Plants require specific amount of nutrients dissolved in water to thrive. For example, it’s good to have a concentration of nitrate (NO3) 10 – 50 ppm and level of phosphate (PO4) from 0.2 to 3 ppm.
When we add certain amount of dry salt (KNO3, KH2PO4, etc.) to a specific volume of a water, you will get specific concentration. For example if you dissolve 1 gram of powder in 1 liter of water, you will get concentration of 1 ppm (Part per Million).
It’s a smart idea to use this unit because 1 ppm is the same in 20 gallon tank and in 100 gallon tank. 100 gallon tank will have 5x more of the powder dissolved in it. Plant doesn’t care, what’s the tank volume, they care only about concentration.
If you have a concentration of some kind of nutrient 20 ppm in your tank and next day, there will be concentration 17 ppm, you can say for sure, that your daily uptake rate is 3 ppm. What does it mean? You have to add 21 ppm (7 days x 3 ppm) of some this nutrient weekly.
Unfortunatelly, it’s not so easy to predict this uptake rate. Test kits aren’t accurate (test kits can measure ppm. Each color in the test match specific concentration in ppm). And you have to test frequently, because if you add more light or another plants, uptake rate will change. That’s why Estimative index method is so popular. EI guarantuee, that there will be allways higher concentration (in ppm) than uptake rate is, so plants won’t be hungry anymore.