Formulation and efficiency of mycorrhizal phosphatic fertilisers

Abstract

Phosphorus is an essential macronutrient for plant development. The direct use of rock phosphate (RP) as a source of P has several advantages: manufacturing costs are lower than those of acidulated phosphatic fertilisers, the energy footprint for manufacture is lower, and no harmful waste streams are associated with fertiliser production. However, direct application of RP has been shown to be rather ineffective agronomically in most conditions. With the growing need to increase agricultural production in a sustainable way and reduce the greenhouse footprint of agriculture, it is important to seek ways to improve the agronomic efficiency of RP applied directly to soils. This thesis aimed to increase the efficiency of RP by combining it with a beneficial microorganism, arbuscular mycorrhizal fungi (AMF), that is known to assist plants in acquiring P from deficient soils. This was achieved by copelletising RP particles with AMF spores with the aid of binders to formulate a potential biofertiliser. A survival analysis was first conducted to understand the viability of AMF spores in several different possible pellet binders, including powders, oils and hydrogels. Some binders were toxic to AMF spores and inhibited spore germination. Five of the binders were selected for subsequent plant growth trials under controlled conditions examining the effect of AMF and binder inclusion on plant acquisition of P from the RP pellets. Three pot trials were undertaken in total. Plants were harvested at the end of their growth period to examine effects of AMF spore inclusion and binder type on biomass responses, mycorrhizal colonisation of roots and changes in plant P acquisition. The first experiment using leeks as the test plant was not successful due to poor and slow plant growth, perhaps due to excessive evaporative losses of water from pots, which led to little root colonisation by AMF. Subsequent experiments using tomatoes and employing different pot and growth conditions were more successful. In these trials, the degree of root colonisation by AMF was quite low, even in positive controls using pure AMF inocula. However, pelletising the AMF spores with RP using specific binders was shown not to inhibit spore viability, and roots that were developed using the biofertilisers were able to be colonised by AMF. Compared to the application of pelletised RP, RP+AMF+binder pellets led to higher plant biomass and P uptake, but were still not as effective in alleviating P deficiency as a soluble P (positive control) treatment. The most effective binders were hydrogels that caused the RP pellet to fully disintegrate on wetting, and this may have assisted RP dissolution in soils and/or plant access to the P in the RP pellet. The relative benefits of AMF inclusion in the RP formulation versus those from using only the hydrogels were assessed in a final experiment with two pellet binders, methyl cellulose and sodium alginate. The results were different depending on the binder, since the inclusion of AMF spores significantly increased plant biomass and P uptake when using methyl cellulose, while there was little effect of mycorrhizal spore inclusion for sodium alginate treatments. This thesis provides a methodology for the preparation of biofertilisers utilising RP, AMF spores, and binder solutions. Evaluation of the novel biofertilisers demonstrated that plant acquisition of P from RP (and hence plant biomass in P-deficient soil) could be enhanced by the inclusion of the AMF and/or binders. Further research needs to be conducted to identify the right combination of AMF, RP, and binders to improve upon these novel biofertiliser formulations.