Effects of subsoil acidity on the shoot and root growth of some tropical and temperate forage legumes

Abstract

The root and shoot growth of four tropical and two temperate summer-growing legumes were assessed when plants were grown in deep profiles of an acidic soil modified by additions of calcium carbonate. Species tested over three harvests were Desmodium intortum, Glycine wightii, Stylosanthes humilis (Townsville stylo), Macvoptilium atvopuvpureum (Siratro), Trifolium repens and Medicago sativa (lucerne). There were large and more immediate effects on root growth, particularly on fine root length, than on shoot growth. The species differed in their root responses to lime, the tropical species in general being more tolerant of subsoil acidity than the temperate species. There were marked differences between species in their responses when expressed as the ratio of fine root length to total shoot weight. The ratio of root weight to shoot weight showed much less variation with lime rate, and it is suggested that the ratio of fine root length to shoot weight is the better indicator of tolerance to subsoil acidity. S. humilis showed little response to lime at any time, and was notable for its length of fine root. Siratro grew well at first but later there was little increase in shoot weight or in length of fine root, although tap root weight increased greatly. Roots of D. intorturn, T. repens and lucerne were slow to penetrate beyond 55 cm depth. At later harvests the root lengths of these species and of G. wightii were highly responsive to lime. Agronomic implications of the results are discussed.