1) As long as you are above the minimum required nutrient level for individual nutrients, do relative differences between the nutrient levels matter?
Nope. You will see a lot of people say that certain nutrients should be in specific ratios. However, when supplying nutrients in excess this does not apply.
2) To determine the amount of a dry fertilizer to add to your stock solution would you do the following:
- Say 120L tank, 900 mL dosing bottle, 30mL doses, and target concentration of nutrient X is 7.5 ppm.
7.5ppm=0.0075 g/L * 120L tank = 0.9g/dose
0.9g/30mL dose = 0.03 g/mL required in the dosing container
0.03g/mL * 900mL dosing container = 27g of dry nutrient X
i.e. add 27g of nutrient X to the container when you're making your stock solution. Is this correct?
That would give you a correct dose provided the fertilizer is 100% nutrient X. However, none of them are. Lets do KNO3 as an example.
Here is the formula we can use.
Dose for 1 liter (mg) = ( Desired Concentration (as ppm) / % of element in fertilizer ) * 100
To see what percentages are in each fertilizer you can use a molar mass calculator.
This will tell you the percentages of each element in a substance.
Enter KNO3 into the calculator link above and you will see that KNO3 has 13.8539% Nitrogen.
Since we want NO3 and not N we have to do a conversion since the weight of N and NO3 are different. To get the weight of each you can enter them in the calculator.
N = 14.0067
NO3 = 62.0049
So to get the factor to multiply by we divide, (here is a list of conversion factors, Conversion Tables
so this step can be omitted)
62.0049 / 14.0067 = 4.4268
This means to convert N to NO3 we multiply by 4.4268
13.8539 (percent of N in KNO3) * 4.4268 = 61.3284
So KNO3 has an equivalent of 61.3284% NO3
Now we can do our calculation...
mg = (7.5 / 61.3284) * 100
mg = 0.1223 * 100
mg = 12.23
So if we need 12.23mg per liter we need 120 times that for 120 liters.
12.23 * 120 = 1,467.6 mg (or 1.4676 grams)
If you want a solution divide the dose size into the container size and multiply by the dose, i.e.
(900 ml / 30 ml ) * 1,467.6 = mg to add to your container
30 * 1467.6 = 4,4028 mg
So we add 4.4028 grams to our 900ml container and dose 30ml each time to raise NO3 7.5ppm
Note I rounded the number throughout these calculations so the numbers won't match calculators perfectly.