<p>Intercropping may increase and stabilise crop yield in heavy Vertosol soils and may assist in the uptake of phosphorus (P) nutrient. Phosphorus niche complementarity is the supposition that there is decreased competition due to complementary use of available P and niche distinction among intercropped species in the form of a given P, in space or in time. Phosphorus deficiency and its unavailability for crops commonly limits crop yield in these soils. Thus, intercropping may be an appropriate means to improve the productivity of low input cropping systems in these soils. </p> <p>This study quantifies biomass yield performance and competition effects, and the capture and utilisation of resources by summer cereals and legumes as sole and intercrops. The principal novelty of this research work reported in this thesis was to investigate the effect of intercropped species (i.e. legumes and cereals) and how phosphorus fertilisers controlled the growth and P uptake (e.g. by sorghum and soybean) of these fertilisers. Thus, both P acquisitions and growth performance processes were evaluated under the interactions with the level of moisture content changes in black vertosol. To address this aim, a one field experiment and two glasshouses experiments assessed P fertiliser and water content on intercrops and corresponding sole crops of soybean and sorghum in irrigated black Vertosol soil. Several methods were used to analyse and present the results of each crop performance, such as univariate analyses, bivariate analysis, indices and graphical methods.</p> <p>The study covered three aspects of P fertiliser in cropping systems. Firstly, in the field two rates of P fertiliser, 0 kg ha^-1 (P0) and 20 kg ha^-1 (P20), were studied, and intercropped sorghum with soybean was compared to sole crops in irrigated black Vertosol soil. Biomass yield (BY), P yield (PY) and nitrogen yield (NY), which are the measure of competition and complementary efficiency, were used for comparison of the relative performance of sorghum and soybean as sole crops and intercropped. The BY, PY and NY were higher by 39%, 48% and 47%, respectively, in intercropped soybean with sorghum than in sole soybean for both P rates, but their value was higher in P20. However, soybean was not aggressive to sorghum; the BY and NY of sole sorghum and mixed sorghum were still on par, while PY was higher by 77% at P0 and 86% at P20 in intercropped sorghum with soybean than sole sorghum. When these results were combined with waterlogged conditions, there were indications of low competition between intercropped species and sorghum appeared to facilitated nitrogen (N) and P uptake by soybean in these conditions.</p> <p>The second phase of the study examined P utilisation and plant growth in a sorghum/soybean intercropping system compared to individual soybean and sorghum plants in a moist Vertosol soil. Soybean was either inoculated with Rhizobium or N fertiliser, and sorghum was N fertilised at 50 kg/ha. One glasshouse experiment was conducted in root boxes (two plants per box). Phosphorus fertiliser was applied between the plants for the sole treatments and half of the mixed treatments and under each plant for the other half of them. Phosphorus fertiliser was tracked using ³²P and ³³P isotope methods. Generally, the water depletion in sorghum was higher and faster than soybean in moist conditions. Intercropping soybean with sorghum in moist Vertosol soil increased its biomass yield and P content in the green parts, thereby improving its performance. Sorghum was more competitive for P fertiliser than soybean when the P fertiliser was added between plants. However, soybean with N fertiliser was unaffected by sorghum competition on P fertiliser when it was applied under the plants. Overall, water uptake by mixed sorghum/soybean roots in moist Vertosol soil appeared to affect the water status of soybean roots, thus increasing their growth and nutrient uptake. </p> <p>Phosphorus utilisation and plant growth in sorghum/soybean combined in pots were compared to separate individual soybean and sorghum plants with different irrigation patterns in the third phase, a single glasshouse study. Three watering patterns were dry, wet and fluctuation from wet to dry (DW). Here, it was demonstrated that biomass yield and P parameters for sole soybean were similar to soybean planted with sorghum in three irrigation patterns. Similarly, the biomass yield and P parameters of sorghum were unaffected by planting pattern in dry and DW irrigation patterns. However, the biomass, P content and P utilisation of mixed sorghum exceeded that of sole sorghum in wet irrigation treatment. Attributes of mixed sorghum were higher than mixed soybean in all treatments. Overall, wet treatment conditions had increased growth and P utilisation of mixed sorghum. In contrast to the previous two experiments, wet and DW conditions had no effect on mixed soybean. It is possible that soybean and sorghum were competing for nutrients in the limited soil of the small pots compared to field and root boxes. In addition, the soil did not flood in the pots because of the high infiltration rates of short pots compared to the other two experimental methods. </p> <p>The findings of these studies indicate the potential for higher productivity and P utilisation of mixed soybean with sorghum in moist black Vertosol by utilising carefully planned water management. Further research is needed in multiple years or locations, and repeated glasshouse studies.</p>