Fertilizer Calculator

Every fertiliser recommendation worth following is written in pounds of actual nutrient per 1,000 square feet. Every fertiliser sold is priced and bagged in pounds of product. The two are related by a percentage printed on the bag, and forgetting to apply it is the single most common error in lawn and garden feeding.

A 26-0-3 lawn fertiliser is 26% nitrogen by weight. The standard single application of one pound of actual nitrogen per 1,000 square feet therefore requires 3.85 pounds of that bag. Somebody who spreads one pound of product has applied a quarter of the intended rate and will conclude, reasonably enough, that the fertiliser did not work.

The error runs in the other direction too, and more dangerously. A 21-0-0 ammonium sulphate is 21% nitrogen; spread at the same 3.85 pounds it delivers 0.81 lb of N, but spread at a "one scoop" habit calibrated for a much weaker organic product it can easily go over. Nitrogen burn is a real and immediate consequence, and it looks exactly like drought damage a week later.

The fix is a single division, and once it is done every other question on this page becomes answerable.

N-P-K looks like three comparable quantities and is not. The first number is percentage of nitrogen by weight, which is what it appears to be. The second and third are not.

The middle number is available phosphate, reported as P₂O₅. Actual phosphorus is about 44% of that, so a 10 on the bag is 4.4% phosphorus. The third number is soluble potash, reported as K₂O; actual potassium is about 83% of it. Both conventions are inherited from nineteenth-century oxide analysis and have persisted because relabelling every bag in the world was never going to happen.

The practical consequence is that "10-10-10" is not balanced in the way it looks. It contains 10% nitrogen, 4.4% phosphorus and 8.3% potassium. And most of the bag is neither: a 50 lb bag of 10-10-10 holds 5 lb of N, 5 lb of phosphate and 5 lb of potash, with the remaining 35 lb being carrier, sulphur, iron, lime and whatever else the formulation uses to make granules that flow.

This is also why a bag with big numbers can be better value than a bag with small ones. Urea at 46-0-0 is nearly half nitrogen; an organic 5-3-2 is a twentieth. Per pound of nitrogen delivered, the difference is enormous — which is not an argument for urea, since the organic product is doing several other things at once, but it is the comparison to make consciously rather than by accident.

Turf nitrogen recommendations converge on a narrow range across the extension services, and the range is smaller than most people spread.

Cool-season grasses — fescue, bluegrass, ryegrass — use 2 to 4 pounds of actual nitrogen per 1,000 square feet a year, split into three or four applications. Purdue, Rutgers and UNH all publish that figure. A low-maintenance lawn does perfectly well on 1 to 2 pounds; a heavily used, irrigated, high-expectation lawn justifies 3.5 to 4. Beyond four, additional nitrogen buys mowing rather than lawn.

The per-application ceiling is one pound of actual nitrogen. That limit exists because soluble nitrogen above it burns and because the surplus is the fraction most likely to be washed off before roots take it up. Slow-release formulations — the coated granules in most bagged lawn products — soften this, which is why some bags legitimately recommend more product than the soluble arithmetic would allow.

Timing matters more than most people expect, and it is counterintuitive. For cool-season grass the two most valuable applications of the year are both in autumn. That is when the plant grows roots and stores carbohydrate, and an autumn-fed lawn greens up earlier and more strongly in spring than a spring-fed one does. Heavy spring nitrogen mostly produces top growth at the expense of the roots that will have to survive summer.

Look at bagged lawn fertiliser and the middle number is very often zero. That is not a formulation fashion; it is regulation.

Phosphorus is the limiting nutrient in most fresh water, which means a small amount reaching a lake produces a disproportionate amount of algae. When that algae dies and decomposes it consumes the oxygen, and the result is the fish kills and closed beaches that follow a warm summer in an over-fertilised watershed.

Established lawns almost never need phosphorus. Soil holds it tightly, it does not leach the way nitrogen does, and most residential soils have accumulated a surplus from decades of general-purpose fertiliser. A soil test will usually show phosphorus as high or excessive.

A dozen or so states now restrict phosphorus in lawn fertiliser, with the common exceptions being new seeding — where a starter fertiliser genuinely helps root establishment — and a documented soil test deficiency. The rules differ by state, so check yours before buying a bag with a middle number, and keep the soil test if you do.

Calculating the right quantity is half the job. Distributing it evenly is the half that shows.

Spreader settings printed on a bag are specific to the product tested on that spreader model. Granule size, density and coating all change how fast material flows through a given gate opening, and a setting transferred between products is frequently 30% or more out. Calibrating takes ten minutes: weigh out the product for a measured strip, spread it, weigh what is left.

Apply half the rate in each of two passes at right angles. Two half-rate passes cover the inevitable variation across the spread pattern, and they hide the light and dark stripes that reveal a single pass with a slightly wrong overlap. It also means a mis-set spreader produces a half-rate mistake rather than a double one.

Sweep granules off hard surfaces back onto the grass. Fertiliser on a drive or pavement has no soil to bind to and goes straight into the storm drain with the next rain — it is the single easiest contribution to water quality anyone with a spreader can make, and it takes a broom.

Water it in lightly after application unless the product says otherwise, and never apply to dormant, drought-stressed or frozen ground. Nitrogen applied to a lawn that cannot use it is a cost, a pollutant and, on frozen ground, entirely wasted.