What is topsoil & why it matters for your lawn
Soil horizons are the classifications of soil layers —an entire discussion for another post. All you really need to know is that the "A horizon" is the uppermost layer of topsoil that contains a rich amount of organic matter where the bulk of soil activity is. This is mainly because of the oxygen availability, in grasslands, this horizon is deeper than in forests because grasses deposit a tremendous amount of organic matter into the soil. These grasslands are where your topsoil comes from.
Topsoil for lawns
Because topsoil is a mined natural resource and therefore expensive, builders often only provide a thin layer of topsoil to new housing plots and is a contributing reason why lawns become dormant in the hot summer. Lawns don't grow well in areas of shallow topsoil but the good news is that grass builds topsoil when it is being properly cared for. Because grass deposits more organic matter into the soil than any other ground cover, proper fertilization increases the speed at which you build healthier soil. Another way to build topsoil is by adding well-aged compost to your lawn.
By encouraging grass roots to grow deeper you are also building deeper topsoil. By mowing your lawn higher off the ground and watering deeply but infrequently you can ensure your grass is rooting as deep as possible.
Fertilizing with synthetic chemicals will deplete your topsoil. This is because all microbial life inhabiting your soil requires both carbon and nitrogen, synthetic fertilizer does not contain enough carbon if any at all. Therefore, microbes in the soil will digest the organic matter in your soil as a source of carbon to wash down all that delicious nitrogen and exhale it as CO2 into the atmosphere.
Cultivation is another way to deplete topsoil, in lawns the most popular method of cultivation is core aeration. Removing cores from the soil introduces a significant amount of oxygen into the soil, dramatically ramping up the microbe's metabolic rates so they digest more of your precious organic matter and release it into the atmosphere.
Lawns require healthy topsoil to thrive, by encouraging deeper grass roots and eliminating the practices that deplete organic matter, you can build healthy topsoil.