Nutrient Cycling in Agroecosystems 63: 43–48, 2002. © 2002 Kluwer Academic Publishers. Printed in the Netherlands.
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Phosphorus availability from phosphate rock and sewage sludge as influenced by the addition of water soluble phosphate fertilizer F. Zapata1,∗ & A.R. Zaharah2 1 Soil and Water
Management and Crop Nutrition Section, International Atomic Energy Agency, Wagramer Strasse 5, P.O. Box 100, A-1400 Vienna, Austria; 2 Department of Land Management, Universiti Putra Malaysia, 43400 UPM, Serdang, Selangor, Malaysia. (∗ Author for correspondence; E-mail:
[email protected] Key words: acid soil, available P, 32 P, phosphate rock, sewage sludge
Abstract Phosphorus (P) inputs are required for sustainable agricultural production in most acid soils of the tropics and subtropics. Phosphate rocks (PR) and organic materials have been suggested as alternative P sources in these soils. Quantitative information on the P availability from sewage sludge (SL) is scanty. Methods to improve the effectiveness of PR such as partial acidulation and compaction with water-soluble P sources have been recommended. The objective of this greenhouse study was to evaluate the relative agronomic effectiveness (RAE) of Florida PR and sewage sludges (irradiated and non-irradiated) applied alone and in mixture with a water-soluble source (triple superphosphate, TSP) at two rates (50 and 150 mg P kg−1 soil). The 32 P isotope dilution technique was utilised to determine the proportion of P in the plant taken up from the P fertilizer treatments. Wheat was grown on an acid loamy sand Dystric Eutrocrepts and harvested 6 weeks after planting. Results on total P uptake and the RAE of the P fertilizer sources tested indicated that the addition of 50 mg P kg−1 soil as TSP was adequate in supplying P to the 6-week-old wheat plants as compared to PR and sewage sludge. Intermediate values were obtained for the mixtures. Similar responses were observed for the high P rate. For a given P rate, phosphorus uptake from PR and SL in presence of TSP was higher than P uptake from these sources alone, indicating an enhancement effect of TSP on the effectiveness of these non-readily available sources. With respect to P uptake from PR applied alone, the relative increases in P uptake from PR due to TSP influence were 52 and 67% for the low and high P rates, respectively. The relative increases in P uptake from SL due to TSP when compared to P uptake from SL alone were 102 and 59% for the low and high P rates of application. Application of a water-soluble P fertilizer together with a non-readily available P source shows an enhancement on the P uptake from the non-readily available P source by the wheat plants. In this experiment the estimated enhancement effects are very likely underestimated.
Introduction Phosphorus deficiency is a major constraint to crop production in tropical and subtropical acid soils and P fertilizers need to be applied to obtain optimum plant growth and crop yields. Direct application of phosphate rocks (PR) has been mainly used in these soils, especially for perennial crops. However, PRs usually do not perform as well as water-soluble P fertilizers with annual crops in terms of yield response (Chien & Friesen, 1992; Hammond et al., 1986; Khasawneh & Doll, 1978). Supplying the early P requirements of a crop with water-soluble P has been shown to be effective for early root development of the plant to utilize
PR more effectively (Chien & Hammond, 1988). This supply of water-soluble P can be made through processes such as by compacting water-soluble P fertilizer with PR, or partially acidulating PR with H2 SO4 or H3 PO4 (Chien and Menon, 1995; Chien et al., 1987; Kpomblekou et al., 1991). The acid generated by hydrolysis of water-soluble P fertilizer would solubilize the unreacted PR, and thus provide extra available P for plant utilisation with time (McLean & Wheeler, 1964). Also, the presence of water-soluble P in such formulations may contribute to a better plant root system resulting in more vigorous seedling, and thus would enhance plant utilization of the insoluble P fertilizer (Chien & Ham-
44 mond, 1988). Using 32 P-labeled single superphosphate (SSP), Xiong et al. (1996) found that uniform placement of SSP and PR significantly enhanced plant growth and P uptake by rye grass grown on two Typic Hapludults, but failed to show that water soluble P increased plant utilisation of a low reactive PR from China. In contrast, Chien et al. (1996), using 32 P labelled triple superphosphate showed the enhancement of plant growth and P uptake from a medium reactive PR by maize and cowpea in a Typic Hapludult. Sewage sludges are the solid by-products of the processing /purification treatments of waste waters mainly from domestic origin and have been used as fertilizer source for many years (Frossard et al., 1996; Kirkham, 1982). Their chemical composition is highly variable, depending on their origin and method of treatment (Frossard et al., 1996). Information on the availability of P from these materials is scarce. This greenhouse study was conducted to evaluate the relative agronomic effectiveness (RAE) of alternative P sources such as PR and sewage sludge applied alone and in mixture with triple superphosphate (TSP), using the 32 P isotope dilution technique.
Materials and methods An acid soil from Hungary classified as Dystric Eutrocrept was air-dried and sieved to pass a 2-mm sieve. This soil is loamy sand, with 8.2% clay; 8.8% silt and 83% sand. The soil pH in water was 5.35; and had 0.27% organic C, 0.025% total N and 7.8 mg P (Olsen) extractable kg−1 soil. The cation exchange capacity as measured by the cobalt hexamine method (Remy & Orsini, 1976) was 2.66 cmol(+) kg−1 ; with exchangeable Ca, Mg and K of 1.82, 0.61 and 0.09 cmol(+) kg−1 soil, respectively. Exchangeable Al was 0.03 cmol(+)kg −1 . The PR used was a finely ground (
