SOIL STRUCTURE, ROOT-SYSTEM AND WATER-UPTAKE OF MAIZE .2. GROWTH AND SPATIAL-DISTRIBUTION OF ROOT SYSTEMS

Four trials were carried out in the field to study the effect on the maize root system of the structure of the ploughed layer. In each case the experimental treatments were the 3 typical ploughed layer structures O, B and C presented in the first article of this series. Treatments affected evolution with time of the depth of the root front, but not maximum root depth measured after silking. Total root weight increased differently between treatments, but root/top + root weight rations were similar in all situations. Growth of the whole plant (roots and tops) was accordingly faster in treatment O than in B and C. Root densities measured at flowering were significantly different in the ploughed layer, where soil structure differs between treatments, but also in the non-tilled layers where it does not. Over the whole profile, the horizontal and vertical spatial distribution of roots was markedly affected by the ploughed layer structure. This can be explained by the spatial variability, at the decimetre scale of mechanical resistance to penetration in the ploughed layer; in non-tilled layers of treatment C, the irregularity of root patterns was due to localized reductions of root density under obstacles located in the ploughed layer ("shadow effect").