The low responsiveness of cotton (Gossypium hirsutum L.) to phosphorus (P) fertilizers in the field where soils have low soil test values for P, suggests that cotton is capable of utilizing soil P pools that are less labile or are below the depth of P fertilizer placement. The objective of this thesis is to understand how the cotton plant acquires P from less labile and subsoil P pools.

The first experiment was conducted using rhizoboxes to examine whether cotton is physiologically efficient at acquiring less labile P pools, through comparison with wheat (Triticum aestivum L.) and white lupin (Lupinus albus L.). Under P-deficient conditions, cotton released negligible carboxylates into the rhizosphere. The P depletion zone of cotton from rhizosphere soil was less than 2 mm, and predominantly from the relatively labile NaHCO3-Pi and NaOH-Po pools. In contrast wheat and white lupin markedly depleted the NaHCO3-Pi and less labile HCl-P pools, and the depletion zone extended to 3 mm. The depletion of NaOH-Po pools by cotton was associated with increased activity of phosphatises in the rhizosphere soil.

A second sand culture experiment investigated the ability of cotton to access P from the sparingly soluble P sources of AlPO4, FePO4 and hydroxyapatite. The experiment revealed that cotton was inefficient in accessing P from all these sparingly soluble P sources. In contrast, wheat showed an outstanding ability to use AlPO4. The 32P reverse dilution technique was then used to determine the variation between these species in accessing sparingly soluble AlPO4 and hydroxyapatite in a Vertosol when N was supplied as different forms. The inefficiency of cotton to access P from these sources was again documented. The percentage of hydroxyapatite recovered by the cotton plant was 8 and 10 times lower than that recovered by wheat and white lupin respectively, while the availability of AlPO4 to cotton was 10 times lower than wheat but 3 times higher than white lupin. Addition of N as NH4-N elevated total plant P uptake from all P sources except in the case of wheat fed with Al-P. The fate of these P sources in the soil was examined in a separate incubation experiment Sparingly soluble Al-P was more ‘labile’ in Vertosols, in comparison to FePO4 and hydroxyapatite which showed little change in availability with time.Hydraulic lift can be a desirable characteristic for P acquisition by crops growing in areas that experience frequent drying of the topsoil. A glasshouse experiment using specialized soil columns tested whether cotton could hydraulically lift water and whether this hydraulically-lifted water facilitates P acquisition from dry topsoil. Hydraulic lift was detected in a Vertosol for 7 days after water was withheld from the topsoil. However, the hydraulic lift detected for the cotton did not aid P acquisition from the dry topsoil. Therefore, it appears that if the topsoil dries out, P uptake by the cotton plant would rely mainly on P acquisition from P sources in the subsoil.

The contribution of subsoil P sources to plant uptake was investigated in two field surveys. Soil samples at various soil depths were collected from continuously-cultivated cereal and cotton paddocks, and corresponding virgin sites, in north-western NSW. Regardless of P fertilizer history, organic and residual P pools at depths of 0 to 45 cm had been invariably depleted following long-term continuous cotton and cereal cropping. With a negative Pbalance, the HCl-P pool below 10 cm also declined significantly. Thus, routine soil P tests, using alkaline bicarbonate extraction solutions with soil samples collected from the topsoil layers (above 10 cm), were less accurate in estimating P responsiveness of cotton to P fertilizers applied on Vertosols. The effect of cropping system, expected from species ariation in root morphological and physiological characteristics, on the soil P accumulation and depletion profile was undetectable.

The results suggest that the low responsiveness of cotton to P fertilizers can be attributed to a number of causes. These include the utilization of organic P pools, subsoil P exploration or possibly mycorrhizal symbioses, rather than any efficient P acquisition from sparingly soluble inorganic P pools in the topsoil. Further research should focus on accurately estimating the contribution of organic P and subsoil P pools to P uptake by cotton plants.