Why were thick clay soils difficult to farm before the heavy-wheeled plow?

Thick clay soils were difficult to farm before the development of the heavy-wheeled plow because of their physical properties. Clay soils are composed of very fine particles, which makes them heavy and difficult to turn over with simple tools like a spade or light, wheelless plows.12

These soils tend to compact tightly under pressure, which drives out necessary air, and they hold water closely adhered to their particles, resulting in poor drainage. Because of these characteristics, clay cannot be worked when wet, and during periods of drought, the soil becomes extremely hard and prone to developing large cracks, making it resistant to cultivation by lighter implements. While early Roman-era plows were effective for the lighter soils of the Mediterranean, they lacked the design—specifically the moldboard—required to effectively invert and manage the heavier, more resistant soils found in northwestern Europe. The introduction of the heavy-wheeled plow provided the necessary mechanical advantage to break and turn these challenging soils, facilitating the expansion of agriculture into those regions.213