Water-Soluble Fertilizer Calculator

One gram of a 20 percent feed in one litre is exactly 200 ppm nitrogen.

Fertiliser to weigh out7.5 ginto 10 L for 150 ppm nitrogen

Check: that gives150 ppmthe calculation run backwards

Per litre0.75 g

Rate as a dilution1 : 1,333by weight of product to weight of water

At 100 ppm instead5 ga typical constant-feed strength

Weekly feed at 4 times strength30 g

Salts build up in a container however carefully the feed is mixed, because water leaves as vapour and the fertiliser stays. Watering to about 15 per cent runoff flushes the excess — without it, a correctly calculated feed still ends in salt damage.

The formula

ppm = grams * 1000 * percent / 100 / litres

Parts per million is milligrams per litre

Liquid feeding is specified in ppm of nitrogen because that is what the plant experiences, and it is independent of how much water you happen to be mixing. One gram of a 20 % nitrogen feed in one litre gives 200 ppm.

Constant feed against periodic

Growers either feed weakly at every watering — typically 100 to 200 ppm nitrogen — or strongly once a week at four or five times that. Constant feed holds the root zone steadier and is easier to get wrong slowly; periodic feeding is easier to get wrong quickly. Either way the total over a month is much the same.

Leaching matters more than the number

Salts accumulate in a container whatever the concentration, because water leaves as vapour and the fertiliser does not. Watering to perhaps 15 % runoff flushes the excess out. Without that, a perfectly calculated feed still ends in salt damage.

Rules of thumb, and the field

Every figure here describes an average season on an average site. Soil type, aspect, drainage, variety and weather each move the answer by more than the arithmetic does, and a grower who has kept records for a few years on one piece of ground knows it better than any formula. Use these to plan an order or size an implement, then trust what the ground has actually done before.