Manganese (Mn) toxicity is a potential limitation to plant growth on acidic and poorly drained soils. Five laboratory experiments using such soils were conducted to examine the influence of soil temperature, pH and water potential on the redox reactions of Mn and the potential for Mn toxicity. The microbial inhibitor sodium azide was used in some experiments to assess the role of microorganisms in these reactions. The reduction of Mn oxides (MnOx) during waterlogging was faster at 20°C and 30°C than at 10°C or 4°C. Sodium azide slowed the reduction of Mn oxides at 20°C and 30°C during waterlogging but had little effect at 4°C and 10°C, suggesting that microbial MnOx reduction during waterlogging was minimal at the lower temperatures. Re-oxidation of Mn2+ in soil drained after severe waterlogging was only observed in soil not treated with sodium azide, indicating that even when very high concentrations of Mn2+ were present, Mn2+ oxidation was still microbial. Prior liming of aerobic soil established lower starting concentrations of water-soluble plus exchangeable (WS+E) Mn2+ and slowed the reduction of Mn oxides during subsequent waterlogging. After drainage, rapid re-oxidation of Mn2+ was observed in all lime treatments but was fastest at the two highest lime rates. In the fourth and fifth experiments, interactions between temperature and water potential were observed. When waterlogged soils were drained to –5 and –10 kPa, re-oxidation of Mn2+ occurred at both 10°C and 20°C. At –1 kPa, there was no net change in WS+E Mn2+ at 10°C, whereas at 20°C, the concentration of WS+E Mn2+ increased, possibly due to the lower concentration of O2 in the soil water at the higher temperature. In the fifth experiment, at 4°C and 10°C there was little or no effect on Mn reactions of varying water potential from –1 to –1500 kPa, but at 20°C and especially at 30°C, both Mn2+ oxidation and Mn oxide reduction were slowed at –1500 kPa compared with the higher water potentials. Overall, the experiments show that a delicate balance between the microbial oxidation of Mn2+ and the reduction of Mn oxides can exist, and that it can be shifted by small changes in soil water potential along with changes in temperature and pH.

Cadmium (Cd) has been identified as a potential contaminant in foods posing health risks to humans and, in Australia, potatoes (Solanum tuberosum L.) have been identified as contributing a large proportion of the average dietary Cd intake. To assess the concentrations of Cd in Australian potatoes and soil factors likely to lead to high Cd concentrations, commercial crops and soils were sampled at 352 sites throughout potato production areas in Australia. Across all states, fresh weight (FW) tuber Cd concentrations ranged from 0.004 to 0.232 mg kg−1 with an overall mean value of 0.041 and a median of 0.033 mg kg−1 (FW). Approximately 92 samples out of 359 (25.6%) exceeded the current maximum permitted concentration (MPC) of 0.05 mg kg−1 (FW) and 18 (5.0%) exceeded 0.1 mg kg−1 (FW). Concentrations of Cd (EDTA‐extractable) in topsoils ranged from 0.01 to 0.59 mg kg−1 with mean and median values of 0.14 and 0.10 mg kg−1, respectively. There was no relationship between Cd concentrations in soil and tubers. Stepwise forward multiple regression analysis of the data indicated that Cl and Zn concentrations in the topsoil, soil pH, and potato cultivar accounted for 57% of the variation in tuber Cd concentrations, with Cl being the dominant factor. Comparison of soil‐plant transfer coefficients (TCs) for Cd with limited international data sets suggests that TCs for Australian soils used for potato production are relatively high.

The response of potato (Solanum tuberosum L.) cultivars Russet Burbank and Kennebec to soil and fertiliser potassium (K) was studied on basaltic krasnozems of north-west Tasmania. Yield increases in response to fertiliser K were recorded at sites with up to 300-400 mg/kg of bicarbonate extractable soil K. The close correlation between relative yield and soil K indicated that soil K can reliably predict fertiliser requirements. Petiole K concentrations at early tuber set increased with fertiliser K at responsive sites; maximum yields were achieved with 12-14% petiole K for Kennebec and 11-13% for Russet Burbank. Petiole K concentrations provide an excellent indication of the K status of a growing crop. Tuber K concentrations increased with both soil and fertiliser K, and yields of 50-80 t/ha removed 180-380 kg K/ha in the tubers. At severely deficient sites specific gravity and crisp colour increased with low rates of fertiliser K, but the general trend was for fertiliser K to reduce specific gravity and crisp colour. Bruising susceptibility decreased with fertiliser K at some sites but the physiological disorder, 'hollow heart', was not influenced by fertiliser K. There were consistent differences between the 2 cultivars. Russet Burbank required higher soil K, had lower petiole and tuber K concentrations and removed less K in the marketable tubers.

Potato tubers can accumulate high concentrations of cadmium (Cd) in edible portions, so that techniques to determine high risk Cd environments are required by growers. The use of combined soil and irrigation water analyses prior to crop planting was investigated as a means to predict risks of Cd accumulation in tubers. Soils and irrigation waters were analysed at 134 sites in the major potato production areas in Western Australia, South Australia, Tasmania, Victoria, and New South Wales. Irrigation waters were analysed for electrical conductivity (EC), major cations, and anions. Cadmium was extracted from soil using aqua regia (1 : 3 HNO3: HCl), EDTA (ethylenediamine-N,N,N′,N′-tetraacetate), DTPA (diethylene-triamine-pentaacetate), 0·01 M CaCl2, 0·01 M Ca(NO3)2, 0·1 M CaCl2, and 1·0 M NH4NO3. The preferred test procedure was validated in a subsequent sampling and analysis program at 39 sites. Irrigation water quality (EC or Cl concentration), measured prior to planting, explained the greatest variation in tuber Cd concentrations. Of the soil test procedures, only Cd extracted by 0·01 M CaCl2 significantly improved the predictive capacity of water EC. These 2 measures explained >55% of the variance in tuber Cd concentrations. The data set were transformed to generate a probability curve for exceeding Cd concentrations of either 0·05 or 0·1 mg/kg fresh weight, the latter being the current maximum permitted concentration (MPC) in Australia for potato tubers. The probability of producing potato tubers exceeding 0·05 and 0·1 mg/kg fresh weight was >50% once irrigation water EC increased above 1·4 and 3·0 dS/m, respectively. Using the relationships developed, growers should be able to quantify Cd risks by a simple test of irrigation water EC prior to planting and, if further precision is needed, also determine CaCl2-extractable Cd in soil.

No abstract

Attributes of 25 Tasmanian sodosols were assessed using field and laboratory techniques to determine changes associated with 4 typical forms of agricultural management [long-term pasture, cropping with shallow tillage using discs and tines, cropping (including potatoes) with more rigorous and deeper tillage including deep ripping and powered implements, and cropping (including potatoes) where the potatoes were harvested when the soil was wet]. Soil organic carbon in the top 150 mm was 2.7% under long-term pasture compared with 1.8% in rigorously tilled cropping paddocks, and microbial biomass C values were 194 and 129 mg/kg, respectively. Readily oxidisable organic C concentrations were 1.8 mg/g and 1.3 mg/g, respectively. Infiltration rate was greater in paddocks with shallow tillage cropping than long-term pasture but was 43% less in paddocks which had grown potatoes and 70% less after a wet potato harvest. Dry aggregate-size showed no change under shallow tillage cropping compared with long-term pasture but decreased significantly in more rigorously tilled potato cropping paddocks. Aggregate stability in all cropped paddocks was nearly 50% less than in long-term pasture paddocks, with values in intensively tilled potato cropping paddocks approaching relatively low levels. Colwell extractable phosphorus (P) increased with all cropping, particularly after potatoes. Lower organic carbon and poorer physical properties were associated with paddocks which had grown potatoes, which adds weight to the view that cropping rotation and associated soil management practices are critical for sustainable management of Tasmanian sodosols. Farmers were surveyed about their views of the condition of their paddocks. They identified more healthy than unhealthy soil attributes under all management histories but reported more unhealthy soil attributes when potatoes were included in their rotation.

Many intensively managed soils contain phosphorus (P) concentrations greater than required for optimum production. Soils with P concentrations in excess of the agronomic optimum can have unnecessary losses of P that can adversely affect water bodies. Reducing excessive soil-P concentrations is important for the economic and environmental sustainability of intensive agriculture, such as the Australian dairy industry. However, little is known of decreases in extractable soil-P concentrations when P fertiliser applications are reduced or omitted from soils with P concentrations and properties representative of intensive pasture grazing systems. Decreases in extractable P (calcium chloride (CaCl2), Olsen and Colwell) were monitored for up to 4.5 years for six Australian grazed pasture soils (Red Ferrosol, Brown Kurosol, Grey Dermosol, Brown Dermosol, Podosol and Hydrosol) with contrasting textures and P-buffering indices (PBI). Sixteen treatments consisting of four initial extractable-P concentrations (Pinit) paired with four ongoing P fertiliser rates (Pfert) were established for each of the six soils, except on an extremely low-PBI Podosol, where a range of Pinit concentrations could not be established. The resultant decreases in P were larger with higher Pinit concentration and lower rate of ongoing Pfert, except in the extremely low PBI Podosol where decreases in initially high CaCl2-P concentrations were large irrespective of ongoing Pfert. There was a greater proportional decrease in the environmentally extractable P compared with agronomically extractable P, with mean decreases in CaCl2-P of 57%, Olsen-P of 25%, and Colwell-P of 12%. The Pinit concentrations, which were well above agronomic optimum, remained above this target. This study advances scientific knowledge of extractable soil-P concentrations when P fertiliser inputs are withheld or reduced from grazed pasture soils, and aids land and catchment managers in estimating likely changes over time.

Concentrations of exchangeable and easily reducible manganese (Mn) were measured monthly for 3 1 months in acidic soils at 6 sites used for cropping in north-eastern Victoria. Manganese concentrations in shoots of wheat (Triticum aestivum), when present, were also measured. Changes in Mn concentrations in the soils and in wheat plants were related to seasonal conditions and to the occurrence of crop yellowing, a chlorotic and stunting disorder which affects cereals grown in the region. Peaks in the concentration of exchangeable Mn occurred either when the soils were waterlogged for a prolonged period or when they were subjected to extreme heating and drying during summer. Shoot concentrations of up to 1200 mg/kg of Mn were measured for plants growing in waterlogged soils. However, in some cases concentrations less than 400 mg/kg, a concentration not considered harmful, were measured in shoots of wheat growing under waterlogged conditions but which still showed symptoms of severe chlorosis and stunting. In the absence of waterlogging, concentrations of Mn in wheat shoots were usually less than 400 mg/kg. The evidence suggests that Mn toxicity is not the primary cause of crop yellowing in north-eastern Victoria, but Mn toxicity, induced by waterlogging, probably contributes to crop yellowing when waterlogging is severe. Soil acidity did not appear to be high enough to cause Mn toxicity in the absence of waterlogging.