The effects of green manured field pea (Pisum sativum L.), low-input (LI) wheat (Triticum aestivum L.) (seeding rate of 40 kg/ha and 85 kg/ha of diammonium phosphate), and high-input (HI) wheat (seeding rate of 100 kg/ha, 85 kg/ha of diammonium phosphate, and 180 kg/ha of urea) sown as rotation crops after cotton on soil quality; cotton growth, yield and nutrient uptake; and gross margins ($AU/ha and $AU/ML of irrigation water) were evaluated from 1993 to 1998 in an irrigated Vertosol in the central-west of New South Wales. Soil quality indicators monitored were aggregate stability (dispersion index), compaction (air-filled porosity), soil resilience to structural destruction (as geometric mean diameter of soil aggregates formed after puddling and drying of soil), exchangeable cations, calcium carbonate, nitrate-N, pH, organic C, development of arbuscular mycorrhiza (AM), and incidence of cotton root diseases (black root rot). In comparison with wheat, field pea increased soil nitrate-N levels during the early stages of the experiment and formed smaller aggregates after puddling and drying, but it was ineffective in ameliorating soil compaction. In contrast wheat was very effective in ameliorating soil compaction. Nitrate-N values under wheat–cotton rotations increased with time such that after 4 years they were similar to that under the field pea–cotton rotation. Soil chemical fertility indicators such as organic C, pH, EC, and exchangeable cations were not affected consistently by either wheat or field pea, whereas minimum tillage, retention of crop residues, and cropping phase (i.e. rotation crop or cotton) affected them more. A net decrease in organic C and an increase in EC was observed with time in all treatments. By sowing either field pea or wheat, the mycorrhizal colonisation of cotton roots was improved. Black root rot incidence was increased 3-fold by sowing field pea, but was not significantly affected by wheat. Cotton lint yield was unaffected by rotation crop, although profitability shown as gross margins/ha and gross margins/ML irrigation water were greater with wheat compared with field pea. Gross margins/ha were in the order HI wheat > LI wheat > field pea, and gross margins/ML irrigation water were in the order LI wheat > HI wheat > field pea. In terms of ameliorating soil compaction, minimising black root incidence, and maximising returns to the cotton grower, wheat is a better rotation crop than field pea. The decision to apply fertiliser and sow wheat at a higher seeding rate will depend on whether land or water is the major limiting factor.
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.
Reduced crop growth rates (‘long-fallow disorder’) can be a feature of long-fallow cotton (cotton, alternating with a bare fallow, is sown every other year). This is usually attributed to decreased development of arbuscular mycorrhiza (AM), although associated soil physical, chemical, and biological properties are very rarely reported. A study was conducted from 1993 to 1997 in a grey, self-mulching Vertosol in the central-west of New South Wales to characterise soil properties under long-fallow cotton with a view to identifying soil factors other than AM that could contribute to cotton growth rate reductions. Soil quality indicators monitored were compaction (bulk density and air-filled porosity), strength (cone resistance), plastic limit, exchangeable cations, nitrate-N, pH, organic C, development of AM, and incidence of cotton root diseases. In comparison with continuous cotton, long-fallow cotton had lower soil strength, and lower plastic limit. Exchangeable Ca and Mg were higher with continuous cotton only in 1994. Higher nitrate-N was also observed with long-fallow during the first fallow phase of the experiment. Long-fallow did not have any significant effect on soil organic carbon. However, a net decline in soil organic C and exchangeable Mg occurred with both treatments. During the cotton phase, subsoil nitrate-N and incidence of black root rot were lower with long-fallow cotton. Uptake of nitrogen by continuous cotton may have been reduced by greater severity of black root rot. Vegetative and reproductive growth, water extraction, and cotton lint yields in long-fallow cotton plots were higher than those in continuous cotton plots. AM development was similar with continuous cotton and long-fallow cotton. Compared with long-fallow cotton, the lower lint yield in continuous cotton was thought to be due to the interactive effects of declining nutrient availability, higher soil strength, and greater severity of black root rot causing decreases in nutrient and water uptake.
The effects of planting cereal or leguminous crops in rotation with irrigated cotton (Gossypium hirsutum L.) on the properties of cracking clay (swelling) soils in the Macquarie and Namoi Valleys of New South Wales, Australia were evaluated during the summer of 1992-3. The observations were made on commercial farmers' fields. The soil properties evaluated were the particle size distribution, the dispersion index, the plastic limit, the percentage of coarse (particle diameter 212-2000 pm) and fine (particle diameter 53-212 pm) particulate soil organic matter, soil respiration rate, soil reactivity, soil aggregate density, pH, nitrate-N and exchangeable Ca, Mg, K and Na. In general,
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