QuestionI have several large areas in my st. augustine grass that are yellow. This is a mature lawn that has been in place over 20 years. The grass is not dead or dying and appears perfectly healthy. I grows just like all of the other grass it is just yellow in color, I am thinking this is either a lime or iron deficency. Any ideas on what might be causing this and how to solve the problem?
Answer Lime and iron can be antagonistic. Here is how it works. Lime raises the pH and iron becomes more and more unavailable as the pH goes up. So we are going to change one thing at a time. It may well be an iron deficiency. If you apply some liquid iron to the yellow area, the iron will be taken up through the leaves and the patch will green up. You can then program this as a routine part of your lawn care. Many commercial companies do it. On the other hand you can do a soil test on the yellow patch. If the iron is present and the pH is too high for good uptake, then you can add the recommended amount of sulfur or aluminium sulfate to lower the pH. This is not as simple as it sounds but it is a more lasting solution to the problem.
If the lawn is 20 years old, it may well be depleted of elements like iron, magnesium, manganese and other minor or trace elements. You should confirm this by soil testing and use a more complete fertilizer blend to replenish the soil.
