Soil micro-organisms are a fundamental component of soil ecosystem services. Plant yields have shown to be increased through processes mediated by fungi, such as increased acquisition of important plant nutrients e.g. phosphate (P) and nitrogen (N). This has seen micro-organisms exploited commercially to create bio-inoculants (BIs). However, it is remarkably difficult to determine the effectiveness of commercially-available BIs that claim to promote crop yields as in most cases the underlying mechanisms responsible for these beneficial effects are unknown. The aims of the thesis were to examine, both within the laboratory and field, the efficacy of some commercially available BIs. Focusing on mechanisms of increased P acquisition mediated by the application of BIs. The effects of soil-P availability on BI performance were also explored. Whilst next generation DNA sequencing was utilised to explore changes in soil fungal assemblages after the introduction of BIs. As illustrated in Chapter 2, there exists much variability in efficacy testing and there is a distinct lack of robust field-based testing of commercial BIs. Chapter 3 consisted of two laboratory-based pot-trials; the first investigated the efficacy of five commercial BIs on Lolium perenne growth, the second explored the effect of an inert carrier media utilised in BI manufacture. Results found that all tested BIs increased grass yields significantly, and while many BIs contain non-living additives, treatments with living microbial fractions were found to have significantly more roots, leading to increased growth per unit P taken up by the grass. The second bioassay found the dual application of carrier media and mycorrhizal spores significantly increased grass yield, the inert carrier media a significant factor with respect to mycorrhizal root colonisation, increasing from 20% to 36%. Chapter 4 explored the phosphate solubilisation and mineralisation potential of the BI products. All BIs successfully mobilised P from recalcitrant P sources (ranging from 164 - 490 mg l-1, for inorganic-P and 0 - 39 mg l-1 for organic-P). A pot trial investigated the phosphorus efficiency ratio of Lolium perenne following application of two inorganic fertilisers of varying solubility, triple super phosphate and rock phosphate. Two of the tested BIs were found to exhibit P mediated growth gains in the form of increased yield, total shoot P, and phosphorous efficiency ratio. Yield and shoot P gains were found to be mediated by differing fractions of the living component of each BI dependent on P source. Chapters 5 and 6 were field trials exploring BI effectiveness on sites of varying P availability. Chapter 5 was on a soil of adequate P (21 mg kg-1). Overall the studies found limited benefits of BI application. Chapter 6 consisted of two field trials, one with a range of P availability (3.3 - 32 mg kg-1), the second with low P availability (8 mg kg-1). Results of the first found positive correlations with the % P and total forage P with increasing plant-available soil-P of BI-treated grass, but no significant increases in yield. The increased P acquisition and lack of yield would suggest that yield limitation was not driven by P. Similarly the results of the second trial found that while some of the BI treated grass yielded moderately higher than controls, there were no significant treatment effects on % P and total forage P, again suggesting yield gains were due to other factors. All BI treatments found to increase the N content of the forage compared to control. Chapter 7 examined the effects of various management practices on soil fungal abundances. Using DNA sequencing, fungal assemblages were found to be significantly affected by both treatment (BI application and N fertiliser), soil type and sampling date. N fertiliser was the only treatment to significantly affect fungal diversity and equitability measures. The study was able to show the potential of NGS technology, Ion Torrent™, for examining changes in fungal communities within the field. The results suggest soils with adequate levels of plant-available P may not see much benefit to warrant the application of BIs, or at least at the application rates recommended. Overall, much work is still required both within science and industry in the development and manufacture of bio-inoculants as a reliable method to increase crop yields