Several factors could cause the yellowing or yellowish appearance of the lawn. The continued use of fertilizer containing phosphorous at high levels will tie up micronutrients such as iron and zinc. Iron and zinc deficiency can cause the yellowing. Soil compaction and cool weather in the growing season may also contribute to the problem. A soil test may reveal that the pH is out of range for St. Augustine, which makes the roots unable to absorb nutrients. If a lack of available iron is the problem, application of chelated iron is recommended. It may take several treatments of chelated iron to correct the problem. Follow label recommendations when using any product. Soil compaction in clay type soils is also common; aeration to allow oxygen into the root zone may help the appearance of the lawn.