ISRIC Report 2010/06 Inventory of P-Olsen data in the ISRIC-WISE soil database for use with QUEFTS

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Niels H Batjes
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This exploratory study presents an inventory of P-Olsen, pHwater, soil organic carbon, organic nitrogen, and exchangeable K values held in the ISRIC-WISE soil profile database. Such profile data are needed to assess location -rather soil type- specific crop yield responses using QUEFTS, an empirical model for the Quantitative Evaluation of the Fertility of Tropical Soils. Regionally detailed datasets of soil variables considered in QUEFTS are needed for (1) further module development/testing and (2) to create spatial data layers to assess regional effects of phosphorus limitation on food security. The focus of this report is on the latter aspect.
Data for available-P are rarely reported in the source materials that underpin the ISRIC-WISE database; they have been measured according to a range of soil type specific analytical methods. Further, these sources seldom include information on land use management and land use history, drivers that strongly influence soil P-levels.
Out of the over 11,000 soil profiles in WISE, only 1147 have data for P-Olsen (0-20 cm) as presently considered in QUEFTS. In part, the corresponding “point data” may be used to develop/test region-specific modules for QUEFTS; 840 of the point data are for soil profiles with pHwater greater than six, corresponding with the pH range for which the P-Olsen method is appropriate. Coefficients of variation for the latter P-Olsen data, clustered according to broad soil classes (FAO, IPCC) as needed for spatial extrapolation, are large (85 to 281%). The present set of median P-Olsen values is not considered representative for any specific geographic area or country, limiting its usefulness for spatial extrapolation based on broad soil classes as defined on e.g. the digital Soil Map of the World.
Selection of the appropriate method to determine so-called plant available-P will vary according to soil type and plant characteristics. In the context of future, global applications that consider QUEFTS or new modelling approaches, different soil fertility – crop yield response modules/functions (i.e. quantitative relationships between chemical soil properties and potential supply of nutrients) will be needed for acidic soil regions where other extraction methods, such as “P-Bray,” are appropriate as a measure for plant available-P. The validity of using the P-Olsen method for the wide pH range presently considered in QUEFTS (4.7-8.0) may need to be re-assessed, in particular for strongly acid soils.

Keywords: legacy soil data, phosphorus, P-Olsen, QUEFTS, ISRIC-WISE database