The effects of phosphorus (P) deficiency on plant symptoms, yield, and components of yield of wheat (Triticum aestivum L. cv. Halberd), P uptake, and the distribution of dry weight within plants of variable P status were examined in 2 glasshouse and 5 field experiments. Apart from stunted growth and depressed tillering, the symptoms of acute P deficiency, most noticeable on older leaf blades, were equivocal; they were not always observed on acutely deficient plants and were absent on moderately deficient plants. In glasshouse experiments, the leaves of acutely deficient plants were spindly, erect, and dark green, whereas in field experiments, the leaves were pale green. In acutely P-stressed plants, leaf senescence, phasic development, and anthesis were delayed. The disorder restricted tiller development and therefore the rate of appearance and the number of leaves per plant. It depressed grain yield principally by reducing the number of fertile tillers. Severe P deficiency depressed shoot growth within 15 days of sowing and ultimately reduced plant height, root mass, and grain yield. In all experiments, shoot yield responses to applied P increased progressively until stem elongation (Zadoks Scale 30) and changed little thereafter. As a result, the external requirement for P (i.e. P level required for 90% maximum growth) increased with time during vegetative development in most experiments. Severe P deficiency also affected the distribution of dry matter between the roots and shoots and between the leaf blades and conducting tissues (sheaths and stems). Both of these responses intensified with advancing plant age. Treatment differences in P uptake in shoots also occurred early in growth and persisted until grain maturity. The partitioning of P between roots and shoots favoured P uptake or retention in the roots of P-deficient plants. Under conditions of acute and moderate P stress, the resources of the wheat plant appear to be directed towards maintaining root growth (at least initially), limiting and delaying shoot proliferation, and maximising the leaf : stem ratio. These regulations appear circumstantially to be adaptive mechanisms for conserving suffiient P to ensure the survival of at least 1 weak, but fertile, tiller on each plant.

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Data from more than 580 field experiments conducted in South Australia over the past 30 years have been re-examined to estimate extractable soil phosphorus (P) levels related to 90% maximum yield (C90) for 7 crop species (wheat, barley, oilseed rape, sunflower, field peas, faba beans, potato) and 3 types of legume-based pasture (subterranean clover, strawberry clover, annual medics). Data from both single-year and longer term experiments were evaluated. The C90 value for each species was derived from the relationship between proportional yield responsiveness to applied P fertiliser rates (determined as grain yield in crops and herbage yield in ungrazed pastures) and extractable P concentrations in surface soils sampled before sowing. Most data assessments involved the Colwell soil P test and soils sampled in autumn to 10 cm depth. When all data for a species were considered together, the relationship between proportional yield response to applied P and soil P status was typically variable, particularly where Colwell soil P concentration was around C90. When data could be grouped according to common soil types, soil surface texture, or P sorption indices (selected sites), better relationships were discerned. From such segregated data sets, different C90 estimates were derived for either different soil types or soil properties. We recommend that site descriptors associated with the supply of soil P to plant roots be determined as a matter of course in future P fertiliser experiments in South Australia. Given the above, we also contend that the Colwell soil P test is reasonably robust for estimating P fertiliser requirements for the diverse range of soils in the agricultural regions of the State. In medium- and longer term experiments, changes in Colwell soil P concentration were measured in the absence or presence of newly applied P fertiliser. The rate of change (mg soil P/kg per kg applied P/ha) appeared to vary with soil type (or soil properties) and, perhaps, cropping frequency. Relatively minor changes in soil P status were observed due to different tillage practices. In developing P fertiliser budgets, we conclude that a major knowledge gap exists for estimating the residual effectiveness of P fertiliser applied to diverse soil types under a wide range of South Australian farming systems.

Summary. Six field experiments were conducted in the Murraylands and Mid North regions of South Australia between 1986 and 1988 to develop plant tests for diagnosing the phosphorus status of barley. The effects of applied phosphorus level on total phosphorus concentrations in youngest emerged leaf blades or whole shoots, and on shoot yield, were examined and critical ranges were established during vegetative growth. Experiments were conducted on a range of soil phosphorus levels with several methods of phosphorus placement and 2 barley cultivars. During early tillering, shoot yield and phosphorus concentrations in youngest emerged leaf blades and whole shoots were increased more by drilling phosphorus fertiliser with the seed (banded) than by spreading the same phosphorus rate over the soil surface and partially incorporating the fertiliser while sowing (broadcast). However, these methods of phosphorus placement did not affect diagnostic relationships between relative shoot yield and phosphorus concentrations in youngest emerged leaf blades and, as a result, critical total phosphorus concentrations were similar for both methods of applying phosphorus fertiliser. Estimated critical total phosphorus concentrations for both Schooner and Galleon barley were similar at each sampling time when allowances were made for minor differences in growth stages between the cultivars; even though phosphorus concentrations in youngest emerged leaf blades and whole shoots were higher in Schooner than in Galleon at adequate phosphorus supply. Between Zadoks scale 13.5 and 16.5 there was a linear decline in critical total phosphorus concentration in youngest emerged leaf blades with increasing plant age. A more variable, but steeper, decline was found for critical total phosphorus concentration in whole shoots. Critical ranges are proposed for total phosphorus concentrations in youngest emerged leaf blades and whole shoots of barley. The decline in critical total phosphorus concentration in youngest emerged leaf blades with advancing plant age was mainly caused by a higher functional requirement for phosphorus at early stages of growth and by increasing dry weight, cellulose and lignin content of youngest emerged leaf blades as plants age. Changes in the spatial distribution of applied phosphorus in relation to root growth did not seem to be important.