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 “Unité Sols et Eaux, Centre de Coopération Internationale en Recherche Agronomique pour le Développement”

(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/l] and anhydrous sodium carbonate Na₂CO₃ [10 g/l]. 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 µm Fine silt (Silt F) 2 µm–20 µm Coarse silt (Silt C) 20 µm–50 µm Fine sand (Sand F) 50 µm–200 µm Coarse sand (Sand C) 200 µm–2000 µm

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TABLE I. GEOGRAPHICAL DISTRIBUTION OF SOILS USED FOR FIELD EXPERIMENTS Geographical

Region

Country Location Soil Taxonomy

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 Hong Ultisol Kandiudult

Asia Thailand Pak Chong Ultisol Oxic Paleustult

Asia Thailand Warin Ultisol Oxic Paleustult

Europe Belarus Minsk Spodosol Sod Podzolic

Europe Hungary Kompolt Mollisol Typic Hapludoll

Europe Hungary Szentgyörgyvölgy Alfisol Typic Ochraqualf

Europe Poland Pulawy Histosol Eutric Histosol

Europe Romania Albota Alfisol Typic Agrudalf

Europe Romania Sanandrei Mollisol Typic Agrudoll

Europe Russia Bryansk Spodosol Sod Podzolic

Latin America Brazil Piracicaba 1 Oxisol Hapludox Latin America Brazil Piracicaba 2 Oxisol Hapludox Latin America Chile Metrenco Ultisol Paleo Humult Latin America Chile Pemehue Inceptisol Distrandept Latin America Chile Santa Barbara Andosol Haploxerand Latin America Cuba Ciego di Avila/Epica Alfisol Orthic Paleustalf Latin America Cuba Ciego de Avila/Univ Alfisol Orthic Paleustalf Latin America Venezuela El Pao Ultisol Typic Paleustult Latin America Venezuela Valle La Pascua Ultisol Typic Paleustult Latin America Venezuela Yaracuy Ultisol Typic Paleustult

27 2.2. pH

The pH in H2O was determined in a suspension of soil and water with a ratio of 1:2.5 (soil:water).

The suspension was stirred from time to time and pH measured after 2 hours of equilibration with a potentiometer. The pH in KCl was measured in the same way as the pH in water, except 1 M KCl solution was used instead of H₂O.

2.3. Total carbon

Following standard method NF ISO 10694 of June 1995: "Dosage du carbone organique et du carbone total après combustion sèche (analyse élémentaire)". Dry combustion (950°C with oxygen) of the sample. Resulting CO₂ is analysed using an infrared cell. (Automatic Analyser LECO CHN 600).

2.4. Total nitrogen

Oxidation of the sample in an induction oven following standard method NF ISO 13878 of July 1998:

"Détermination de la teneur totale en azote par combustion sèche (analyse élémentaire)" and resulting N is analysed using a thermal conductivity cell. (Analyser LECO PF 428).

2.5. Total phosphorus

Dissolution of the sample ("Mise en solution totale par attaque acide") was made according to the standard NF X 31-147 of July 1996 and analysis ("Dosage du phosphore") according to the standard NF ISO 11263 of July 1995. Digestion of the sample with HF/HNO3 was performedin a microwave oven. Phosphorus was analysed using a colorimetric method (Molybdenum blue) with an automatic analyser Alliance Integral.

2.6. Cation exchange capacity

The CEC method used hexamine-cobalt trichloride (Co (NH₃)₆ Cl₃) [4]. Soil (3.5 g) was shaken in 70 ml of solution of hexamine-cobalt trichloride for 2 hours. The concentration of this solution should be adjusted taking into account the forecasted CEC of the sample between 2 and 5 CEC. The pH is measured on an aliquot of the solution after centrifugation after filtering the remaining suspension. Ca, Mg, K, Na, Al and Mn are analysed in the filtrate using ICP (Inductively Coupled Plasma). The CEC is calculated after titration of the cobalt remaining in the solution.

2.7. Available phosphorus

2.7.1. P Olsen [5] and P Bray II [6]

Bray II and Olsen methods were chosen as reference chemical extraction methods for the project and performed on the soil samples received.

For Olsen P, 5 g of soil are suspended in 100 ml of sodium bicarbonate 0.5M NaHCO3, pH = 8.5 and the suspension shaken for 30 minutes. The P content in the filtrate is analysed using colorimetric method (Molybdenum blue), with an automatic analyser Alliance Integral.

For Bray II P, 2 g soil mixed with 14 ml of the P extracting solution consisting of 0.03 mole/L sodium fluoride (NaF) and 0.1 mole/L of HCl and shaken for 40 seconds. After filtration, the P content is analysed using a colorimetric method (Molybdenum blue with boric acid) with an automatic analyser Alliance Integral.

2.7.2. P iron strips, Pi [7, 8]

In this method iron hydroxide impregnated paper strips are utilised as sink for phosphate ions in solution. It is a method to testing for available soil phosphorus developed at IFDC. The latest modified version was used for the analyses [8].

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2.7.3. P resin [9]

The analyses for available P by the resin method were performed on some samples at the Centre for Nuclear Energy in Agriculture (CENA), Piracicaba, Brazil, thanks to the collaboration of T. Muraoka.

Results are given in mg P/dm³ of soil.

2.7.4. ³²P exchange kinetics method [10]

J.C. Fardeau made the analyses by the ³²P exchange kinetics method at the “Centre d’études nucléaires” (CEN), Cadarache, France. The E₁ value or quantity factor, i.e. the amount of isotopically exchangeable phosphate within one minute, was utilised as index of available soil P.

3. RESULTS AND DISCUSSION