Why are clay soils difficult to work when wet or dry?

Clay soils are classified as "heavy" in horticulture because their very fine particles make them difficult to cultivate. Their physical behavior changes significantly depending on moisture levels:1

• When wet: Clay soils hold water tightly against their particles, leading to poor drainage. They become sticky and, under pressure, tend to compact tightly, which drives out essential air. This lack of aeration and the tendency to become waterlogged make them difficult to work with tools.12
• When dry: During periods of drought, clay soils lose their moisture and become extremely hard. They often develop large cracks, which prevents them from being worked satisfactorily.1

To improve the workability of clay soils, gardeners often incorporate humus—such as compost, leaf mold, or manure—to help lighten the soil structure and improve porosity.13