YELLOW PATCHES  Three years ago we planted St. Augustine (Floratam) sod. The soil is sandy, pH 6.5. The grass is thick and in most areas looks healthy. In one area the grass blades are mixed deep green and light green. We find no trace of disease and cannot understand what is happening. The problem seems to be spreading. Any ideas on what is happening? - Lake County

ANSWER: Your County Extension Office can help. They would be happy to look at a sample which you bring in. The hours of this service in Lake County are 9:00 to 4:00, Monday through Friday. Because your Floratam has no specific symptoms of disease (e.g., leaf spots), Turfgrass Pathologist Dr. Monica Elliott suggests you evaluate soil nutrition. Therefore, when you visit the Lake County Cooperative Extension Service Office (352-343-4101), please pick up a Soil Testing Laboratory Form. You can collect a soil sample from your lawn by following the instructions on the form, and then mail both to Gainesville. The cost of soil analysis is $5 or higher, depending on which nutrients you have tested. According to Turfgrass Soils Scientist Dr. George Snyder, pH indicates the availability of other nutrients, such as iron, so you don't have to analyze for all the nutrients. In sandy soil, the pH can change rapidly due to calcium in your irrigation, thus an earlier pH test may no longer be accurate. A new soil test may give you an idea of possible problems in nutrient availability. It will not be definitive, and will not exclude other possible problems, such as diseases. Your description of a mixture of deep green and light green leaf blades may be a clue, depending on what kind of mixture you are referring to. If the scale is large, a foot or more in diameter, it could indicate buried debris which you can detect by probing. If the scale is very small, e.g., youngest leaves are light green and older leaves are dark green, then iron or other micronutrient deficiency is possible. Then again, by explaining this and showing a sample at the County Extension Office, they might be able to figure it out for you. Each of Florida's 67 counties has an Extension Office, listed in the White Pages in the County Government section under "Agriculture" or "Cooperative Extension Service."