Tolerance to acid soil conditions of the velvet beans Mucuna pruriens var. utilis and M. deeringiana

Abstract

Fast growing, climbing leguminous cover crops such as the velvet beans can be used to reclaim weed-infested, degraded soils in the humid tropics, especially land covered by the grass Imperata cylindrica; they climb over the grass leaves and shade the grass out if their cover lasts long enough. Tolerance of two species of velvet bean to eroded soils was investigated by removing topsoil and directly sowing into the subsoil; plots where topsoil was not removed were used as a control. The response to small amounts of P fertilizer and lime was also tested. Removal of the topsoil resulted in retarded growth of both species, in increased dry matter content of the shoot, in decreased specific leaf area and in increased leaf weight ratio, due to shorter internodes. Six weeks after planting the leaf area index (LAI) was about 1.2 where topsoil was retained, sufficient for a shading effect on Imperata. Where topsoil had been removed, the LAI was only 0.6. Mucuna pruriens var. utilis showed a faster aboveground growth than M. deeringiana; the species did not differ in tolerance to eroded soil. Small amounts of P fertilizer had no significant effect on the growth of both Mucuna species. Shoot: root ratios, on a dry weight basis, were much lower when topsoil had been removed, about 3.7 and 2.4 for M. p. utilis and M. deeringiana respectively, compared to 6.2 and 3.3 where topsoil was retained. Removal of topsoil led to reduced Mg and to increased Al concentrations in roots, and to increased levels of Mn and Al in shoots. In the second year no effect of lime or residual effect of P application was found on growth of Mucuna or Imperata. Removal of the topsoil had little effect on the growth of weeds after the cover crop had been harvested. Due to the high Al tolerance of Imperata, reclamation by Mucuna will be less effective if the topsoil has been lost by erosion.