|I have an established St.Augustine lawn shaded by very large oak trees. The lawn is thick and grows quickly but has several variations in color from yellow to very dark green. I use quality fertilizers and and still have the problem. I have used ironite with very little improvement.|
|The problem is iron chlorosis due to your high pH soils. Other contributing factors include high phosphorus levels (the middle number on a fertilizer bag), and cool and/or wet soil conditions.
Fertilize you turf with 21-0-0, an acidifying fertilizer at a rate of 5 pounds per 1,000 square feet in mid-April and June. Then in mid-October, use a product similar to 15-5-10 at a rate of 7 pounds per 1,000 square feet. Return your clippings to the turf by mowing with a mulching mower. Water only every 5-7 days applying 1/2 to 1 inch of water when you do. A coffee can or rain gauge can be used to determine how long it takes to apply this amount.
Chelated iron products can be used as a quick fix to the iron deficiency. Applying a pound of wettable sulfur per 100 square feet through a hose end sprayer is another good idea to help in the longer term. This should be done in early spring and again in late fall when temperatures are cooler.