Phosphorus fractions in Rengam soil as influenced by phosphate rocks applied

Information concerning phosphorus fractions in Malaysian soil is very limited. Past study of P in Malaysian soil had excluded occluded-P and organic-P fractions measurement. Therefore, the objective for this study was to measure soil P fractions and study P dynamic in soil as affected by P fertilizer applied. Phosphorus fraction study was conducted at Ladang Bukit Lawiang estate, near Kluang, Johor. The oil palm (Elaeis guineensis Jacq.) (D x P palm) is eight years old and producing about 20 ton FFB haE-1 yrE-1. This soil is a Rengam series (Typic Paleudult) and was selected because it is commonly used for oil palm production in Peninsular Malaysia and available soil P was low (1.5 mg P kgE-1). Two phosphate rocks (Christmas Island PR and China PR) and a triple super phosphate (TSP) were selected for this study. The rates of phosphate rocks (PR) applied were ) and 4.5 kg Christmas Island PR equivalent palm E-1 yr E-1 and PR were applied overall broadcast on the month of May annually. Soil samples were collected rom )- to 15- and 15- to 30- cm soil depth from control plots and plots that were applied with Christmas Island PR, China PR and TSP. Inorganic P (nonoccluded aluminium P (Al-P) and iron P (Fe-P), occluded Al-P and Fe-P, and calcium P (ca-P) measurements were conducted using the modified procedure of Chang and Jackson. Total P (inorganic plus organic P) was measured using the perchloric acid (HCLO4) digestion method and organic P was calculated as the difference between total and inorganic P. Soil P concentration in 0- to 15- (p = 0.01) and 15- to 30- cm (p = 0.04) soil depth were significantly affected by fertilizer P source x soil P fraction two-way interactions. Application of China PR increased Ca-P by 19.0 and 19.7 times in 0- to 15- and 15- to 30- cm soil depth, respectively. However, Christmas Island PR and TSP applied on Rengam soil only increase Ca-P by 2.2 and 1..4 times, respectively at the top 15- cm soil depth. At 15- to 30- cm soil depth. Christmas Island PR increased Ca-P by 3.0 times, but TSP did not increase Ca-P in 15- to 30- cm soil depth. The increase of Ca-P for soil applied with China PR was due to lower solubility of China PR as compared to Christmas Island PR and TSP. Christmas Island PR and TSP applied on Rengam soil increased nonoccluded-P by 16% and by 7% when China PR was applied at the top 15- cm depth. However, at 15- to 30- cm soil depth, China PR, Christmas Island PR and TSP increased nonoccluded-P by 54.8, 46.4, and 54.6%, respectively. China PR applied to soil reduced organic-P by 26% in the top 15- cm soil depth, however, application of Christmas Island PR and TSP increased organic P by 37 and 12%, respectively. Occluded-P fraction was not affected by fertilizer P applied measured in soil. Total P was highest in the top 30- cm soil profile when China PR was applied and is about 2.6 to 2.8 times greater than total P measured in control plot. Total P was lowest when TSP was used as P fertilizer. Christmas Island PR was intermediate. In general, this study showed that source of PR used as P fertilizer for oil palm soil could affect P fractions in soil. Continuous use of China PR could increase Ca-P fraction and reduce organic-P fraction. In contrast to Christmas Island PR and TSP could increase nonoccluded-P and organic-P fractions.