Effects of prolonged shaking periods and contents of iron oxides on dispersion of plinthitic soil in Nigeria

The dispersion of sesquioxide-rich or plinthitic soils is difficult and not completely achieved within routine dispersion period. Two prolonged shakin g periods of 24 and 48 hours were tested to see which gives a more complete dispersion of two plinthtic soils-a drier Plinthustalf and a more humid Pl inthustult. Iron oxides which are the main aggregating agents in plinthite were also determined and the iron ratios calculated to relate their content s to the rate of dispersion of the soils. Results showed that prolonged sha king for 24 hours gave greater dispersion which changed the textural classi fication of the soils from the routine time (5 minutes shaking) to finer te xtural classes. Shaking for 48 hours increased the clay content significant ly (P = 0.001) over that of the 24 hours but textural classification did no t change further. The soil clay was aggregated mainly into silt-size partic les as the clay content increased at the expense of the silt fraction on pr olonged dispersion period. Both the amorphous and crystalline inorganic iro n (Fe) oxides were responsible for aggregation in the soils. and their effe cts were enhanced by desiccation. Dispersion was therefore more difficult i n the drier Plinthustalf than in the more humid Plinthustult.