Phosphorus Use-Efficiency by Cotton Grown in an Alkaline Soil as Determined Using32Phosphorus and33Phosphorus Radio-Isotopes

32Phosphorus (P) and 33P radioisotopes were used to identify the contribution of soil and fertilizer P sources to P uptake by cotton (Gossypium hirsutum L.) grown in an alkaline soil representative of those used for growing cotton in Australia. Phosphorus fertilizer application only increased P concentration in the plants during leaf expansion, but had no effect on biomass production, P uptake at later growth stages sampled, or lint yield. However, at flowering, more than 50% of the P taken up was derived from within the fertilized band of soil, even though recovery of the 45 kg P ha−1 applied was low (1.2–3.4%). Phosphorus fertilizer application did not increase lint yield of cotton in this soil. Although P fertilizer recovery was poor, P fertilizer remained in an available form throughout the experiment indicating that pre-plant applications can remain effective. However, they will only be beneficial when soil P is limiting to crop growth.