The soil microbiome contains fungi which deliver key functions in all ecosystems. In agricultural contexts, a number of benefits relating to crop yield, health and morphology are directly derived from soil fungi. This has in recent years resulted in the commercial utilisation of such fungi as biostimulants, which are purported to deliver crop benefits. A key difficulty remains that efficacy and functionality of such products remains very difficult to quantify, and where beneficial effects have been observed, the modes of action responsible remain unknown. Much remains unknown concerning interactive effects between microbes, soil and crops. There is a tendency for existing studies to depend on a small number of growth indicators (chiefly yield), and in this thesis, a case is presented that future research should consider a greater number of morphometric variables and plant health indicators in plants treated with biostimulants in order to reveal more useful insights into the effects and efficacy of these products. In this study, biostimulant treatments applied to an experimental Lolium perenne crop were associated with significant increases in plant area, perimeter, height and biomass, and were also linked to improved crop phosphorus uptake in the leaves and roots. The most effective biostimulants studied consisted of mycorrhizal fungi and plant growth-promoting bacterial formulations biofixed to diammonium phosphate and zeolite carriers. These these growth benefits were largely attributable to mineral effects as opposed to the live component, but the results suggested a possible persistence effect of improved plant growth-promotion, since similar yields were obtained over the course of three non-destructive cuts undertaken over a three month period. Similar treatments on an Arabidopsis thaliana experimental crop were associated with similar morphometric results, but this time the crop P uptake was lower in biostimulant treated crops than in the controls, suggesting a possible metabolic cost to the plant associated with the treatment. Finally, a high-throughput DNA metabarcoding method was used to examine fungal community structure shifts associated with sustainable intensification approaches in a field trial to contextualise the findings of the previous chapters, and examine the effects on soil fungal communities of representative management approaches anticipated in the future that will involve biostimulants. The approach was also able to demonstrate divergent microbial communities, increased AMF abundance with sustainable intensification approaches and an increase in the relative abundance of economically important plant pathogens. Overall, while potential exists for biostimulants in agriculture, much work is necessary to improve consistency, viability and the knowledge base before they can be used with confidence.