8.6 is highly alkaline. You are just out of the desired range for St Augstine grass. The range is quite broad, 5.0 to 8.5, however it is not good to be on the extreme ends of this spectrum. I suggest you add some gypsum and humic's to your soil to increase the acidity. Keep adding lime until your reach the 6.5 to 7.0 range. Currently your soil is preventing your grass from absorbing the nutrients from the fertilizer. Once you increase the acidity you should see a dramatic increase in the overall health appearance of your grass.
The good news is this problem is easily fixed with multiple applications of lime. The bad news...you wasted a bunch of fertilizer. Sandy soils have excellent drainage. This means your fertilizer literally went down the drain. Chances are you might see some of the benefit from the fertilizer you already applied, unfortunately it will not be equal to the amount of money you already spent for it.
Traditional fertilizing practices (dry broadcast) allow the fertilizer to sit on top of the sand. Good for your type of soil and you current watering practices (it can only dissolve so fast). Fertigation mixes the fertilizer with water, sending the fertilizer right where the water goes. If you have sandy soil it is easy to overwater. With your fertigator, water more frequently for shorter periods of time. This will keep the application of water on the foilier portion of the plant material and within the root zone. Soaking your roots will not benefit your landscape, it will just waste water and fertilizer...