Water Soluble Fertilizer Calculator
Turn a target feed strength in ppm nitrogen into the exact weight of water soluble fertilizer to dissolve, at final strength or in an injector stock tank.
🌱 What is a Water Soluble Fertilizer Calculation?
Water soluble fertilizer is a dry, fully dissolving blend designed to be delivered in irrigation water rather than spread on the ground. Because it is applied as a solution, its strength is quoted not in kilograms per hectare but in parts per million of nitrogen in the finished feed. This calculator does the arithmetic that links those two worlds, converting a target ppm into the exact weight of product to dissolve, either directly into a finished batch or into the concentrated stock tank that a fertilizer injector draws from.
The people who need this calculation are greenhouse growers running constant liquid feed, nursery operators mixing stock tanks for a Dosatron or Anderson injector, hydroponic and fertigation growers preparing reservoirs, and serious home gardeners feeding containers where every drop of nutrient arrives in the water. All of them face the same three linked questions: how much product per litre of finished feed, how much stronger the stock has to be to survive the injector ratio, and how many kilograms that actually means for the tank sitting in front of them. Getting any of the three wrong by a factor of ten is easy and expensive, since an overdose scorches roots while an underdose quietly stunts a whole crop before anyone notices.
Two conventions cause most of the confusion. The first is that fertilizer labels report phosphorus as P2O5 and potassium as K2O rather than as the elements, so a bag reading 20-10-20 delivers only about 44 ppm of actual phosphorus when it is delivering 100 ppm of phosphate. The second is that the ratio between nutrients is fixed by the product, not by the grower. Choosing a target of 200 ppm nitrogen from a 20-10-20 blend automatically commits you to 100 ppm phosphate and 200 ppm potash, and the only way to change that balance is to change fertilizer or add a second one.
This calculator reports all of it together: the final-strength rate in grams per litre, the imperial equivalent in ounces per 100 US gallons, the stock concentration, the kilograms for your tank, the phosphate and potash that ride along in both oxide and elemental form, and a warning if the concentrate you have specified is too strong to actually dissolve. A rate table from 50 to 300 ppm and a chart of rate against nitrogen analysis show how the numbers shift if you change target or product. For granular fertilizer spread across a field or lawn rather than dissolved in water, use the fertilizer calculator instead.
📐 Formula
📖 How to Use This Calculator
Steps
💡 Example Calculations
Example 1 — Greenhouse 20-10-20 at 200 ppm on a 1:100 Injector
20-10-20 fertilizer, 200 ppm N target, 1:100 injector, 20 L stock tank
Example 2 — Plug Seedlings on a 1:200 Injector
15-5-15 fertilizer, gentle 100 ppm N target, 1:200 injector, 10 L stock tank
Example 3 — Direct Mixing a 100 L Batch, No Injector
24-8-16 fertilizer, 250 ppm N target, mixed directly at 1:1 into a 100 L tank
❓ Frequently Asked Questions
🔗 Related Calculators
How do I calculate ppm from water soluble fertilizer?
Divide the target ppm of nitrogen by ten times the nitrogen percentage to get grams per litre. For 200 ppm N from a 20-10-20 fertilizer, that is 200 divided by 200, which is 1.00 g/L. In imperial units, divide the target ppm by 0.75 times the nitrogen percentage to get ounces per 100 US gallons, giving 13.33 oz.
How much 20-20-20 do I need for 200 ppm nitrogen?
Exactly 1 gram per litre of finished feed, or about 13.33 ounces per 100 US gallons. This is the benchmark most greenhouse growers memorise, because a 20 percent nitrogen product at 1 g/L always lands on 200 ppm N regardless of the phosphate and potash numbers on the bag.
What is an injector ratio and how does it change the mix?
A fertilizer injector draws concentrated stock solution into the irrigation line at a fixed ratio, such as 1:100 or 1:200. The stock must therefore be that many times stronger than the finished feed. A 1 g/L final rate on a 1:100 injector needs a 100 g/L stock, and on a 1:200 injector it needs 200 g/L.
How much fertilizer goes into a stock tank?
Multiply the final-strength rate in grams per litre by the injector ratio, then by the tank volume in litres, and divide by 1000 for kilograms. A 20 litre stock tank feeding a 1:100 injector at 1 g/L final strength needs 1 times 100 times 20 divided by 1000, which is 2.000 kg of fertilizer.
Why does 1 ounce per 100 gallons give 75 ppm per percent of nutrient?
One US gallon of water weighs about 8.345 pounds, so 100 gallons is roughly 834.5 pounds, or 13352 ounces. One ounce of a pure nutrient in that mass is about 75 parts per million. Multiplying by the percentage of nutrient in the product scales it, so 1 oz of a 20 percent product per 100 gallons gives 15 ppm.
What is a normal ppm nitrogen feed rate for greenhouse crops?
Constant liquid feed for most greenhouse ornamentals runs 100 to 250 ppm nitrogen, with plugs and seedlings at the low end near 50 to 100 ppm and vigorous crops such as poinsettia at 200 to 300 ppm. Intermittent feeding uses higher rates applied less often, typically 250 to 400 ppm.
How do I convert P2O5 to elemental phosphorus?
Multiply the P2O5 figure by 0.4364, because phosphorus makes up 43.64 percent of the mass of phosphorus pentoxide. Similarly, multiply K2O by 0.8301 for elemental potassium. Fertilizer labels in most countries still use the oxide convention, while soil and tissue tests usually report the elements.
What happens if the stock solution exceeds the solubility limit?
Undissolved fertilizer settles at the bottom of the tank, so the injector delivers a weaker feed than intended while the sediment slowly clogs screens and the injector body. Most water soluble fertilizers dissolve to somewhere between 300 and 700 g/L in warm water, and appreciably less in cold water.
Can I mix calcium nitrate with other fertilizers in one stock tank?
Not in a concentrate. Calcium reacts with sulfates and phosphates to form insoluble calcium sulfate and calcium phosphate, which precipitate out and block the injector. The standard practice is two stock tanks, with calcium-based fertilizers in tank A and sulfate or phosphate fertilizers in tank B, both injected into the same line.
Should I base the rate on nitrogen or another nutrient?
Nitrogen is the industry convention, because it is usually the nutrient driving growth and the one crop recommendations are written around. The calculator therefore takes ppm nitrogen as the target and reports the phosphate and potash that come along with it, which is the ratio fixed by the product you chose.
Does this calculator work for hydroponics and fertigation?
Yes. The arithmetic linking ppm, percentage analysis and dilution ratio is the same whether the solution feeds a hydroponic reservoir, a drip fertigation line or a greenhouse injector. For recirculating hydroponics you should also track electrical conductivity, since salts accumulate as water is taken up.
What units does this calculator use?
Fertilizer analysis is entered as percentages of N, P2O5 and K2O, the target as ppm nitrogen, the stock tank in litres and solubility in grams per litre. Results are given in grams per litre, ounces per 100 US gallons, grams per litre for the stock, and kilograms of product per stock tank.