Growth response of Grevillea robusta A. Cunn. seedlings to phosphorus fertilization in acid soils from Kenya
Karanja N.K. | Mwendwa K.A. | Zapata F.
Three experiments were conducted to assess the response of Grevillea robusta to phosphorus fertilization using acid soils showing low P levels from Eastern (Gituamba-Andosols) and Western (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 kg soil, and 32P isotope dilution techniques were used to assess P uptake in the shoot harvested at 3 and 6 MAT (months after transplanting). Application of MPR to the Andosol reduced height and root collar diameter of G. robusta 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 VAM upon harvesting at 12 months. At 3 MAT 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 P fertilizer 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.
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