High gradient magnetic separation of some soil clays from Nigeria, Brazil and Colombia.

Summary Phosphate sorption characteristics of the tropical soil clays described in Part I were determined. Estimates of native labile phosphate in the clays were similar to those calculated from whole soil data but under similar experimental conditions clays tended to sorb more exchangeable and less non‐exchangeable phosphate than whole soils. This may imply that short‐term measurements in soil underestimate the proportion of added phosphate that is labile and hence underestimate potential residual responses to fertilizer. Relationships of sorbed phosphate with iron (Fe ox ) and aluminium (Al ox ) extracted by acid‐ammonium oxalate and with carbon differed between clays from Nigerian soils and those from South America. Exchangeable phosphate was related to Al ox in both groups and to Fe ox in the Nigerian clays. In both groups of clays non‐exchangeable phosphate was related positively to Al ox but its relationship with carbon was negative in the South American clays and positive in those from Nigeria. These relationships are compared with those in whole soils in which the ratio of metal to carbon in organic‐metal complexes is important.