<p>Mycorrhizal symbioses are considered the most important mutualism on Earth. This symbiosis occurs between plant roots and mycorrhizal fungi in the rhizosphere. Mycorrhizal hyphae facilitates the exploration of a greater volume of soil by plant roots and increased water and nutrient uptake, especially phosphorus (P) which is immobile in the soil. This consequently improves plant growth, particularly at early growth stages. Cotton (<i>Gossypium hirsutum</i> L.) is a mycorrhizal dependant crop and mycorrhization clearly improves early growth and nutrient uptake. Although mycorrhizal fungi also alleviate abiotic stresses, adverse abiotic soil condition might restrict mycorrhizal colonisation and associated nutrient uptake. One possible adverse environmental condition is elevated soil sodicity. In Australia the majority of cotton growing regions are sodic. Sodicity creates adverse physical and chemical conditions, including waterlogging, hard-setting, high bulk density, high pH, and high soil solution sodium (Na), which may affect mycorrhizal colonisation of cotton plants.</p> <p>This thesis aimed to investigate the percentage of root length colonised by mycorrhizae and nutrient uptake of cotton in a range of naturally sodic soils. This thesis also attempted to estimate the relative hyphal contribution to early P uptake of cotton in sodic soil conditions.</p> <p>A series of glasshouse experiments was conducted in order to assess mycorrhizal colonisation and a number of different colonisation techniques in moderately (ESP 10– 15) and highly (ESP>15) sodic soils. Standard techniques to inoculate cotton plants in sodic soils were unsuccessful until live hyphal cultures were introduced. </p> <p>In a separate glasshouse experiment, changes in mycorrhizal colonisation and nutrient uptake of cotton in a range of naturally non-sodic and low-sodic soils from cotton production areas in southern Queensland and northern New South Wales, with different exchangeable Na percentages (ESP) (ranged between 1.4 and 9.8) was investigated. Linear mixed model analysis showed minimal effects of sodicity, when ESP was less than 10, on mycorrhizal colonisation, associated plant growth and nutrient uptake. Principle component and regression analysis showed that other sources of variation including soil pH and soil extractable P, rather than sodicity, might drive cotton colonisation in Vertosols with low to moderate ESP. The colonisation percentage was positively linearly correlated with P, Mg, and Zn uptake of cotton plants.</p> <p>An isotope experiment was established to assess the mycorrhizal colonisation and nutrient uptake of cotton plants under highly (ESP 21) and less (ESP 7) sodic soil conditions with two rates of applied P. The relative hyphal contribution to P uptake was quantified using dual isotope labelling techniques (³²P and ³³P). Root colonisation and P uptake of mycorrhizal cotton plants reduced by 16% and 20%, respectively, in highly sodic soil as compared to plants in low sodic soil, however, the relative proportion of P delivered via hyphal pathways (³²P from root-free hyphal compartment) was similar. Under high P conditions, the relative increase in the proportion of ³³P (root + hyphae compartment) taken up by inoculated plants was greater in the low sodic soil relative to the high sodic soil. Mycorrhization improved early seedling vigour, and nutrient uptake. </p> <p>Overall, these results confirmed that early growth and nutrient uptake of cotton, especially P and Zn, benefits from mycorrhizal association in both non-sodic and sodic soils. However, mycorrhizal colonisation establishment in sodic soils is not as straight forward as in non-sodic soils. In the absence of fresh hyphal material, reliance on spore germination to colonise cotton roots might be unsuccessful in moderately- and highlysodic soils. Therefore, the presence of a fresh mycorrhizal hyphal network within the inoculum source may play an important role in initiating colonisation of cotton roots in sodic soils with ESP greater than 15. </p> <p>These results indicate that higher levels of sodicity restrict mycorrhizal colonisation of cotton. Reduced colonisation and hyphal exploration of the soil, possibly due to the physical and chemical constraints imposed by highly-sodic soil, rather than poorer mycorrhizal function, might be one of the responsible factors for limited early P uptake of cotton in highly-sodic soil. However, mycorrhizal colonisation and associated nutrient uptake (P, Zn, Ca, Mg, K, Mn) of cotton was not dominated by sodicity and associated physical/chemical conditions (waterlogging, high bulk density, high pH, high soil solution Na) occurring in soils when ESP is less than 10. Further investigation into mycorrhizal spore density, species diversity and mycorrhizal proliferation under sodic soil conditions is warranted.</p>