Extending the reach of crushed-rock fertilizers to Africa: Alternative potassium fertiliser using nepheline syenites from Malawi

Abstract

Africa has one of the most disadvantaged agricultural sectors in the world due to high poverty levels and high fertiliser import costs from the northern hemisphere. This study was conducted to assess the potential of nepheline syenites from Malawi, representing others from the rift tectonic settings of Africa, for their potential as potash sources. This was a multidisciplinary study which used a combination of remote sensing, airborne and field gamma-ray geophysics, and petro-geochemical techniques to assess the potential of nepheline syenite as fertiliser. Petrological and geochemical analyses of rock samples and soil samples were conducted for determination of K content and to confirm the presence of nepheline and other associated minerals within the rock and soil samples. Petro-geochemical results show the presence of nepheline in most of the samples and this agrees well with expectations from the geophysical and remote sensing digital terrain model results. The study has also discovered the presence of davidsmithite ((Ca,□)2Na6Al8Si8O32) an uncommon silicate mineral of the nepheline group, associated with the heterovalent replacement of Ca2+ for K+. Plant growth trials using this study’s nepheline syenite and crushed-rock from other parts of the world have shown that Malawi’s nepheline syenite is able to release K for plant growth. Although different intrusive complexes are not homogenous, the results show that, generally, nepheline syenites from Malawi have similar geochemistry to those in other parts of the world, some of which have been used as crushed-rock fertilisers. This thesis, therefore, provides an initial reference material on how geophysical and remote sensing techniques can be used to delineate nepheline syenite occurrences in the East African Rift System as a potassium source. This thesis further shows Malawi’s potential as a source of crushed-rock potash fertilisers. It also provides pilot information for further research in this area because agro-geology is a new discipline in both Malawi and Africa as a whole.