Phosphorus fractions and their relationship to weathering indices in vertisols

The Ca-P, Al-P, Fe-P, occluded, organic, available and total P were estimated in pedons of seven soil series of Vertisols and their relationship was ascertained with the smectite content and crack volume. These were taken as mirror image of weathering. The Ca-P, available, organic and total P were higher in the younger Vertisols of Taswaria and Taswaria sodic series, derived from calcicgneiss complex, whereas the Al-P, Fe-P and occluded P fractions maintained their higher concentrations in the older Vertisols of Patan and Aklera series, originating from basalt. Vertisols of Chambal and Kota series, developed on basaltic alluvium, were intermediate between these two types. Variations in parent materials failed to account for the close resemblance of Vertisols of Bhatewar series, developed from gneiss complex, to that of Patan and Aklera series, derived from basalt, with respect to the fractions of phosphorus. Calcium-P, available, organic and total P decreased while Al-P, Fe-P and occluded P registered a reverse trend with increasing smectite content. However, the similar relationship did not hold good in Chambal and Kota series, having higher smectites and higher Ca-P, organic and total P as compared to the Vertisols of Bhatewar series. A plot of crack volume against forms of phosphorus indicated that the Ca-P, organic and total P decreased while the Al-P, Fe-P and occluded P fractions showed opposite trends with crack volume. The crack volume was a better index of weathering, representing not only the smectite content but also its complete profile, for explaining variations in phosphorus content. It may be used as a pedotransfer function for gauging the availability of phosphorus elsewhere in Vertisols.