The best method for improving soil under already-installed sod is to aerate the site. The plug holes should back-filled with organic matter such as peat moss or ground leaves. If the soil is low in salts, composted manure can also be added. A soil test can determine whether the soil in high in salt. Gypsum is used to help reclaim sodic (high-sodium) soils since sodium can be flushed out of the soil with water after application of gypsum. Gypsum is a salt and its addition to soil can raise the salt content if it is not flushed out with water. Instead, sulfur can be added to the soil to improve soil structure and help lower pH at the rate of half a pound per 1000 square feet every four to six weeks starting in mid April. The best recommendation is to obtain a soil test to help monitor nutrient content of the soil to help improve structure and fertility.