If the St. Augustine is yellowing it is usually due to a lack of iron. High pH and phosphorus levels in the soil can cause this. Iron supplements can help as can spreading peat moss over the turf 1/3 inch deep and then watering it in well.
It may also be due to a lack of water or a lack of nitrogen. If you fertilize even just once a year that will provide some needed nutrients. If you water enough to maintain moderate soil moisture and avoid serious drought that is enough too.
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