No abstract

The effect of the incubation of zinc (Zn) applied to the soil on Zn uptake and the Zn concentrations in chemical extractants was studied. In a glasshouse experiment using a Zn-deficient gravelly sandy loam, the effect of recently applied Zn was compared with that of Zn incubated with the soil for 15 days at 40°C on growth and Zn uptake by navy beans (Phaseolus vulgaris cv. Gallaroy). At the second harvest (33 days after sowing), the dry weight of shoots of recently applied Zn was consistently higher than that of incubated Zn, except at the highest rate of 1 J.lg Zn g-l soil, where yields were similar. Comparisons of the slope of the linear regressions of Zn uptake as a function of rate of application showed that incubated Zn was approximately 80% as effective as recently applied Zn.A laboratory experiment measured the decrease in Zn concentration in HCl, EDTA, DTPA, and dilute CaCl2 with incubation for up to 8 days at 40° C in four contrasting soils from Western Australia and Queensland. An addition of 2·5 J.lg Zn g-l soil increased the concentration of Zn in all extractants at all times of incubation compared with the untreated soil. The recovery of the added Zn was generally highest with HCl and lowest with 0·002 M CaCb and decreased exponentially in all extractants with increasing time of incubation in all soils. The order of the rate of decrease in Zn concentration for all extractants was krasnozem > gravelly sandy loam> sand> sandy clay loam. The model, Y = Ct B , where C and B are constants, was used to describe the relationship between the recovery of added Zn and time of incubation.

Seven field experiments were carried out during 1980 and 1981 to determine the nitrogen fertilizer requirements of potatoes grown in basaltic krasnozem soils in North Queensland. Rates of 0, 40, 80, 160 and 320 kg nitrogen/ha were compared in all experiments. Comparisons of urea and ammonium nitrate as fertilizer forms and basal and split application methods were also carried out in four of the experiments. Total yields of fresh tubers from nil-nitrogen plots varied from 18.1 to 29.7 t/ha and nitrogen applications increased these yields at all sites to levels varying from 136 to 325% of the control plots. Ninety-eight per cent of maximum yields calculated from quadratic functions were produced by rates of basally applied urea nitrogen varying from 108 to 205 kg/ha. These rates were poorly correlated with relative yields and topsoil (0-20 cm depth) nitrate nitrogen but were well correlated with nitrogen in 20-50 cm depth. Nitrogen application increased the average tuber weights from 135 to 179 g but reduced the specific gravity of tubers. Splitting nitrogen applications reduced average tuber weights.

A field experiment was established to define the phosphorus (P) requirement for establishment and maintenance of a mixed legume pasture (Stylosanthes scabra cv. Seca, S. hamata cv. Verano, S. guianensis cv. Graham, Macroptilium atropurpureum cv. Siratro) introduced into a native grass pasture on an infertile duplex red earth. Rates of 0, 5, 10, 20, and 40 kg P/ha were applied to separate plots in year 1 (1982), 2, and 3. In year 5 (1986 growing season), half of each plot that had received 20 and 40 kg P/ha in year 3 was refertilised at the original rate to ensure that maximum yields were defined. Bicarbonate- or acid-extractable soil P concentrations of 8 mg/kg were sufficient for 80% maximum legume yield. The residual value of applied P in the surface soil, as measured by soil analysis, decreased exponentially, but an initial application of 40 kg P/ha was still sufficient to produce near-maximum legume yield after 5 years. Phosphorus application increased the dry matter yield of legume. During the establishment phase (years 1 and 2 after planting) yields reached maximum at 10 and 20 kg P/ha, respectively, but increased linearly in subsequent years. When the original rates were reapplied in year 5, peak yield occurred at 20 + 20 kg P/ha, and there was no difference between this yield and that from plots receiving 40 kg P/ha in year 1. Native grass yields increased with P application only in years 4 and 5 of the experiment. Stylos demonstrated good tolerance to low P supply. In year 1, 80% of the total legume yield consisted of Graham stylo and Siratro, whereas in subsequent years, Seca and Verano made up 70 and 20%, respectively, of the total, irrespective of treatment. Yield of legume at nil P, relative to maximum, increased from 5% in year 1 to 42% in year 5.

The zinc (Zn) content of particle size fractions of 12 mainly Zn deficient soils was measured by extraction with three contrasting extractants. The soils, which ranged from sands to a black earth, were from Western Australia and Queensland and particle size fractions (clay, silt, fine sand, coarse sand) were obtained by sieving and sedimentation after ultrasonification of soil suspended in deionized water. The extractants were concentrated HNO3/H2SO4/HClO4 (acid extractable or AE-Zn), DTPA and 0.002 M CaCl2. For each extractant, Zn contents of the fractions and whole soils were correlated with organic carbon and ammonium oxalate extractable Fe and Al. The AE-Zn concentrations in whole soils were 0.6-132 mg kg-1 and high clay soils had higher concentrations (mean 54 mg kg-1) than low clay soils (mean 2 mg kg-1). After fractionation, lowest AE- and DTPA-Zn were found in coarse sand fractions and concentrations increased with decreasing particle size. Clay plus silt fractions contained 60-99% of the whole soil AE-Zn and 76-93% of the whole soil DTPA-Zn. The CaCl2-Zn concentrations were very low (<5.0 �g kg-1) for all soils. In whole soils, DTPA-Zn was only a small proportion, 3.2% and 1.8%, of the AE-Zn in the low clay and high clay soils, respectively. The CaCl2-Zn was generally less than 2% of the DTPA-Zn in whole soils. In whole soils, AE-Zn was correlated with oxalate extractable Fe and with oxalate A1 (r = 0.72 and 0.71, respectively; P <0.01), whereas DTPA-Zn was correlated with oxalate extractable Fe (r = 0.82; P < 0.01). The AE- and DTPA-Zn were correlated with organic carbon only in some fractions. The DTPA- and CaCl2-Zn were not correlated with AE-Zn content, nor was DPTA-Zn correlated with CaCl2-Zn in whole soils or fractions (P<0.05). Dispersion of the soils with ultrasonification in the absence of dispersing agents was not as effective as dispersion with conventional mechanical/chemical dispersion. The percentage of the soil recovered in the clay fraction after sonification was 23-78% of that recovered by the conventional method. Fine and coarse sand contents were similar for either method, indicating that incomplete dispersion of clay by ultrasonification resulted in higher silt contents.