The effect of P and AMF on cotton yield and yield components of cotton Honours thesis The soils of the Ord River Irrigation area (ORIA) are extremely old and naturally low in available Phosphorus. The production of cotton in the area requires the application of fertilisers to provide a profitable yield. The P requirement for cotton has been studied extensively with critical limits of P reported to range from 5 to 12 mg/kg with Phosphorus availability declining as soil pH increases. An optimum of 80 kg/ha of double superphosphate (17.5% P) was required on virgin soil to meet the nutrition requirements of a dry season cotton crop grown in the ORIA. Breaking down the yield into yield components helped to provide an understanding of which components contributed most to the increase in yield. The increase in lint yield is primarily attributed to bolls/m2 and to a lesser extent by lint/boll. Percentage fruit retention contributed most to the increase in bolls/m2 and lint/seed contributed most to the increase in lint/boll. The number of fruiting sites and seeds/boll do not contribute much to the increase bolls/m2 and lint/boll, respectively. P was the most significant treatment in the glasshouse study. Plants which had P applied had higher P uptake and dry weights. AMF did not contribute to growth or P uptake in the glasshouse study.