Growth response of Grevillea robusta A. Cunn. seedlings to phosphorus fertilization in acid soils from Kenya

Abstract

Three experiments were conducted to assess the response of G. robusta to phosphorus fertilization using acid soils showing low P levels from Eastern (Gituamba-Andosols) and W estern (Kakamega-Acrisols) Kenya. In the first experiment P was applied as Minjingu phosphate rock (MPR-13%P) at 0, 52 and 77 kg P per ha into 5 kg of soil. In the second experiment 2 g vesicular arbuscular mycorrhizae (VAM) soil + root inoculum per 5 kg soil was included in addition to the same MPR rates using Kakamega soil only. In the third experiment, MPR and triple superphosphate (TSP) were added to 5 kg Kakamega soil at a rate of 25.8 mg P per kgsoil, and 32 P isotope dilution techniques were used to assess P uptake in the shoot harvested at 3 and 6 MA T (months after transplanting). Application of MPR to the Andosol reduced height and root collar diameter of G. r obusta significantly (p < .05) as compared to the control. Significant increases (p < .05) in height and root collar diameter where P was added compared to control were recorded with the Acrisol. Soil interaction with P fertilizer rates was highly significant (p < .001) for both height and root collar diameter growth. The roots were not infected with V AM upon harvesting at 12 months. At 3 MA T the percentage P derived from the MPR and TSP was 3% and 6% respectively. P uptake decreased significantly (p < .05) between 3 and 6 months. The results indicated that addition of Pfertilizer in the Acrisols was probably required at the early stages of G. robusta growth but further research and particularly root studies are required to ascertain the above observations