Your best option for improving the soil conditions in an existing lawn is a combination of aeration and topdressing. Aeration is the removal of cores of soil. This reduces the density of the soil. Topdressing is the application of organic material to the surface of the turf. The purpose of topdressing is to build an ideal soil layer over the years and to fill in low areas that have developed. Topdressing may need to be done annually for several years if the underlying soil is particularly poor.
The ingredients of the topdressing mixture should be fairly dry; thorough mixing is essential before use. If the topdressing is lumpy, then pass it through a 1/4 inch mesh sieve.
Peat—Buy a fine-grade sphagnum or sedge peat. Well-decomposed leaves are a satisfactory substitute. Garden compost is best avoided—weed seeds can be a problem.
Loam—This is soil that is neither hard like clay nor sandy. The consistency should be medium to fine. (Organic material should be less than 1/4 inch in size.)
Sand—Sea sand is not suitable as it must be lime-free. The particle size should not be too large—avoid coarse grit.
Here are the proportions:For lawns on heavy soil – 1 part peat, 2 parts loam and 4 parts sandFor lawns on loamy soil – 1 part peat, 4 parts loam and 2 parts and For lawns on sandy soil – 2 parts peat, 4 parts loam and 1 part sand
The best time to topdress your lawn is early autumn. Aerating the area a day or two before topdressing will greatly increase the benefits obtained in heavy or compacted soil.
Spread the topdressing mixture at the rate of 3 pounds per square yard, using a spade to put down small heaps over the surface. This topdressing must be worked well into the surface so that it is knocked off the grass blades and sifted down to soil level. On no account should the grass be smothered. The topdressing may be spread by a garden rake. It is essential to spread the topdressing evenly over the entire surface so that depressions are filled in and no new bumps are created.