| Design and operation of smallholder irrigation in South Asia |
|Chapter 6 - Irrigability|
|Soils problems on irrigation|
Lateritic soils are also the product of leaching, but differ from podzols in that humic acids are not involved in the leaching process, and the break-down of soil minerals is not as complete. Lateritic soils occur extensively in upland tropical areas on acidic parent rock, including granite. They are of characteristic red-yellow color and granular in texture, with a small amount of clay mineral. They are readily erodible, and consequently may be of shallow depth (10 to 20 cm), particularly in undulating terrain. However, depth may be a meter or more in higher rainfall areas where weathering has proceeded more rapidly. The soil transitions into less weathered, fragmented, parent rock.
Lateritic soils have moderately low fertility and low moisture retention capacity. Particularly where of shallow depth they would be classified, by most criteria, as poorly suited to irrigation. However, such soils occur extensively in some areas, including the granitic portion of the Deccan, in central India Under rainfed conditions cultivation is precarious, usually limited to the monsoon season, and generally at subsistence level. Where water can be made available multiple cropping becomes possible and productivity is substantially increased. The shallow depth and low moisture retention capacity remain a problem, however, necessitating a short irrigation interval. Land shaping for irrigation in such conditions requires considerable care, in order to avoid removal of topsoil.
An important factor in management of such soils under irrigation is the role of the subsoil, both in storage of soil moisture, which is available to plants by capillary rise or through root penetration, and as a potential contributor to deepening of the topsoil by mixing in the course of cultivation over the years. Work on agricultural research stations in areas of these soils has demonstrated the effectiveness of this process.