N is Nitrogen
P is Phosphorus
K is Potassium
A 15-15-15 solution contains 15% Nitrogen, 15% Phosphorus, 15% Potassium. A 20-10-5 solution contains 20% Nitrogen, 10% Phosphorus, 5% Potassium. The percentage of the solution not used by N-P-K is trace elements and inert material.
If you are buying hydroponic nutrient, get the powder kind that mixes with water. It is much cheaper over the long run when you compare it with already mixed solutions.
An all purpose hydroponic nutrient solution with secondary elements like calcium, sulphur and magnesium and trace elements boron, copper, molybdenum, zinc, iron, and manganese will get you through all stages of growth.
But depending on the stage of growth, you can adjust different nutrient levels needed at different times to optimize growth.
Higher amounts of N are needed when the temperature will be below 80 degrees in the grow room during vegetative growth. 20-20-20, or 23-19-17, or 12-6-6, or something similar, with trace elements should do it.
If temperatures are higher than 80 degrees in the grow room, you need not worry about more N in the formula during vegetative growth.
During flowering the plant needs lots of P, regardless of temperature. 15-30-15, or 5-20-10, or 2-4-3, or something similar, with trace elements should do it.
Do not over fertilize your plants. Too much fertilizer will kill your plants. If you under fertilize, plants will take longer to grow but will not die. Follow the mixing instructions on your hydroponic solution package, if you aren't sure, use less rather than more.
As water evaporates an is absorbed by the plants, your water reservoir level will drop. Add tap water that has been aged 3 days or longer to the reservoir. I don't add nutrient solution when I top up the tank, some people do.
Change the nutrient solution every 2 weeks. That is, discard the old solution and clean out the reservoir, pumps, and other equipment that is used with HOT WATER. After cleaning, add tap water that has been aged 3 days or longer to the reservoir then add nutrient solution. You only need to clean the cups and tubing the plants are in before you start a new crop.
The old solution that you are discarding can be used to water house or garden plants. This will at least double the growth rate if you usually water your plants with regular tap water.
