Short answer: because calcium reacts with phosphates and sulfates when they are concentrated together. The reaction forms solids that settle to the bottom of the bottle, and the plant can no longer use those nutrients. Keeping them in separate bottles prevents it.
What happens in a single bottle
Calcium is usually supplied as calcium nitrate. Phosphorus and sulfur come from compounds such as potassium phosphate and magnesium sulfate. In a strong concentrate, calcium combines with sulfate to form calcium sulfate (gypsum) and with phosphate to form calcium phosphate. Both dissolve poorly, so they drop out as sediment.
Why it is fine once diluted
At the much lower concentration of a finished nutrient solution, these elements can share the same water without forming solids. That is why both parts end up in your Kratky container together, as long as each is diluted first.
How to use a two-part nutrient
- Fill your mixing container with the full volume of water.
- Add part A and stir until it is fully mixed.
- Add part B and stir again.
- Measure EC, then adjust pH.
Never pour part A and part B into the same small cup or syringe, and never add them to a small amount of water before topping it up.
How one-part nutrients avoid the problem
One-part powders keep the ingredients dry, so they cannot react until they meet water. Some one-part liquids use formulations that limit the reaction. Either way, follow the label: if it says to add slowly or stir well, there is usually a reason.
Signs something went wrong
Cloudiness or white sediment straight after mixing usually means the parts met at high concentration. Start again with fresh water rather than trying to fix the batch.
Related guide: choosing a nutrient for Kratky hydroponics.
