St. Augustine grass turns yellow due to a depletion in the amount of nitrogen the plant is able to absorb. As a nitrogen-rich grass, St. Augustine requires that soil be properly watered and contain the right balance of nitrogen and iron. Over-watering grass leads to nitrogen depletion.
St. Augustine grass maintains its vibrant green hue when it has the proper balance of nutrients to properly replicate chlorophyll. As a plant that heavily relies on nitrogen, the grass begins to die and turn yellow when this substance becomes diluted. A yellowing lawn likely has a soil composition that lacks enough nitrogen or is simply being given too much water.
Upon noticing that St. Augustine grass is turning yellow, the best solution is to add more nitrogen as well as an iron treatment to strengthen the shoots and the color. Nitrogen is only needed for therapeutic purposes or during initial planting, but iron needs to be sprayed on a yard every two to four weeks to help the grass reach its richest green. Slow-release nitrogen deposits also help create greater longevity in the lifespan and total health of each shoot, so combining a soil-based application and regular sprays every few weeks replenishes nitrogen stores and ensures a vibrant lawn.