Effects of surface moisture supply on the subsoil nutritional requirements of lucerne (Medicago sativa L.)

Abstract

During glasshouse experiments, lucerne plants were grown in deep (85 cm) reconstituted profiles of a granitic soil. The plants were subjected to controlled degrees of drought in surface layers, while a water table was maintained at 85 cm depth. The effect of placing monocalcium phosphate, sodium borate, and calcium carbonate (lime) in the moist subsoil (55 cm depth) was compared with that of application to surface layers, either frequently moistened or dry. Within 8 weeks of commencing surface drought the phosphorus concentration in the shoots had fallen to much lower levels than that in plants grown in surface-moist soil. Deep placement of monocalcium phosphate increased the phosphorus content and yield of the droughted plants, but the effects were greatly enhanced when the deep phosphate band was placed within a soil layer amended with lime and borate. These nutritional effects did not occur with surface-moist plants. The evidence indicated that the combined placement of phosphate, lime, and borate in the subsoil was particularly effective because of a marked increase in lateral root proliferation within the amended layer. Lime did not appear to influence phosphate availability directly through soil chemical effects. The results emphasize the importance of subsoil nutritional characteristics in stabilizing lucerne yields over intermittently wet and dry periods.