Impact of soil, plant, and fertilizer properties and processes on the dissolution and agronomic effectiveness of Moroccan phosphate rock: A thesis submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy at Lincoln University

Abstract

Acidic soils account for up to 40% of arable land globally and over half of cropland in Africa, and most of the latter are phosphorus deficient to varying degrees. The bioavailability of phosphorus in acid soil agroecosystems is severely restricted by irreversible conversion of soluble fertilizer phosphorus by reaction with aluminium and iron hydroxides. Over 70% of the world’s known phosphate rock reserves are located on the African continent (mainly in Morocco), and it is possible that direct application of sparingly soluble Moroccan phosphate rock may be more appropriate than water-soluble phosphorus as a fertilizer for use in impoverished acid soils in Africa. Previous research has demonstrated that “high reactivity” phosphate rocks from deposits in the Americas (Sechura, North Carolina) and Tunisia (Gafsa) are suitable and effective for direct application, while Moroccan phosphate rock is considered to be less reactive. The main objective of the research described in this thesis was to investigate the dissolution and bioavailability of Moroccan phosphate rock in acid soils using New Zealand high country grassland soil as an exemplar. This involved conducting a combination of laboratory incubation and controlled environment experiments to examine the effect of phosphate rock mineralogy, soil conditions (pH, water content, calcium status, phosphorus retention capacity), plant species (grass, legume), and fertilizer technology on the reactions and agronomic effectiveness of Moroccan phosphate rock. Results showed that the relative reactivity of Moroccan phosphate rock from different mines/deposits was more closely related to particle size than apatite mineralogy, and Khouribga-B was found to be the most reactive of the Moroccan phosphate rocks investigated. Significant phosphate rock dissolution only occurred when soil pH was less than or equal to 5.5 and soil water content exceeded 50% of water holding capacity. Phosphate rock dissolution and plant availability was very limited when soil pH exceeded 6, while optimum dissolution required a soil water content more than 75%. The agronomic effectiveness of phosphate rock was greater for a legume iv (blue lupin - 69% compared with water-soluble phosphorus) than for a non-legume (perennial ryegrass - 46% compared with water-soluble phosphorus). This was mainly attributed to enhanced mobilization and uptake of calcium by the legume, although increased root exudation of low molecular weight organic anions such as citrate and malate could have contributed. The inclusion of blue lupin as a short-term green manure crop significantly improved phosphate rock utilization by perennial ryegrass due to enhanced dissolution and uptake by lupin. This suggests that Moroccan phosphate rock is more likely to be an effective fertilizer for legume crops in Africa, although legumes could be employed indirectly to enhance the phosphorus nutrition of non-legume crops via their use as green manure crops. Co-granulation of Moroccan phosphate rock with single superphosphate was shown to enhance the agronomic performance of the phosphate rock which was attributed to acid generated because of proximal monocalcium phosphate dissolution within the granule. The findings of this study provide evidence that direct application of Moroccan phosphate rock has the potential to contribute to improving phosphorus supply and nutrition in African acid soil agroecosystems, although this remains to be verified by further field-based research in different environments.