VERTICAL-DISTRIBUTION OF MAIZE ROOTS IN RELATION TO PERMANENT SOIL CHARACTERISTICS

The vertical distribution of maize roots was studied in four contrasti ng soils, (arenosols, luvisols, planosols and vertisols) by using in-s itu root mapping on vertical planes. The relationship between root con tact frequency and depth was different for each soil, with a relativel y low field-to-field variability within each soil type. The general as pect of this relationship did not change appreciably for three years i n arenosols, with a low colonization in sandy layers probably being du e to mechanical barriers. The relationship was consistently non-monoto nic in luvisols and planosols, because of the sparse colonization of s andy layers. In planosols, these layers were traversed by some primary roots, which were essentially clustered in animal burrows. The distri bution of root contact frequency was closer to an exponential function in vertisols. In these soils rooting depth and colonization of deep s oil layers showed a marked increase during two dry years compared with a wet year. This was probably due to a denser net of shrinkage cracks and slickensides, where roots were essentially located in dry years. These results raise the possibility of modelling the decrease in root distribution with depth using soil information and climatic characteri stics.