Seeds of the winter annualBromus tectorumlose primary dormancy in summer and are poised to germinate rapidly in the autumn. If rainfall is inadequate, seeds remain ungerminated and may enter secondary dormancy under winter conditions. We quantified conditions under which seeds enter secondary dormancy in the laboratory and field and also examined whether contrastingB. tectorumgenotypes responded differently to dormancy induction cues. The study also extends previous hydrothermal time models for primary dormancy loss and germination timing inB. tectorumby using similar models to account for induction and loss of secondary dormancy. Maximum secondary dormancy was achieved in the laboratory after 4 weeks at –1.0 MPa and 5°C. Seeds in the field became increasingly dormant through exposure to temperatures and water potentials in this range, confirming laboratory results. They were released from dormancy through secondary after-ripening the following summer. Different genotypes showed contrasting responses to dormancy induction cues in both laboratory and field. To examine secondary dormancy induction and release in the field in terms of hydrothermal time parameters, we first created a model that allowed mean base water potential (Ψb(50)) to vary while holding other hydrothermal time parameters constant, as in models for primary dormancy loss under dry conditions. The second model allowed all three model parameters to vary through time, to account for changes (e.g. hydrothermal time accumulation) that could occur simultaneously with dormancy induction in imbibed seeds. Shifts in Ψb(50) could explain most changes in dormancy status for seeds retrieved from the field, except during the short period prior to dormancy induction, when hydrothermal time was accumulating. This study illustrates that hydrothermal modelling, and specifically changes in Ψb(50), can be used to characterize secondary dormancy induction and loss inB. tectorum.

The differences in soil properties and crop yield on slopes of varying steepness in intensively cropped paddocks with red ferrosols (Humic Eutrodox), which had evidence of erosion over many years, were determined at 5 sites in north-west Tasmania. We found that soils on the steeper slopes, which have had greater soil loss, had significantly lower topsoil carbon concentrations than soils on flatter slopes. We found that significant redistribution of soil had occurred on the sites in this study. Areas with concave slope positions had over-thickened topsoils. However, the effect of erosion on the topsoil thickness of eroded slope positions was masked by the uniform depth of cultivation across paddocks. Penetration resistance increased gradually with depth on all transects, indicating no compacted layers or plough pans on these intensively cropped sites. Soil physical properties did not significantly change from non-eroded, level areas to steep, eroded parts of the paddocks, even although considerable soil loss had occurred from the steep slopes. We attribute this to the gradational nature of these red ferrosols, which characteristically have strongly structured subsoils. Crop yield was significantly correlated with slope but was not significantly different on land up to 18% slope. We anticipate that continued erosion at current rates on this less steep land will inevitably reduce crop yields. We found a significant correlation between organic carbon, total exchangeable bases, extractable K, and crop yield, but no correlation between available water content and crop yield. We conclude that soil organic matter contributes to productivity through its effect on nutrient concentrations. Total exchangeable bases appeared to provide the best and simplest explanatory model to predict crop yield. We propose a set of 4 tests for future assessment of soil condition on these red ferrosols.

Attributes of 21 Vertosols in 2 different regions of Tasmania were assessed using field and laboratory techniques to determine differences associated with 3 local forms of agricultural management (long-term pasture, rain-fed cropping and irrigated cropping). Vertosols in the northern Midlands had better physical properties (lesser bulk density and penetration resistance, and greater porosities and water holding capacities), poorer nutrient status (lower pH, exchangeable bases, and extractable P) and better biological properties (greater organic carbon (OC), carbon fractions F1 and F3, and more worms) than south-eastern Vertosols. When adjusted for clay content, cropped sites had less soil OC than pasture sites at 0–75 mm depth. Readily oxidisable (fraction F1) carbon in the surface 75 mm was 3.6 mg/g and 6.9 mg/g under long-term pasture compared with 2.5 mg/g and 3.9 mg/g in irrigated cropped paddocks on south-eastern and Midlands sites, respectively. Soil organic carbon values were positively correlated with physical and chemical soil properties. Long-term pasture paddocks showed stronger structural development and had smaller aggregates than cropped paddocks, which had more larger clods. Vane shear strength and penetration resistance were less in rainfed cropped paddocks compared with long-term pasture but this effect was not apparent on irrigated cropped paddocks. Farmers considered that a majority of their soil attributes were healthy under all management histories but strategies for maintaining organic matter levels and minimising clod formation by tillage are essential for long-term sustainable use of these Vertosols.

Bromus tectorum is a highly invasive annual grass. The fungal pathogen Pyrenophora semeniperda can kill a large fraction of B. tectorum seeds. Outcomes in this pathosystem are often determined by the speed of seed germination. In this paper we extend previous efforts to describe the pathosystem by characterising secondary dormancy acquisition of B. tectorum. In the laboratory approximately 80% of seeds incubated at –1.0 MPa became dormant. In the field, seeds were placed in the seed bank in late autumn, retrieved monthly and dormancy status determined. The field study confirmed the laboratory results; ungerminated seeds became increasingly dormant. Our data suggest that secondary dormancy is much more likely to occur at xeric sites.

Selected soil properties and paddock management characteristics were measured for 121 potato and poppy crops in north and northwest Tasmania to see if variation in these characteristics explain variation in crop yield. The soil properties we selected were those that previous work found had changed the most as a result of cropping and, therefore, may be affecting yield on the particular soil type. The soil properties and management characteristics that were significantly correlated with crop yield varied with crop and soil type. None of the soil characters had correlation coefficients greater than 0.63, probably reflecting the capacity of individual farmers to overcome particular soil limitations through their management of tillage, nutrition, irrigation, weeds and pathogens. On ferrosols, a visual score of soil structure was significantly correlated with potato yield (r = 0.57) and exchangeable aluminium was significantly correlated with poppy yield (r = 0.63). Exchangeable calcium (r = 0.54) and penetration resistance (r = 0.38) correlated positively and topdressed nitrogen (r = –0.49) correlated negatively with poppy alkaloid assay, an important determinant of overall poppy yield. On dermosols, depth to 2000 kPa penetration resistance (r = 0.60) and fertiliser P (r = –0.67) were correlated with potato yield, structure score correlated with poppy yield (r = 0.59), and penetration resistance with poppy assay (r = 0.52). On sodosols, fertiliser K (r = –0.41 and r = 0.55) and N (r = –0.45 and 0.42) correlated negatively with poppy yield and positively with poppy assay. On clay loam soils such as dermosols and ferrosols, increased topsoil cloddiness appears to be having a deleterious effect on crop yield. Cloddiness is readily assessed on these soils using the structure scorecard, which could therefore become a practical diagnostic test for farmers and advisers.