AVAILABILITY OF NICKEL ALONG A TOPOSEQUEN CE OF ULTRAMAFIC SOILS IN NEW-CALEDONIA

Readily available nickel concentrations extracted by KCl M and DTPA 0. 005 M + CaCl2 0.01 M at pH 5.3 were determined for two types of oxidic soils from a toposequence on peridotite. The first soil type belongs to a piedmont formation and the second is of alluvio-colluvial origin. Results indicate that the quantities of nickel extracted are nearly n on-existent in the first soil type but may attain 120 mu g g(-1) (NiDT PA) in the second. The greater availability of nickel in the second ca se may be attributed to 3 factors: (i) a greater abundance of goethite and an absence of haematite, (ii) the existence of reducing condition s and (iii) the presence of weatherable silicate minerals. Nickel phyt otoxicity may have a detrimental effect on plants not adapted to these edaphic conditions.