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MapsSoil quality in rainfed lowland riceby Stephan Haefele and Robert HijmansRainfedlowland rice Aagroecosystemsare characterizedby fields that areflooded for at leastpart of the growingseason, but thatare not irrigated.Asia has about 46million hectaresof rainfed lowlandrice, constituting Calmost 30% of theglobal rice area.Rice production inthese ecosystems—often hamperedby drought,submergence, andproblem soils—isassociated withlow productivity,and with ahigh incidenceof poverty.Recent technological advances,such as the development of stresstolerantrice varieties and improvedcrop-management options, can helpboost yields substantially. However,such benefits depend stronglyon the quality and availability ofnatural resources, particularly soiland water. Here, we present mapsthat help characterize soil qualityand soil-related constraints inrainfed lowland rice ecosystems(excluding deepwater rice) in Asia. 1,2Map 1. Soil quality in areas where rainfed lowland rice is grown in Asia.We transformed the data ofthe Soil map of the world 3 bycreating four soil quality groups. 4The first two groups—“good” and“poor” soils—do not have majorsoil chemical constraints but differin their degree of weathering (thephysical and chemical breakdown ofsoil over time) and, therefore, theirindigenous soil fertility (the amountof nutrients the soil can supply). Thethird group—“very poor”—representshighly weathered soils that arelikely to have multiple soil chemicalBDconstraints to crop growth (such asacidity, severe phosphorus deficiency,and iron and aluminum toxicities).The last group—“problem soils”—combines the most frequently citedsoil problems, including acid-sulfate,peat, saline, and alkaline soils,which partly cause low fertility, andpartly cause soil chemical toxicity.Although not widespread,problem soils are locally important,especially in northern India (sodicsoils); in some coastal lowlands ofBangladesh, Myanmar, Thailand,1Our approach is based on that of Garrity DP, Oldeman LR, Morris RA. 1986. Rainfed lowland rice ecosystems: characterization and distribution.In: Progress in rainfed lowland rice. International Rice Research Institute (IRRI), Los Baños, Philippines. p 3-23.2For more details, see Haefele SM, Hijmans RJ. 2007. Soil quality in rice-based rainfed lowlands of Asia: characterization and distribution. In:Aggarwal PK, Ladha JK, Singh RK, Devakumar C, Hardy B, editors. Science, technology, and trade for peace and prosperity. Proceedings of the26th International Rice Research Conference, 9-12 October 2006, New Delhi, India. IRRI, ICAR, and NAAS. p 297-308.3Food and Agriculture Organization of the United Nations: The digital soil map of the world. An updated version is available from www.iiasa.ac.at/Research/LUC/luc07/External-World-soil-database/HTML/index.html.4To do this, we simplified the Fertility Capability Soil Classification (FCC) system, which classifies soil types into agronomically relevant groups.See Sanchez PA, Palm CA, Buol SW. 2003. Fertility capability soil classification: a tool to help assess soil quality in the tropics. Geoderma114:157-185.30 Rice Today January-March 2009
Map 2. Distribution of rainfed lowland rice soil quality area in Asia. Each dot is colored to represent the assumed dominant soil class.and Vietnam (saline and acid-sulfatesoils); and in coastal regions ofBorneo, Sumatra, and New Guinea(acid-sulfate and organic soils)(Map 1A). Fertile soils withoutmajor constraints are relativelywidespread in India, Bangladesh,western Myanmar, Java, Cambodia,northern China, and Korea (Map 1B).Poor soils without major constraintsare frequent in eastern India, SriLanka, coastal lowlands of Borneoand Sumatra, and New Guinea (Map1C). Very poor soils with considerablesoil constraints are particularlycommon in the eastern parts ofMyanmar, most of Thailand, Laos,Vietnam, Malaysia, Borneo, Sumatra,and Cambodia (Map 1D). In India,very poor soils are mostly limited tothe west coast and to mountainousregions in the north and northeast.We overlaid the soil groups withthe distribution of rainfed lowlandrice area (Map 2). 5,6,7 The resultsshow that only about one-third ofrainfed lowland rice is grown onrelatively fertile soils, slightly lessthan one-third grows on soils withlow indigenous soil fertility, andslightly more than one-third isproduced on soils with considerablesoil constraints often combined withvery low soil fertility. Rainfed lowlandrice in Southeast Asia is much morelikely to be on poor soils with varioussoil constraints compared with Southand East Asia. About 7% of rainfedrice is grown on problem soils suchas acid-sulfate or saline soils.Soil quality is a major constraintcontributing to the low productivityof rainfed lowland rice ecosystems.However, spatial variation shouldnot be ignored: farmers growingrainfed lowland rice in Southeast Asiaare much more likely to encountervery poor soils with various soilconstraints than farmers in Southand East/Northeast Asia.Dr. Haefele, a soil scientist, andDr. Hijmans, a geographer, workat IRRI.5See The where and how of rice on pages 19-21 of Rice Today Vol. 6, No. 3.6This type of analysis is hampered by the spatial resolution of soil data. The digital soil map of the world was updated recently but it does notinclude detailed soil data that are available nationally, but even these data suffer from large variability within mapping units. In the future,this will probably be addressed by predictive spatial modeling of soil properties.7Soil characteristics can be strongly modified by local geomorpholgy and hydrology. Integration of soil characteristics with land properties suchas slope and climate is a next step we want to take in the characterization of Asia’s rainfed lowland rice ecosystems.Rice Today January-March 2009 31
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