Nutrient removal is quite often neglected in closed hydroponic systems since it is difficult to measure and control. The reason is that not all nutrients are removed at an equal rate by plant roots. The essential nutrients can be grouped into 3 categories based on the rate of uptake from the nutrient solution.
- Elements are actively absorbed by roots and can be removed from solution in a few hours such as NO3, NH4, P, K, Mn
- Elements have intermediate uptake rates and are usually removed from solution slightly faster than water is removed such as Mg, S, Fe, Zn, Cu, Mo, C
- Elements are passively absorbed from solution and often accumulate in solution such as Ca and Boron.
Great care is needed to prevent toxic build-up of elements since they are not absorbed in the same ratio as they are replenished by the refill solutions, be it either computer controlled or done manually by hand.
Regular water analysis can be done, however, the accuracy of water analysis is not sufficient enough in order to alter the refill solutions. One would need at least four analyses per sample to obtain an average reading of the contents of the water. This is especially true for the micro elements where readings below 1ppm are needed. A 10% deviation is quite substantial for the micro elements.
Low concentrations are difficult to monitor and control. Typical measurement errors associated with water analysis done by AA-Spec instruments are (in mM), K:3%, Ca:1%, S:2%, P:3%, Mg:3%, Fe:10%, Mn:10%, Zn:20%, Cu:175% and Mo:3000%.
The total amount of nutrients in solution can easily and accurately be determined by measuring the electrical conductivity of the solution. However, because of the differential rate of nutrient uptake, conductivity measurements mostly measure the calcium, magnesium and sulphate remaining in solution. The micronutrients contribute less than 0.1% to electrical conductivity.