Soil phosphorus availability in selected sao hill forest plantations, Iringa, Tanzania
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Date
2001
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Sokoine University of Agriculture
Abstract
A study was undertaken to investigate soil phosphorus availability in selected Sao Hill Forest Plantations located in Iringa region. Tanzania. Pine and Eucalyptus are the major species in Sao Hill Forest Project. Soil samples were collected from three adjacent stands of pine, eucalyptus and pine-eucalyptus mixture and analysed for physical and chemical properties. Composite top soil samples were collected from each stand at a depth of 0 to 20 cm and used for bioassay experiment in which three lex els of N and four levels of P were faclorially arranged. Eucalyptus saliyna seeds were directly sown into pots containing 1.8 kg of soil (Oven dry w eight), w atered to 60% field capacity for the rest of the growth period (12 weeks). The soil samples before and after the experiment were extracted with different extractants to estimate available P. The soil samples were also subjected to a fractionation procedure to estimate the amount of P in different pools. Soil analysis results showed that the soils were strongly acidic (pH caciz 4.2 to 4.5). The soil exchange sites were dominated by aluminium (33%) and the soils were deficient in P (< 10 mg P kg'1). The amount of P extracted by Resin. Bray 1. Bray 2 and iron oxide filler strips were significantly (P< 0.05) affected by P fertilizers. Iron oxide filler paper strips extracted the highest amount of P al any P rale, followed by the Resin method. Plant P uptake was significantly and highly correlated with Bray-2-P (r=0.70), resin P (r=0.64), Bray-l-P (r=0.63) and iron oxide filter strips (r=0.59). The results indicated that in the untreated soils, adsorption maximum was lower in the mixed stand but this stand had the highest bonding energy. The potential buffering capacity was lowest in eucalyptus stand. The adsorption maximum was little affected by fertilizer amendments while as expected the bonding energy decreased w ith increasing fertilizer rate. Most of the added P was recovered as NaHCOj-P., fraction accounting for 43% in the pine stand and 40% in the mixed stand. In the eucalyptus stand. 27% of the applied P was recovered as NaHCOj-Pi and to 25% as NaHCOj-P,,. Most of the extraction methods correlated well with at least one of the soil fractions. The application of P fertilizers had a significant (P<0.05) effect on seedling height in the soils from all the stands but the height measured at the 9dl week (H3) showed a significant interaction between soil type and applied P fertilizer. Root I collar diameter, total and components dry matter of eucalyptus seedlings were significantly different (P<0.05) al different levels of P fertilizer. Total and components P contents were significantly (P<0.05) affected by P fertilization and also there was a significant interaction betw een soil source and P fertilizer. The soils from different stands significantly (P<0.05) affected foliar P concentration of E. saligaa seedlings indicating that the pine stand was P deficient. Application of N and P fertilizers to E. saligna significantly increased N and P foliar concentrations and there was a strong positive correlation between foliar P concentration and growth variables confirming P deficiency. It is concluded that the acid soils of Sao Hill are severely P deficient and that application of P fertilizers is likely to improve growth and yield of forest plantations in the Project
Description
Dissertation
Keywords
Soil phosphorus availability, Sao hill forest plantations, Iringa, Tanzania