Root orientation of four sorghum cultivars: application to estimate root length density from root counts in soil profiles

The root length density (RLD) distribution in the soil is a key factor for water and nutrient uptake but its direct assessment is not easy. A field method is presented here to assess RLD of sorghum (Sorghum bicolor L. Moench) from the root intersection density (RID) and taking root orientations into account. The method was developed with four varieties cropped in 2006 in Bambey (Senegal) on a sandy soil, and validated on the basis of independent data obtained on two other varieties cropped in 2008 in Thiès in Senegal on a sandy loam soil. Sets of 1 × 10 m cubic undisturbed soil samples were collected at different positions and at three different dates for the four varieties during the first experiment. RIDs of thick and fine roots were assessed on the sides of the soil cubes and RLDs were measured in the volumes. Fine roots appeared to be isotropic when thick roots were horizontal near the surface and gradually became vertical in deeper horizons. RLD was tenfold lower for thick roots than for fine ones. This led to the development of a model to determine RLD from RID. The relationship for all roots was modeled as RLD = RID × CO, where the root orientation coefficient (CO) is equal to 1.97, indicating that the global root distribution is almost isotropic (CO = 2 for isotropic conditions). The model was validated on the basis of independent data from the second experiment. This model enables RLD mapping and description of RLD variability on sorghum from RID observations on trench profiles (with 10 × 10 cm or 5 × 5 cm surface area used for RID measurement).