Phosphorus (P) is one of the most limiting and important elements in crop production, yet often has limited availability in the soil. Manufactured inorganic P fertilisers are required to improve soil and crop P supply but their use depletes finite reserves of rock phosphate and impacts on water quality and ecosystem biodiversity. Bio-inoculants have a potential role to increase soil P supply and reduce dependence on expensive fertilisers. The objective of this thesis was to further understand the role of mycorrhizae (M) and P solubilizing bacteria (PSB) bio-inoculants and external P sources (super phosphate (SP), struvite (AMP) and rock phosphate (RP)on phosphorus availability in soil and their effects on the growth, yield and P uptake of barley. Field experiments on low P status soils in 2010 and 2011 demonstrated the potential for the use of bio-inoculants (PSB and M) in mobilizing P from soil and significantly (P <0.05) enhancing P uptake to increase growth of barley, and to a lesser extent, grain yield. It was postulated that bio-inoculant effects in the field were compromised by the presence of native M and PSB. Glasshouse pot experiments were conducted with a range of growth media: horticultural sand (zero P), field soil (low P status but with native micro-organisms) and heat sterilized field soil. These demonstrated the effects of bio-inoculants without the presence of native M and PSB, and to a lesser extent in the presence of native micro-organisms, in terms of increased plant root and shoot growth, grain yield and tissue P concentration. Across all experiments bio-inoculants (M and PSB) increased the effectiveness of water soluble SP, partially soluble AMP and insoluble RP. M and PSB were equally effective. In combination with these external P sources, bio-inoculants (M and PSB) significantly (P < 0.05) increased yield, P concentration and total P uptake, plant dry matter and concentration of P in the grain compared to P fertilizers without bio-inoculants. However, applications of P fertilizers reduced the colonization of roots by mycorrhizae. The potential role of P uptake enhancing bio-inoculants in reducing external inputs in agriculture is discussed.