Fertilizer calculation basics for hydroponics

I’m often asked by growers and students alike to walk them through the calculations for setting up a hydroponic fertilizer recipe. While the math itself is pretty straightforward, there are several key points to take into account, including: percent elemental composition of a fertilizer, injector ratios, size of stock tank, and compatibility of fertilizer salts in stock tanks. This alert will cover the basics of fertilizer calculations when using dry fertilizers.

by Neil Mattson

Step one: how much fertilizer to achieve a ppm target?
We’re going to begin using metric units (milligrams, grams, liters) but I’ll show how to convert to U.S. standard units. Let’s say we want to use commercial calcium nitrate to supply 100 ppm nitrogen (N). There are different forms of calcium nitrate but let’s assume the form we are using contains 19% calcium (Ca) and 15.5% nitrogen (N) (these values come from the product label). Let’s calculate how many milligrams (mg) of calcium nitrate we need to achieve 100 ppm N in 1 liter (L) of water.

The basic principle we follow is that 1 ppm equals 1 mg/L. (This is an inherent property of physics. 1 L of water at room temperature weights 1 kg or 1,000,000 mg, therefore 1 mg of a given element in 1,000,000 mg of water becomes 1 ppm). Then we have to take into account that a given fertilizer contains only some fraction of element. In our case calcium nitrate contains 15.5% nitrogen. To calculate how many mg needed per 1 L of water divide the target value by the percent of the element.

Click here for the full e-GRO Edible Alert.