Influence of phosphorus on selected desmodium growth and nodulation parameters.

Abstract

Phosphorus is known to affect growth and formation of nodules although its role in the process is less understood. Several workers have, however, related phosphorus nutrition with rhizabial activities in leguminous plants. In other studies, it has been shown that nodules are stronger sinks for phosphorus-compared roots, shoots and even young mature leaves. We initiated this study to determine the effect of phosphorus on selected desmodium species growth and nodulation parameters in a pot experiment. A 2 by 8 factorial experiment was laid in a complete randomized design (CRD) at KARICYMMIT Research Station located in western Kenya. Pots were each filled with 7.5 kg 2 mm sieved dry soil. The treatments consisted of two desmodium species (D. intortum and D. uncinatum) and 8 levels of phosphorus applied at 0, 10.3, 20.6, 30.9, 41.2, 51.5, 61.8 and 72.1 mg P kg-1 soil. The experiment was replicated 3 times. Plants were top dressed with urea (at 85.5 mg N pot-1) at 21 days after emergence (DAE). Shoots were harvested twice during the growth period (at 15 and 28 weeks after planting) while dry root weight and nodulation parameters were determined at 34 wap. Results showed that shoot biomass produced in the second harvest was 21% and 42% higher than production in the first harvest for D. uncinatum and D. intortum, respectively. Root biomasses was significantly (p=0.045) increased at 41.2 mg P kg-1 soil compared to the control treatment in the D. uncinatum plant while D. intortum had significant increase in root weight at ≥30.9 mg P kg-1 soil. Addition of phosphorus in excess of these rates did not translate into gain in biomass except at 72.1 mg P kg-1 soil. Application of phosphorus at ≥20.6 mg P kg-1 soil significantly (p=0.039) increased the number of nodules produced by D. intortum species from an average of 37 (control) to 86.8 nodules-1 pot at 51.5 mg P kg-1 soil. The number of active nodules obtained in the control treatment was significantly (p=0.041) low compared to that obtained at 30.9 mg P kg-1 soil in the D. uncinatum species. Whereas D. uncinatum produced few but heavy nodules, D. intortum had relatively large number and light nodules, phosphorus rates not withstanding. Relationships between nodule number and root biomass are also discussed