SHRINKAGE OF BARE AND CULTIVATED SOIL

The surface subsidence of shrink-swell soils may used to estimate the soil profile water content and soil heaving. Roots are known to affect cracking patterns by anchoring the soil mass, thus influencing soil shrinkage. Our hypothesis was that plant root will influence soil shrinkage measured by vertical surface subsidence. We tested this hypothesis by measuring shrinkage in a large weighing lysimeter for bare soil with no root system and for soils under wheat (Triticum turgidum L.) and alfalfa (Medicago sativa L.) crops possessing fibrous and tap root systems, respectively. The volume-loss shrinkage curves for both bare and cultivated soil were round to conform to the straight-lines model, with distinct changes in shrinkage zones, in spite of uneven water-content distributions in the profile. The shrinkage characteristic (i.e., the differential change in bulk volume divided by the change in volume water) was greater for fallow (0.677) than for wheat (0-380) and alfalfa (0.377), with the cultivated conditions being similar. These data suggest that plant roots may have large effects on in situ soil shrinkage rates and the water-content zones throughout which they occur. This result means that a soil's shrinkage characteristic may change depending on the cropping condition. It also means that shrinkage measurements from soils without systems (e.g., small cores or clods) are not necessarily representative of actual shrinkage properties observed in the field.