For saving space and soil, this method also has several
other benefits, including no soil-borne diseases, no
weeds to pull and no soil to till, run-of-the-mill side
benefits of soil-less gardening.


The essential nutrients can be put into 3 categories based on how quickly they are removed from solution. Group 1 elements are actively absorbed by roots and can be removed from solution in a few hours. Group 2 elements have intermediate uptake rates and are usually removed from solution slightly faster than water is removed. Group 3 elements are passively absorbed from solution and often accumulate in solution.

Approximate uptake rates of the essential plant nutrients.

GROUP 1 Active uptake, fast removal NO3, NH4, P, K, Mn
GROUP 2 Intermediate uptake Mg, S, Fe, Zn, Cu, Mo, C
GROUP 3 Passive uptake, slow removal Ca, B

One of the problems with individual ion monitoring and control is that the concentration of the group 1 elements (N, P, K, Mn) must be kept low to prevent their toxic accumulation in plant tissue. Low concentrations are difficult to monitor and control. Table 2 shows typical measurement errors associated with the use of ICP emission spectrophotometry for analysis of hydroponic solutions. Nitrogen cannot be measured by ICP-ES. Accuracy for the macronutrients is good, but solution levels of B, Cu, and Mo cannot be accurately measured by ICP-ES. The calculations in this table are for a typical refill solution, not for the low concentrations that should be maintained in the circulating solution. The measurement errors for K, P, and Mn can be 10 times higher because the solution levels are lower.

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 sulfate remaining in solution. The micronutrients contribute less than 0.1% to electrical conductivity.


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