Having a clear idea on ppm is a must for dosing the correct amount of fertilizer in your planted tank. Aquatic plants need a precise amount of macro and micro nutrients dissolved in water to thrive. The concentration of nutrients dissolved in water is measured by ppm.
When we add a certain amount of salt in a specific amount of water, we get a specific concentration. This concentration is measured with ppm unit. PPM stands for Parts Per Million. If we add 1 gram salt to 1 liter of water, we will get a concentration of 1 ppm. As an example, dissolving 1 gram of Potassium Nitrate KNO3 with 1 L of water will provide 1 ppm concentration of KNO3.
PPM is widely used for dosing fertilizers because ppm remains same whether it is a 20 gallon tank or a 100 gallon tank. It doesn’t vary on tank volume. 100 gallon tank will have 5x more salt dissolved in it than the 20 gallon tank. All plants care about is the concentration, nothing more.
Suppose, in your tank, you have 20 ppm concentration of a certain nutrient. If, in the next day, the concentration becomes 17 ppm, then the daily uptake of that nutrient is 3 ppm. That means, you’ll have to add 3*7 = 21 ppm of that nutrient on a weekly basis. But, measuring uptake is not easy at all. It is extremely hard to accurately measure daily uptake as it changes with light intensity and number of plants.
Comment below if you have any question and I’ll try to answer as soon as I can!