Adaptive attributes of tropical forage species to acid soils. IV. Differences in shoot and root growth responses to inorganic and organic phosphorus sources

In highly weathered acid soils, low supply of phosphorus (P), a major plant nutrient, severely limits pasture establishment and production. Previous r esearch indicated that inherent differences in efficiencies of P acquisitio n and use exist between tropical forage grasses and legumes when grown in a cid soils. These differences in P acquisition between grasses and legumes m ay result from their ability to use sources of less available P from infert ile acid soils. We tested this hypothesis by conducting a greenhouse study. The main objective was to determine differences in shoot and root growth r esponses between the grass Brachiaria dictyoneura CIAT 6133 and the legume Arachis pintoi CIAT 17434 when grown under monoculture or in grass+legume a ssociation with different sources of inorganic and organic P. Two acid soil s of contrasting texture (sandy or day loam) were amended with different so urces of P: di-calcium phosphate (Ca-P), aluminum phosphate (Al-P), phytic acid (organic-P), and cow manure (dung-P). Except for Al-P, which was appli ed at two rates (20 and 100 kg P ha(-1)), the sources were applied at 20 kg P ha(-1). After 75 days of growth, shoot, and root biomass production, dry matter partitioning, leaf area production, total chlorophyll content in le aves, soluble protein in leaves, total root length, and proportion of legum e roots in grass+legume association were determined. Greater differences fo r shoot and root growth characteristics were found between the grass and le gume than between the two types of acid soil. Shoot biomass production per unit soil surface area of both species was higher with Ca-P than with Al-P, organic-P, or dung-P. With Ca-P, the grass produced twice as much shoot an d root biomass than did the legume. But, the legume responded to the source s of less available inorganic and organic P by extending its leaf area. The refore, not only do wide differences exist between the grass and legume in their ability to grow on infertile acid soils amended with sources of relat ively less available inorganic and organic P, but also the legume is better adapted to such soil conditions.