Farming can cause clay to physically degrade
The physical properties of clay soils degrade when sodium (Na) and to a lesser extent magnesium (Mg), displace calcium (Ca). Na and Mg form weaker bonds between the clay platelets (very small, flat minerals) than those formed by calcium.
Good soil structure
Platelets bonded in aggregates with large pore spaces
Poor soil structure
Platelets dispersed with no pore spaces
Cultivation breaks these weakened Na and Mg bonds, causing the clumps of platelets (aggregates) to disintegrate into individual platelets. The loose platelets disperse into a suspension then settle and fill air spaces in the soil. This results in:
Poor drainage
Reduced aeration
Water logged surface
Slippery boggy conditions
Loss of surface nutrients
Surface crusting
Water stress in the ripening period
Solid, hard surface when dry
Difficult cultivation
Increased fuel usage cultivating
Poor weed control
Greater surface runoff
Physically degraded clays yield less because of
Difficult cultivation leads to less timely sowing
Cloggy sead beds reduce germination
Surface curst prevents emergence
Compaced, waterlogged clay with poor aeration reduces root growth
Weed control is less effective
Less water infiltrates soils, increasing water stress and early burning off
Subclover is unable to bury seed resulting in poor regeneration
Gypsum improves soil structure by providing calcium that displaces some of the sodium and magnesium. Calcium increases the strength of the bonds between the clay platelets. This allows platelets to aggregate together into crumbs forming soft, friable, well draining soil.