Experiments were carried out under controlled conditions to investigate the physiological bases for species differences in yield and nutrient responses to variations in phosphorus supply. Buffel grass (Cenchrus ciliaris), and to a less extent Mitchell grass (Astrebla elymoides), showed a much larger yield response to increasing phosphorus supply than mulga grass (Thyridolepis mitchelliana). Mitchell and mulga grasses had much lower relative growth rates than buffel grass. Mulga grass required a lower external phosphorus concentration for optimal growth than Mitchell and buffel grasses; this was attributed to its superior system for absorbing and transporting phosphate from low concentrations, but was not associated with any yield advantage, yield being related more to the photosynthetic than to the nutritional characteristics of the plants. Differences between species in their external phosphorus requirements for growth and their distribution in semiarid Queensland are discussed.

The effect of the external phosphorus concentration on the pattern of root development in mulga, Mitchell and buffel grasses was examined. During seedling and early vegetative growth, the pattern of root growth suggested that the emergence of axes, their extension and branching were coordinated in such a way that the mean rate of extension and distance between branches remained fairly constant for each component of the root system. Phosphorus deficiency did not greatly affect the length of the seminal axis or the number of its primary laterals, but resulted in a significant reduction in the length of these laterals; the length and number of nodal roots were also greatly reduced. Species differences in the pattern of root development are discussed in relation to seedling drought survival on infertile soils. *Part I, Aust. J. Agric. Res., 26: 423 (1975).

The influence of phosphorus application on the growth and drought survival of buffel grass on a sandy red earth in south west Queensland was studied in a series of pot experiments. An omission nutrient trial showed phosphorus was the principal nutrient limiting seedling growth, the severity of the deficiency increasing with time. A further pot experiment showed that at rates greater than 24 kg P ha-1 luxury consumption occurred. The critical phosphorus concentration for the plant was estimated to be 0.26 per cent. The available (0.01 NH2SO4 extractable) soil phosphorus concentration corresponding to this critical tissue concentration was 25 p.p.m. Beyond the 2-leaf seedling stage, the seedling depended increasingly on external phosphorus for growth. Under conditions of phosphorus deficiency, phosphorus absorption rates were insufficient to maintain tissue phosphorus at the concentrations necessary for healthy growth, and relative growth rates were low. In the presence of phosphorus, absorption and growth rates increased. Maturity of phosphorus deficient plants was characteristically delayed but with little yield loss. The addition of phosphorus increased seedling growth rate, root development and drought survival. The depth of root penetration is considered to be the major factor responsible for survival.

Changes in tissue nutrient concentration and nutrient uptake of a C3 native grassland and a sown C4 buffel grassland community, growing on infertile sandy red earth soils near Charleville, were studied. The seasonal pattern of litter production of grasslands in the grazed and ungrazed situation was also investigated. Studies in litter decomposition of Mitchell grass are reported. The decline in the above-ground biomass growth rate of both communities over the summer growing period corresponded with a similar trend in the uptake of nitrogen and phosphorus. Moreover, tissue shoot phosphorus concentrations of all perennial grass species were always low. The total amount of nutrient absorbed (nitrogen, phosphorus) was about 35% greater for the buffel compared with the native grassland community. Both communities could be contrasted with respect to the efficiency of nutrient absorption and nutrient utilization. Continued overgrazing led to a reduction in litter and root yield. As little biomass was produced by the community over the autumn-winter period, the litter production rate could be expressed as a function of herbage yield at the commencement of the period and was found to be relatively independent of grazing pressure. The agronomic and ecological implications of these results are discussed in relation to the management of these grasslands. __________________ *Part I, Aust. J. Agric. Res., 29: 773 (1978).

Field and controlled environment studies were undertaken to determine the seasonal variation in vesicular-arbuscular mycorrhizal (VAM) propagules under different types of vegetation in the mulga (Acacia aneura) shrublands of south-western Queensland and how inoculation with VAM affects the growth and response to phosphorus fertilizers of several grasses common to this region. A most probable number (MPN) technique was used to estimate the number of viable VAM propagules under mulga shrubland and native pasture. There was a pronounced rise in MPN at a native pasture site from June to November, peaking at 1 propagule/g soil, before declining between November and May to non detectable concentrations. In the mulga shrubland site, MPN remained very low (less than 0.2 propagules/g soil) throughout the study. Pasture species in the field were screened for VAM infection in both winter and summer. Only three of the 14 species sampled were infected with VAM in winter (June) after a long drought. In contrast, 10 of the 14 species sampled in December possessed VAM-infected roots. A pot experiment was conducted to examine the influence of VAM inoculation on growth and the response to phosphate fertilizer of several important pasture grasses in pastures derived from mulga shrublands. The treatments comprised a factorial combination of inoculation or non-inoculation with VAM, four grass species (Aristida armata, Cenchms ciliaris cv. USA, Digitaria ammophilla, and Thyridolepis mitchelliana), and four rates of P fertilizer designed to range over very deficient to non-limiting for each species. All species except D. ammophilla produced growth responses to VAM inoculation. Though C. ciliaris and T. mitchelliana responded to VAM inoculation only in soil unamended with P fertilizer, A. amata showed growth responses across all P rates examined, suggesting some factor other than P was limiting this species. The growth response of the grasses to VAM inoculation was poorly correlated with the percentage of root infected with VAM. D. ammophilla had the highest levels of root infection (32%) despite producing no growth response to VAM. In contrast, T. mitchelliana had less than 2% of the root infected with VAM. All species had thin fibrous roots with long (0.35-0.47 mm), frequent root hairs. The differential growth responses to VAM inoculation of A. armata compared with the other grasses may provide a management strategy to control Aristida ingress into pastures established from mulga shrublands.