EFFECTS OF PHOSPHORUS AND POTASSIUM ON N-2 FIXATION (N-15-DILUTION) OF FIELD-GROWN CENTROSEMA-ACUTIFOLIUM AND C-MACROCARPUM

The effects of three levels of phosphorus (5, 40, 75 kg P ha-1) and potassium (0, 30, 60 kg K ha-1) on growth, N2 fixation, nodulation and acetylene reduction in Centrosema acutifolium and C. macrocarpum were studied during establishment on an Oxisol in the Eastern Plains of Colombia. Nitrogen fixation was estimated by the N-15 dilution technique, using Melinis minutiflora as the reference plant. Phosphorus limited growth and N2 fixation to a greater extent than did potassium. Phosphorus supply increased dry-matter production, on average, by 193%, N concentration in shoot tissue by 10% and percentage of nitrogen derived from the atmosphere (%N(dfa)) by 15% at 14 weeks after sowing. These increases resulted in 259% more N2 being fixed at 75 kg P ha-1 than at 5 kg P ha-1. In contrast, K supply enhanced dry-matter production by 85% and the amount of N2 fixed by the same proportion. Severe P deficiency quickly led to a strong reduction in nodule weight, whereas K deficiency decreased nodule weight per unit root weight only at later stages of growth. With a supply of 75 kg P ha-1, with or without K, the legumes derived over 87% of their N from the atmosphere. The percentage N derived from the atmosphere of C macrocarpum was more sensitive to P deficiency (63%N(dfa) with 5 kg P ha-1) than that of C acutifolium (76%N(dfa)) at 10 weeks. Phosphorus was considered to have a direct effect on N2 fixation, which would make it necessary to take into account its effect on the symbiosis when screening legumes for tolerance to low soil fertility.