounts. A 10-10-10 product is 10 percent nitrogen, so 10 pounds of it contains 1 pound of actual nitrogen. That is why the calculator asks for the first number only: once you know how much nitrogen you want, the product amount follows from simple division. Nitrogen drives leafy growth, phosphorus supports roots and flowering, and potassium supports overall vigour and disease resistance. A soil test is the only reliable way to know which of the three your soil actually lacks — many garden soils already have plenty of phosphorus, and adding more does nothing for the plants while contributing to runoff. Your local extension service can test your soil cheaply and tell you what to apply.

How much nitrogen do plants actually need?

Most vegetable beds and lawns are maintained with 0.5 to 1 pound of actual nitrogen per 1,000 square feet per application, repeated through the season rather than applied all at once. Heavy feeders such as tomatoes, corn and squash sit at the upper end; light feeders such as herbs, beans and peas (which fix their own nitrogen) need little or none. Houseplants need far less again, and a weak liquid feed every two to four weeks in the growing season is usually enough. The calculator’s default of 1 pound per 1,000 square feet is a common maintenance figure, not a prescription for every situation — the label rate for the specific product always takes precedence, and a soil test takes precedence over both. Over-fertilising is worse than under-fertilising: excess nitrogen produces soft, leafy growth that attracts aphids, delays fruiting, and can burn roots and leaves.

Granular or liquid — which to choose?

Granular fertiliser is spread on the soil and releases nutrients over days or weeks as it dissolves; slow-release granules last longer and are harder to over-apply in one go. Liquid feed acts fast — useful for a quick correction or for container plants that are watered often and leached of nutrients — but it washes out faster and is easier to overdo. Compost and well-rotted manure are the gentlest option of all: they feed slowly, add organic matter, and are very hard to over-apply. Whatever the form, the rules are the same: apply to moist soil, keep product off foliage, water it in, and never fertilise a plant that is dormant, drought-stressed or freshly transplanted. Do not fertilise in the middle of a heatwave, and stop feeding most perennials and shrubs by late summer so new soft growth is not caught by the first frost.

Common mistakes

The most frequent mistake is reading the label rate as the amount of nitrogen rather than the amount of product — a 10-10-10 bag applied at 10 pounds per 1,000 square feet delivers only 1 pound of nitrogen, not 10. The second is feeding on a calendar instead of by need: a plant that is growing well in rich soil does not need more fertiliser just because a month has passed. The third is fertilising dry soil, which concentrates salts around the roots and can burn them — water first, feed second. If your plants look pale or stunted despite feeding, the cause may be pH, drainage or a pest rather than a nutrient shortage; the plant disease checker and pest identifier helper can help narrow it down. For the soil itself, see the compost ratio calculator and the soil volume calculator, and check the planting calendar for what to feed and when.