Question: Seemingly overnight, two of my repotted plants leaves have turned a light yellowish-green in the center growth.
Answer: When we got this question, we immediately thought back to one of our local club meetings the week before. A member brought in a “problem plant”, whose outer rows of leaves were its usual shade of green, but whose center rows were nearly yellow-green. Just by chance, this was “soil test” night, where members can have the pH of their soil tested. Her soil tested at a pH of 5.4, very acidic, and far below the optimum (about 6.8, where 7.0 is neutral). When this was pointed out to her, she then told us that she had recently repotted a number of plants into a commercially sold soil mix bought at a local department store. There couldn’t have been a better example of the consequences of too low (or high) a pH if we had planned such a a program!
When pH is too low (acid) or too high (alkaline), many of the nutrients become “locked up” and can’t be used by the plant. Even with proper fertilization, plant roots are unable to take-up important nutrients and, simply put, the plant “starves”. The result is the yellowish foliage growth that appeared once the overly-acid soil mix was used by this grower. The consequence of too high a pH can be the same. This is a problem for those of us with very hard, alkaline, water. For example, our water usually tests at a pH of 7.4 or higher. To correct for this, we simply add enough vinegar so that the pH is as close to neutral (7.0) as possible. For our showplants, we use “distilled” water that we collect from our dehumidifier (in the winter) or our air conditioner (in the summer).
Acidic water is a less-common problem. Rain water can be quite acidic for some. One product that we know of is “pH up” a product of Dyna-Gro. This is a fertilizer supplement (a formula of 0-0-8) that raises water pH. A supplement of “pH down” is also available (a 1-5-0 formula) and we have heard positive comments–if vinegar isn’t for you. We have also heard of some growers using products available at tropical fish stores to correct for this.