Most people assume pH control is essential, but there is considerable misunderstanding about the effect of pH on plant growth. Plants grow equally well between pH 4 and 7, if nutrients do not become limiting. This is because the direct effects of pH on root growth are small, the problem is reduced nutrient availability at high and low pH. The recommended pH for hydroponic culture is between 5.5 to 5.8 because overall availability of nutrients is optimized at a slightly acid pH. The availabilities of Mn, Cu, Zn and especially Fe are reduced at higher pH, and there is a small decrease in availability of P, K, Ca, Mg at lower pH. Reduced availability means reduced nutrient uptake, but not necessarily nutrient deficiency.
Unfortunately, hydroponic systems are so poorly buffered that it is difficult to keep the pH between 4 and 7 without automatic pH control. Phosphorous (H2PO4 to HPO4) in solution buffers pH, but if phosphorous is maintained at levels that are adequate to stabilize pH (1 to 10 mM), it becomes toxic to plants. Plants actively absorb phosphorous from solution so a circulating solution, with about 0.05 mM P has much less buffering capacity than the fresh refill solution that is added to replace transpiration losses. Figure 2a is a titration curve of fresh refill solution compared to the recirculating solution. Six mmoles of base were required to raise the pH of fresh solution from 5.8 to 8, but only 1 mmole of base raised the pH of the circulating solution to 8. Figure 2b shows the slopes (derivatives) of the lines in Figure 2a. Figure 2b clearly shows poor buffering of the circulating solution between pH 5 to 9; small amounts of acid or base rapidly change the solution pH. The fresh refill solution is buffered by phosphorous, which has its maximum buffering capacity at pH 7.2. This point is called the pKa of the buffer and it is the point at which half of the phosphorous is in the H2PO4 form and half is in the HPO4 form. In other words, the phosphate ion absorbs and desorbs hydrogen ions to stabilize the pH. Unfortunately, phosphorous is quickly removed from the solution.