Fertiliser calculator
Fertiliser sums start from one question: how many ppm of a nutrient do you want in the irrigation water? 1 ppm is 1 gram in a tonne of water. Divide the target by the fertiliser’s content and you have the grams per tonne. Add the injection ratio and the tank size to see how many kilos go in the stock tank, whether the stock stays below the solubility limit, and what it does to the EC.
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A worked example
A soilless tomato greenhouse in Kumluca. The recipe calls for 150 ppm of calcium (Ca), given as calcium nitrate (15.5-0-0, 26.5% CaO). The fertigation unit runs at 1:200, the stock tank holds 2,000 litres, and one valve zone takes 36 m³/h.
- As the oxide 150 ppm Ca ÷ 0.7147 = 210 ppm CaO
- Per tonne of water 210 ÷ 0.265 = 792 g of calcium nitrate
- Into the stock tank 792 g × 200 ÷ 1,000 = 158 g/L; 317 kg in a 2,000-litre tank. Far below the solubility limit (about 1,200 g/L).
- What else it brings 123 ppm of nitrogen (N); the EC goes up by about 0.95 mS/cm
- Injection 36 m³/h ÷ 200 = 180 L/h; one tank treats 400 tonnes of water, about 11 hours of watering
- Tank calcium nitrate goes in tank A; sulphates and phosphates in tank B
[The example’s numbers await Perge’s check.]
How it is worked out
Fertiliser bags give phosphorus as P₂O₅, potassium as K₂O, calcium as CaO and magnesium as MgO; recipes and water tests mostly give the element. Bring both to the same unit first: P = P₂O₅ × 0.4364, K = K₂O × 0.8301, Ca = CaO × 0.7147, Mg = MgO × 0.6031. Nitrogen is always written as N.
1 ppm is 1 gram in a tonne (1,000 L) of water. So the fertiliser per tonne = target ppm (as the oxide) ÷ the fertiliser’s content. For example, 100 ppm of K₂O from potassium nitrate at 46% K₂O takes 100 ÷ 0.46 = 217 g.
The fertigation unit feeds the stock into the water at a set ratio. At 1:100, 1 litre of stock goes into 100 litres of water, so the stock is 100 times stronger than the irrigation water. Stock strength (g/L) = g per tonne × ratio ÷ 1,000. If that is more than the fertiliser can dissolve, the rest settles on the bottom. On cold nights the limit drops further.
We give the EC rise from the makers’ 1 g/L tables, so it is approximate; the water’s own salts change it, so check with an EC meter. Calcium nitrate never shares a tank with sulphates or phosphates: in a strong solution gypsum and calcium phosphate settle out and block filters and emitters. That is why there are two tanks: calcium nitrate in tank A, sulphates and phosphates in tank B. Potassium nitrate and urea can go in either.
Questions
How do I work out fertiliser ppm?
ppm means grams per tonne of water. Fertiliser per tonne (g) = target ppm ÷ the fertiliser’s nutrient content. For example, 100 ppm of nitrogen from calcium nitrate at 15.5% N takes 100 ÷ 0.155 = 645 g. If the target is written as the element (Ca, K, P, Mg), turn it into the oxide first.
How much fertiliser goes in a tonne of water?
There is no single number: it depends on the crop, the growth stage and what the water already holds. For each fertiliser, the amount = target ppm ÷ its content. All the fertilisers together take the water’s EC to 1.5–3 mS/cm for most crops; check the end result with an EC meter.
Can calcium nitrate and magnesium sulphate go in one tank?
No. In a strong stock, calcium and sulphate settle out as gypsum (calcium sulphate). Put calcium nitrate in tank A and magnesium sulphate in tank B. Once they are diluted in the irrigation water, they cause no trouble together.
What does an injection ratio of 1:100 mean?
The fertigation unit puts 1 litre of stock into 100 litres of water. The higher the ratio (1:200, 1:500), the stronger the stock and the longer a tank lasts, but the closer you get to the fertiliser’s solubility limit.
What do P₂O₅ and K₂O on a fertiliser bag mean?
Fertiliser labels give phosphorus and potassium as oxides. As the element: P = P₂O₅ × 0.4364, K = K₂O × 0.8301. So 0-52-34 MKP holds 22.7% phosphorus and 28.2% potassium.
How much potassium nitrate dissolves in water?
About 316 g in a litre of water at 20 °C, and about 209 g at 10 °C. Keep the stock well below that limit so crystals don’t form when the tank cools on winter nights.