ABSTRACT
Irrigation level is believed to influence root distribution along the soil profile. There is no known reason for the phenomena. The aim of this study was to see how water content at the soil surface and around the root crown of sorghum (Sorghum bicolor L. Moench) affects root distribution with depth. The work was done in an experiment where an array of soil surface and deeper soil layer water contents were established under the plant, and their effects on root and plant growth measured. Variable soil water profiles were achieved by growing plants in soilfilled vertical PVC tubes of varied diameters, lengths and distances between the soil surface and a water table. It was found that when the top soil (top 3 cm) was wet (at least above 70% of field capacity) crown roots initiation and establishment proceeded at potential rates, resulting in a large number (14) of crown roots which grew to a depth of 30–40 cm, at 24 days after emergence. Irrespective of deeper soil layer water contents, when the soil surface water content was low, crown root numbers were markedly reduced (from 14 to 6) and existing crown roots increased in length (from 38 to 87 cm), at 24 days after emergence. Total crown roots’ length per plant was approximately the same, irrespective of the soil moisture regime. When soil surface moisture was low, plants had several thick distorted crown root initials that apparently did not penetrate the hard dry soil surface. It is concluded that by limiting crown root number per plant, a dry soil surface causes compensatory growth in existing roots, which subsequently reach deeper soil layers. Thus, the soil surface moisture content, as affected by irrigation (or rainfall) frequency, exerts control over sorghum root distribution along the soil profile.
