Management of heavy cracking clays

Wide-cracking dark coloured Vertisol, Nicaragua

Soil water management, tillage, cropping systems, and nutrient management pose special problems in heavy cracking clays. These soils mainly belong to Vertisols and vertic subgroups of other soils and occur mainly in (sub) tropical areas with a pronounced dry season. Management of soil water is the important aspect of soil management in the semi-arid tropics. The poor internal drainage and extremely slow hydraulic conductivity, leads to water logging, and delay in planting. Extreme consistency properties of the soil permit tillage operations within a narrow soil moisture range only: because soils are sticky when wet and hard when dry. Use of tillage implements under wet conditions may result in soil sticking to implements and the formation of large clods.  Wide deep cracks in the dry season, permit easy entry of rainfall and water moves freely into the cracks. Although chemically rich these, soils under sustained, high-input systems, may suffer from fertility problems on account of limited availability of N.P and micronutrients.

The presence of heavy clays may be associated with a microrelief called gilgai that is a result of the continuous churning of these soils. Infrastructure and buildings may seriously be damaged in the long run, if they are built on these soils.

Besides the need to get as much of the rain as possible into the soil for use by the crop, there is the need to provide adequate surface drainage to avoid plant injury or slow growth from water logging once the cracks have closed and infiltration rates have slowed down.  A traditional and relatively early method was the cambered bed. A cambered bed can be formed by ploughing up and down so that the soil is turned inwards to the centre. Cambered beds have been used successfully in many areas of Africa.

In very dry areas, water logging is less likely to be a problem, and tillage is important to get every drop of water in the soil, and minimize runoff and evaporative losses. The roles of ridges and furrows are reversed: water is designed to run off the ridge, sometimes suitably broadened, which then becomes a water harvesting device designed to lead runoff into the furrow in which the crop is planted. To block the water movement in the furrow, the ridges may be 'tied' at intervals with a cross ridge, although in occasional unusually wet years the ridges may be untied.

Cropping system is an important aspect of heavy cracking clays management. The ICRISAT system of post-harvest ploughing followed later by seedbed preparation and dry seeding before the rains has been found to be successful in regions with predictable rains. Proper management and timing of cultivation practices are crucial for the management of heavy cracking clays.