Lack of Iron Causes Yellowing
The yellow streaks and spots are caused by a lack of iron in the leaf. It is called iron chlorosis. Iron chlorosis turns a St Augustine lawn into a patchwork of unsightly yellow spots and streaks. Iron chlorosis is common during the summer months when rapid growth exceeds the plant's ability to extract Iron from the soil. There is no permanent fix for iron chlorosis. The only solution is to apply supplemental iron in a form that the St Augustine grass can absorb.
Foliar Applied Liquid Iron
The best fix for the iron chlorosis is foliar applied liquid iron. The best and most expensive supplement is chelated iron. A good alternative is iron sulfate - which not only is absorbed by the St Augustine grass plant but also promotes iron to be released from the soil (acid-forming)
I also use acid-forming fertilizers to create micro releases of iron. Most of the soils in Pinellas are alkaline (high pH). Iron is readily available in neutral to slightly acid soils (low pH). Acid forming fertilizers will lower the soil pH – if only for a brief time – and releasing iron to be used by the plant.
Iron Absorption is Slow
Iron absorption is slow and playing “catch-up” takes time – there is no “spray today – lawn looks great tomorrow” scenario. For best effect, small amounts of preventative foliar applied liquid iron should be applied year-round to iron-deficient lawns - not just when yellow spots and streaks appear. Applying small amounts throughout the year helps build up iron in the plant and enables the St Augustine grass plant to "coast" through the rapid growth of summer without going "yellow".
For the homeowner, this is a difficult problem to solve. The best solution is to apply acid-forming fertilizers with high iron content every 2 – 3 months. The iron content can be found in the analysis on the container. Millorganite is a good (but expensive) source and a very good product to keep the yellow out. With Millorganite be prepared to use 50 - 100 lbs. per thousand square feet or about 5 – 10 bags for an average lawn. Millorganite has a noticeable musty smell when applied to lawns during the rainy season.
Professional Solutions – Custom Formulations for Pinellas County
Throughout the year, I use a water-soluble blend of acid-forming fertilizers plus a chelated iron. This foliar applied liquid application used throughout the year will prevent yellow spots and streaks in even the most difficult lawns. During the nitrogen blackout periods, I rely on the more expensive chelates to keep the grass green and to mask the effects of low nitrogen.