ORSTOM U R 2B c/o DPVE, *CEA, DPVE, CEN Cadarache, 13 108 Saint Paul 16s Durance, France, f CIRADjIRAT, BP 1163, Lomk, Togo and SORSTOM UR ID, 70 route d'Aulnay, 93 143 Bondy cedex, France S U M M A R YPotassium release and fixation were studied on Ferralsols (typic Eutrustox) of Southern Togo both by isotopic exchange with 42K+ ions and by exchange with 1 M ammonium acetate. Experiments were conducted on soil samples taken from surface (0-30 cm) and subsoil (80-100 cm) layers of four plots. These plats, with the same soil type, were subjected to various kinds of use: forest and crops, with or without fertilizers. A single wetting and drying of the soil samples increased the amounts of exchangeable K. This exchangeable K release could stem from clay protonation during drying. However, the K release is insufficient to allow permanent intensive agriculture without K fertilizer addition. Soil samples were therefore treated with KCI in order to study K fixation. About 20% of the applied K was fixed and the percentage increased with wetting and drying. The fixation can be explained by the presence of about 2% of interstratified clay minerals in these kaolinitic soils. Comparison of the kinetic characteristics of the isotopically exchangeable K with the amounts of ammonium exchangeable K revealed the existence of several kinds of exchange site for K ions.
This paper deals with the pedogenesis, the classification, the characteristics and the geography of New Hebrides soils, and lastly with some relationships between these soils and the vegetation. The following pedogenetic features are emphasized: the New Hebrides soils derive mainly from basic volcanic rocks. They are often young or frequently rejuvenated by volcanic ash. They are differentiated in three main groups, according to a climatic sequence due to the tradewinds: ferrallitic soils in the wetter southeast part, fersiallitic soils in the drier northwest part, and andic soils on the 'perhumid’ highlands. The wetter the climate or the younger the volcanic ash, the more andic are the soils. A correlation is attempted between the soil units of the French pedological classification and those of the soil map of the world by the F. A. O. Next, the characteristics and the geographical distribution of the thirteen main groups of New Hebrides soils are briefly outlined. Finally, some relationships between some soils and the native vegetation are briefly reported.
Abstract. - Two soils, one from Aoba (New Hebrides), the other from Tenerife (Canarian Islands), the rejuvenation of which by a fall of volcanic ashes has been dated (by 14 C), show that, in a humid tropical or subtropical environment, the recent rejuvenation leads to pedological forms relatively less maturated than the normal soils type. In Aoba, they are one thousand years old, saturated and slighty evoluted andosols, in Tenerife appear andic brown earths that are less than 9000 years old and bury tropical paleosols. It is noteworthy in Aoba, that only one thousand years has been enough to form halloysite, whereas Aomine and Miyauchi (1963) estimated tah 8000 years have been necessary in the case of the Japanese andosols.
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