Clay soil is made of at least 50% clay particles. These particles are smaller than sand or even silt, giving clay a different texture than these soils. Lawns grown on clay soils suffer more severely from certain problems that most lawns are prone to and have a unique set of difficulties other lawns don't share.
Excessive water retention is one of the biggest problems that clay soil lawns face. Because clay soils are made up primarily of small particles, there is very little space between the individual grains of clay. As a result, it is difficult for water to penetrate into the soil. Rainwater tends to pool on the surface, keeping it muddy for longer periods of time. Once the water does penetrate, it drains very slowly, keeping the clay saturated for much longer than other soils. This can damage and weaken roots by depriving them of oxygen and cause root rots.
Clay soil is naturally dense because of its small molecules. It is very difficult for the roots to penetrate the dense soil, so the roots tend to remain shallow and weak, opening the lawn up to opportunistic weeds. Because of their shallow root systems, clay lawns are also more easily damaged by foot traffic, drought and other stresses. Clay soil requires frequent aeration to prevent excessive compaction and poor drainage.
Although clay soils are often nutrient rich, it can be difficult for plants to get those nutrients. Beneficial microorganisms and decomposers (such as worms) do not flourish in clay soil, so they cannot break down nutrients to a useful form for the plants.
When clay gets wet and dries again, it tends to form a solid, rock-like mass. Frequent heating and cooling can cause this mass to crack, damaging the lawn and weakening the root systems.
For plants, springtime is determined more by soil temperature than by air temperature. Because clay soils are so dense, they take a long time to warm up--particularly in cold environments where they can store frozen water all winter. This can shorten the growing season for your lawn, giving you less time to establish and strengthen your grass.