They contain very small, densely packed particles that hold moisture but don't allow much air space for plant roots.
Clay soils are just the opposite. They contain very small, densely packed particles that hold moisture but don't allow much air space for plant roots. Compost helps separate those tiny clay particles so water can drain more freely and plant roots can get needed oxygen. If it's not clear which type of soil you have, send a sample to a state-certified soil-testing lab for analysis.