Contents & Foreword, Characterizing And ... - IRRI books

Benefits and limitations of soil phosphorus mappingIn the 1970s, an undeveloped upland and lowland area of Sitiung (Mimpi Plain) inWest Sumatra was forwarded as a potential area for settlement of transmigrants andfor upland research coordinated as part of the Benchmark Soils Project conducted bythe University of Hawaii (Buurman and Sukardi 1980). The area was mapped at amoderate resolution (1:40,000) for soil type by CSAR by taking soil samples frommore than 50 locations scattered throughout the area in 1976-77 (Soil Research Institute1979). The mapping project demonstrated the benefits of mapping for agriculturaldevelopment policy and soil classification, and the limitations of low-resolutionmapping conducted by CSAR.Some of the sites in Sitiung were classified in detail with samples being takento a depth of 180 cm. This detail enabled several soils in the region to be reclassifiedamong the classes brown tropical forest, Latosol, and podzolic (Buurman and Sukardi1980). Soil profiles from 88 sites in the same region were later used to map topsoilproperties including KCl-extractable Al, extractable P (25% HCl), sand and clay contents,total P, and pH, and 109 scattered samples were used to map variation in siltcontent. Actual soil property values were translated into maps by krigging to give 268points within the sample area (Trangmar et al 1984). The soil P status maps fromSitiung showed that extractable soil P was low to medium throughout the region (mainly40–160 mg P kg –1 ) with high P sorption capacity compared with other tropical Oxisolsand Ultisols. Mapping showed that native extractable soil P in uplands and lowlandswas correlated positively with silt content (r = 0.52**) and negatively with sand content(r = –0.55**) at 0 to 15-cm depth.The relationship between soil P and soil texture in Sitiung demonstrates thatCSAR’s original soil P recommendation maps completed in 1971 (Widjaja-Adhi,personal communication), which were based on soil type rather than directly measuringsoil P in each field, were well founded. However, the variation in soil P in Sitiungshows that a weakness in mapping conducted by CSAR may be in the resolutionsince the maps are only 1:250,000–1:500,000 scale. This low resolution may explainwhy not all field trials gave the responses as expected based on the soil maps. Forexample, P response trials conducted near Indramayu and Subang in 1987-88 respondedto P application despite the area being classified as having high P status (SriAdiningsih, unpublished).Good correlation between actual yield and expected yield based on the soilnutrient status maps may also be limited because the maps do not distinguish irrigated,rainfed, or tidal lowlands. All the lowland types are presented on one map andrecommendations that follow from the mapping exercises and field trials apply to alllowland rice-growing conditions. This may limit the usefulness of the maps as a toolfor formulating P recommendations since average grain yields and farmers’ aversionto the risks involved in investing in fertilizers differ between cropping systems.The fourth edition of soil P maps shows that much of the lowlands of Lampungin Southern Sumatra have a high soil P status. This was confirmed by a soil survey of32 irrigated lowland sites conducted in 1998 before beginning field trials in a programknown as the Acid Soils Project (Kasno, unpublished). The survey showed that,178 Clough et al