XA0201061 STANDARD CHARACTERIZATION OF SOILS EMPLOYED IN THE FAO/IAEA PHOSPHATE PROJECT D. MONTANGE CIRAD-CA GEC, Montpellier, France F. ZAPATA International Atomic Energy Agency, Vienna Abstract In the frame of the FAO/IAEA networked research project, the agronomic effectiveness of natural and modified phosphate rock (PR) products was evaluated using nuclear and related techniques under a variety of soil, climate and management conditions. In addition to the local soil analyses, it was decided to make a standard characterization of the soils employed in the project to gather direct and comparable information on the relevant soil properties affecting the suitability of PRs for direct application and to better interpret the results from the agronomic evaluation, including the creation of a database for phosphate modelling. This paper describes the standard characterization of soils, that was mainly made at CIRAD, Montpellier, France. A total of 51 soil samples were analyzed froml5 countries including Belarus (1), Brazil (2), Chile (3), China (20), Cuba (2) Ghana (6), Hungary (2), Indonesia (3), Kenya (1), Malaysia (1), Poland (1), Romania (2), Russia (1), Thailand (3) and Venezuela (3). Methods of analyses used for the soil characterization included textural class, pH, chemical analysis for total N and P, and exchangeable elements (CEC, saturation). Available P was measured using 4 methods including Olsen, Bray n, Pi paper and Resin. Available P measurements using resin method were made at CENA, Piracicaba, Brazil. The soil P dynamics was described using the 32P isotope exchange kinetic method at CEN Cadarache, France with the same soil samples. As a result of the worldwide distribution of the soils employed hi the project, the results showed a very large diversity in each of the measured soil characteristics. The analysis of the data focused on the most representative tropical acid soils, i.e. Ultisols and Oxisols. Inceptisols have also been included because most of them were acid and located in the tropics and subtropics. Results are synthesized and analyzed with particular emphasis on: i) identification of the most relevant soil characteristics affecting PR dissolution and their agronomic potential in tropical acid soils; and ii) suitability of methods for routine testing of available P hi these soils. The study demonstrates that pH and soil P status parameters, in particular P fixation capacity index and P concentration in aqueous extract are the main soil properties affecting PR dissolution hi these tropical acid soils. Additional related characteristics are clay content, organic matter content, and % Ca saturation. In the case of high P fixing soils, it would be also useful to have information on total and extractable Fe and Al. The methods of determination of available P were only relatively compared. In order to compare the P measured by these methods (extracting solutions or isotopic method) and the P actually available to the crop, yield data of field experiments are needed, as well as the response of the crops to the P fertilization. Further studies are needed with PR application to tropical acid soils to: a) establish relationships between crop yields and soil P supply in field experiments, and ii) group soils P supply into categories based on the probability of crop response to P fertilization. 1. INTRODUCTION Phosphorus (P) deficiency is widespread in acid soils of the tropics and subtropics. Also, most of these soils have a high P sorption capacity. Thus, they require substantial inputs of P fertilizers for adequate crop growth and optimum production of food and fibre. Manufactured P fertilizers like super- phosphates are mostly imported and supplies to the resource-poor farmers are limited and expensive. Many phosphate-bearing mineral deposits exist worldwide. Phosphate rocks vary widely in mineralogical, chemical and physical properties, and consequently in their reactivity and agronomic potential. Under certain conditions, direct application of phosphate rocks (PRs) is an agronomic and economically attractive alternative to the use of manufactured phosphatic fertilizers. Upon application 24 of a PR to a soil, both the rate of P dissolution and fate of the released P are affected by the soil properties determining the chemical reactions and the P demand by the growing crops. Among the soil properties, soil pH, exchangeable Ca, P-sorption capacity and organic matter are reported to be the main factors affecting the agronomic potential of PRs [1]. Recognizing the importance of overcoming main constraints to agricultural production in the tropics and subtropics, the Joint FAO/IAEA Division of Nuclear Techniques in Food and Agriculture established a research network of 21 participating scientists from developing and industrialized countries and implemented a project during 1993-1998 with a main objective to evaluate the agro- nomic effectiveness of rock phosphates through the use of P-32 and related techniques [2] Participating scientists represented benchmark sites with regard to the worldwide distribution of chemical infertile acid soils especially in the tropics and subtropics in order to test the agronomic effectiveness of indigenous and imported PRs under a variety of soil and crop conditions. As part of the project, participating countries performed a characterization of the soils and PRs utilized in their research. In addition, a standard characterization of the soils and PRs employed in the project was made with financial support of the World Phosphate Institute (IMPHOS) to gather direct and comparable information on the factors affecting the suitability of PRs for direct application and to better interpret the results from the agronomic evaluation, including the creation of a database for phosphate modelling. This paper describes the standard characterization of the soils employed in the project. Results are synthesized and analysed with particular emphasis on: i) identification of the most relevant soil characteristics affecting PR dissolution and their agronomic potential in tropical acid soils; and ii) suitability of methods for routine P testing in these soils. 2. MATERIALS AND METHODS A total of 51 soil samples from 15 countries participating in the FAO/IAEA Co-ordinated Research Project on "The use of nuclear and related techniques for evaluating the agronomic effectiveness of P fertilizers, in particular rock phosphates" were collected for chemical and physical analyses. All samples were surface (topsoil) soil samples used for routine soil P testing. The full list of soil samples grouped according to the geographical region of origin is shown in Table I. Their worldwide distribution was as follows: 27 samples (53%) were from Asia, 10 (20%) from Latin America, 7 (14%) from Africa and 7 (14%) from Europe. The routine physical and chemical analyses of the soil samples were carried out at the "Unite Sols et Eaux, Centre de Cooperation Internationale en Recherche Agronomique pour le Developpement" (CIRAD), Montpellier, France. All the analyses were performed on the "fine earth" fraction (samples passed through a 2 mm sieve). The determination of the physical and chemical properties was made following the French standard methods briefly described below [3]. 2.1. Particle size distribution Automatic granulometer GRANULOSTAT: Standard method AFNOR X 31-107. Destruction of the organic matter by wet digestion using hydrogen peroxide (H2O2). Soils were dispersed and suspended in sodium hexa-metaphosphate [40 g/1] and anhydrous sodium carbonate Na2CO3 [10 g/1]. Clay and silt particles were determined by sedimentation using a Robinson pipette. Sand particles separated from the clay and silt by a sieve and subsequently fractionated by wet sieving (rinsing the sample with water in a nest of sieves). The following particle size fractions were considered: Clay < 2 urn Fine silt (Silt F) 2 um-20 urn Coarse silt (Silt C) 20 um-50 urn Fine sand (Sand F) 50 |j.m-200 am Coarse sand (Sand C) 200 um-2000 u: 25 TABLE I. GEOGRAPHICAL DISTRIBUTION OF SOILS USED FOR FIELD EXPERIMENTS Geographical Country Location Soil Taxonomy Region Africa Ghana Abena Oxisol Typic Haplorthox Africa Ghana Ankasa Oxisol Typic Haplorthox Africa Ghana Ayinase Oxisol Typic Haplorthox Africa Ghana Boi Oxisol Plinthic Eutrodox Africa Ghana Kwaben Ultisol Typic Hapludult Africa Ghana Tikobo Ultisol Typic Hapludult Africa Kenya Kakamega Ultisol Typic Hapludult Asia China Changxing Inceptisol Typic Haplaquept Asia China Gaonan Aridisol Typic Calciorthid Asia China Guangzhou Inceptisol Typic Haplaquept Asia China Huazhou Ultisol Typic Hapludult Asia China Jingde Ultisol Typic Hapludult Asia China Jinhua Ultisol Typic Hapludult Asia China Jinxian Inceptisol Anthraquic Eutrocrept Asia China Kunming (red clay) Inceptisol Typic Eutrocrept Asia China Kunming (shale) Ultisol Typic Hapludult Asia China Liujiang Alfisol Typic Rhodudalf Asia China Taihe Inceptisol Anthraquic Eutrocrept Asia China Yangling Mollisol Typic Haplustoll Asia China Yingtan Ultisol Typic Hapludult Asia China Yiyang Ultisol Typic Hapludult Asia China Yongdeng Mollisol Typic Calciustoll Asia China Yuzhong Entisol Typic Ustipsamment Asia China Zhanjiang Inceptisol Typic Eutrocrept Asia China Zunyi 26 Ultisol Typic Hapludult Asia China Zunyi 27 Entisol Typic Udorthent Asia Indonesia Batu Marta Ultisol Typic Paleudult Asia Indonesia Pasar Jumat Ultisol Typic Kandiudult Asia Indonesia Pusaka Negara Inceptisol Endoaquept Asia Malaysia Rengam Ultisol Typic Paleudult Asia Thailand Kho