Abstract

Investigations conducted on Malawi soils ranging from highly weathered to less weathered soils show that P build-up due to fertilisation is much higher in soils managed by commercial estates than in soils managed by smallholder farmers. This suggests a need to monitor potential environmental impacts of the P build-up in the soils. The P build-up may be monitored using soil P tests used as a basis for P fertilisation. The present investigations have shown that the efficacy of selected soil tests in predicting P uptake was in the following increasing order: Olsen P > Nelson P > Bray P1=Resin P > Mehlich 3 > (0.5pCa + pH₂PO₄)eq. However, when potentially mineralisable soil organic P  was included in the regressions of P uptake on each of the soil P tests, the accuracy of the soil tests was in the following increasing order: Bray P1 > Olsen P > Mehlich 3 > Nelson P > Resin P > (0.5pCa + pH₂PO₄)eq. The effectiveness of Bray P1 extraction method may be attributed to the acidic nature of the extractant (pH 2.6), which increases the activity of phosphate ions in soil solutions following dissolution of Ca-, Al-, and Fe-bound P forms and both the complexation of Al and Fe by humic substances and the precipitation of Ca (CaF₂) by the fluoride (F−) component of the extractant. Mehlich 3 is used in Malawi as a basis for P fertilisation. It could also be used for monitoring potential environmental impacts of P build-up in Malawi soils.

Key words: Phosphorus status, extractants, ferruginous oils, ferralitic oils, alluvial soils, potentially mineralisable soil organic P.