Soil basics

    Soil is actually a blend of minerals, air, water, and organic matter. Most soils are about 45 percent mineral, 25 percent air, 25 percent water, and 5 percent organic matter.  The minerals may be clay(tiny particles), sand(large particles), or silt (intermediate size particles).
     Loam, the ideal topsoil,k is a mix of large and small mineral particles, with about 40 percent of the particles sand, 40 percent silt, and 20 percent clay. Loam is relatively dark colored, damp, and loose.  The dark color indicates that organic matter is plentiful in the soil, although still only around 5 percent of the total. Dampness indicates that the soil can hold moisture, a result of the clay and organic matter.
     Because loam soil is loose, it has plentiful pore space for water and air to pass through to nourish roots and for roots to grow in. Large particles, such as sand, help loosen the soil. Organic matter helps bind soil particles into larger granules, which also creates porosity.
    The sand, silt, and clay content of your yard may not be ideal, but you can help it become more airy and moist, and you can add organic matter to it. First, however, you need to learn the type of soil making up your ground.

Take a good look at your soil

A laboratory soil test is the best way to find out the exact makeup of your soil, but you can get a quick idea of its composition before sending off a sample. Dig hole about 12 inches deep. Pay attention when you put the shovel in the ground. If it meets resistance, the soil may be compacted. (Compacted topsoil is an indication that the organic matter content of the soil may be low.) Finish digging the hole, and then look at the layers in the soil from the top down.
1. There should be a layer of decomposing leaves and grass and other organic matter on the top. If not, your soil may not have the organic content it needs.
2. Look below the layer of humus to see how deep the topsoil is. At a mininum, you'll need a 4-inch-thick layer of topsoil. Deeper is better, but the most you'll need is around 8 inches.
3. Also check to see how dark the topsoil is. The darker it is, the richer in organic matter and the more fertile it is. If it is light colored, it is sandy and less fertile.
4. Subsoil, the layer below the topsoil, is pretty much just dirt. The same mixture of clay, sand, and silt is there, but the brew of organic matter and simple life forms is missing. Quite simply, there isn't enough there to support a healthy lawn.