Never combine concentrated hydroponic nutrients in very little water
Two- and three-part nutrients are in separate “parts” for a certain reason. If the “parts” are mixed when they’re still concentrated or in very little water, a white precipitate will form. And depending on the nutrients’ formulation, this could occur within 60 seconds or so. Most of the precipitate is normally calcium sulfate. The longer the dilution is delayed, the tougher (or even impossible) for dissolution to occur. Plants could only make use of nutrients that are completely dissolved in plain water. So the precipitate is the food which plants won’t be able to get.
So to prevent this from happening, make sure you add most of the water prior to combining your nutrients; and stir very well before adding each part.
Which should come first, A or B?
The first part to add should be the one which contains phosphate. Note that the sequence of adding nutrient part could impact the stability of the nutrient solution, especially if the water is high in alkaline. “Alkalinity” (carbonate and bicarbonate) is a component of natural water which causes high pH level. Adding nutrient dose to water with high alkalinity will significantly reduce the stability of some nutrient species, which include iron, calcium, zinc, copper, sulfate, and manganese.
So, rather than attempting to pre-adjust the water’s pH level (usually a very tough thing to do — adjusting the pH is better performed after all additives and nutrients have been added) it’s best to first add the part of the nutrient that will lower the water’s pH level the most. Usually, this is the part containing phosphate. In 2-part nutrients,often this is Part B. So Part B comes prior to Part A.
But see to it that you check with the brand you’re using. Keep in mind that “part” without phosphate will typically have very little effect on the pH level; and it often contains iron which is very unstable at pH levels way above 6.5. Take note that in 3-part hydroponic nutrients, the phosphate is at times dispersed across 2 bottles. So to be really precise, know which has the highest phosphate and then add that one first.
Add the same amounts of each nutrient part
Try not to measure roughly. Note that any excess in one nutrient won’t compensate for the lack in another. In a 2-part nutrient, adding less than required for part B, for instance, can cause a deficiency in more than half the nutrients needed. Such issue is compounded with two- and three-part nutrients, as the volume of doses for every part will be around 1/2 to 1/3 (respectively) of what it will otherwise be if one-part is used. So without the right measuring tools, when small tanks are used, the error in the dosage could be significant.