T. B. Weaver

Cotton-based rotation systems on a sodic Vertosol under irrigation: effects on soil quality and profitability

An experiment was established in 1993 on a sodic Vertosol (Vertisol, Typic Haplustert) at Merah North, north-western New South Wales, to evaluate the sustainability of selected irrigated cotton (Gossypium hirsutum L.)-rotation crop sequences. Crop sequences were selected following discussions with local cotton growers. The indices used to evaluate sustainability included soil quality, microbiology, yield and profitability. This paper presents data on soil properties [soil organic C, structure as air-filled porosity of oven-dried soil, exchangeable Ca, Mg, K and Na, pH, electrical conductivity (EC 1:5 ) and EC 1:5 /exchangeable Na in the 0-0.6 m depth], lint yield and profitability (as gross margins/ha and gross margins/ML of irrigation water). The 6 cropping systems sown after minimum tillage were: continuous cotton (R1), long-fallow cotton (R2), cotton-green manured faba bean (Vicia faba L.) (R3), cotton-dolichos (Lablab purpureus L.)-green manured faba bean in the first year followed by cotton-wheat (Triticum aestivum L.) (R4), cotton-dolichos (R5), cotton-fertilised dolichos (with P and K removed by cotton replaced as fertiliser) (R6). In 1996, air-filled porosity of oven-dried soil was highest with R4 at the surface but lowest with Rl in the 0.15-0.30 m depth. In subsequent years, air-filled porosity of oven-dried soil was higher with R2 and R4 in the deeper depths, although differences between cropping sequences were small. Air-filled porosity of oven-dried soil increased between 1996 and 1998 in all treatments, and was probably caused by the change from intensive to minimum tillage in 1993, irrigation with moderately saline water and application of gypsum resulting in an increase in EC 1:5 /exchangeable Na. In general, differences in soil properties such as soil organic C, exchangeable Ca, Mg, K and Na, pH, electrical conductivity (EC 1:5 ) and EC 1 :5 /exchangeable Na between cropping sequences were far less than those which occurred with time. The key changes were decreases in pH, exchangeable sodium percentage, exchangeable cations and organic C between 1994 and 1996, and increases in air-filled porosity of oven-dried soil, EC 1:5 and EC 1:5 /exchangeable Na between 1996 and 1998. A decrease in air-filled porosity of oven-dried soil occurred between 1998 and 1999 as a consequence of preparing land and sowing cotton under very wet conditions. R1 had the highest cumulative gross margin/ha and R3 had the lowest. R2 had the highest cumulative gross margin/ML of irrigation water and R3 again the lowest. Among crop sequences, R2 and R4 gave the best returns with respect to both land and water resources.

Continuous Cotton and Cotton-Wheat Rotation Effects on Soil Properties and Profitability in an Irrigated Vertisol

ABSTRACT The combination of cotton (Gossypium hirsutum L.)-wheat (Triticum aestivum L.) rotations and minimum tillage is known to be superior to cotton monoculture in maintaining soil quality in eastern Australian Vertisols. Furthermore, sowing cotton into retained wheat stubble can minimize erosion and runoff, reduce off-field movement of pesticide residues and nutrients, and protect cotton seedlings from attack by heliothis moths. However, the consequences of sowing irrigated cotton into standing wheat stubble on soil quality and profitability are unknown. The objective of this study was to quantify the changes in several soil quality indices, viz. soil organic C (SOC), salinity, sodicity, structure, exchangeable cations, and profitability due to sowing irrigated cotton into standing wheat stubble. The sodicity indices measured were ESP (exchangeable sodium percentage) and EC1:5/exchangeable Na, and structural indices were in situ macroporosity at field capacity and bulk density. Soil properties and profitability were measured between 2000 and 2003 in a furrow-irrigated experiment at the Australian Cotton Research Institute (ACRI), near Narrabri, in New South Wales, Australia. The treatments were continuous cotton sown after conventional (disc-ploughing and chisel ploughing, followed by ridging) or minimum tillage (slashing of cotton plants after harvest, followed by root cutting and bed renovation with a disc-hiller), and minimum-tilled cotton-wheat. Between 1985 and 1999 cotton was sown after wheat stubble was incorporated but after 1999 it was sown into standing wheat stubble. Conventional cotton was sown in all plots between 1985 and 1999, whereas Roundup-Ready™ cotton was sown from 2000 onwards. Continuous cotton sown after conventional tillage had lowest SOC, exchangeable K, in situ macroporosity at field capacity and EC1:5/exchangeable Na, and highest ESP and bulk density. The reverse occurred with cotton-wheat sown after minimum tillage. Differences in these soil properties were greater between tillage systems than between cotton-wheat and continuous cotton cropping systems. Nonetheless, minimum-tilled cotton-wheat had higher SOC and exchangeable K than minimum-tilled continuous cotton. Sowing cotton into standing wheat stubble improved salt leaching. By 2003, cumulative gross margins were similar in all three cropping systems. At the same time, in comparison with both continuous cotton systems, cumulative variable costs were about 20% lower with minimum tilled cotton-wheat. Similar profits can be realized with less intensive cotton production systems which at the same time degrade soil quality less and have lower production costs. Analysis of weed control costs suggested that over-the-top application of Roundup in Roundup-Ready™ cotton resulted in early season control of weeds in the short-term and decreasing weed control costs over the longer-term.

Soil water storage, drainage, and leaching in four irrigated cotton-based cropping systems sown in a Vertosol with subsoil sodicity

Comparative studies of drainage and leaching in irrigated cotton (Gossypium hirsutum L.) based cropping systems in Australian Vertosols are sparse. Our objective was to quantify soil water storage, drainage, and leaching in four cotton-based cropping systems sown on permanent beds in an irrigated Vertosol with subsoil sodicity. Drainage was inferred using the chloride mass-balance method, and soil water storage and leaching were measured with a neutron moisture meter and ceramic-cup water samplers, respectively, from September 2005 to May 2011 in an ongoing experiment. The experimental treatments were: CC, cotton monoculture, summer cotton with winter fallow; CV, cotton–vetch (Vicia benghalensis L.) rotation with vetch stubble retained as in-situ mulch; CW, cotton–wheat (Triticum aestivum L.), with wheat stubble incorporated and a summer–winter fallow; and CWV, cotton–wheat–vetch, with wheat and vetch stubbles retained as in-situ mulch and summer and spring fallows. Soil water storage was generally highest under CW and CWV and least under CV. An untilled short fallow (~3 months) when combined with retention of crop residues as surface mulch, as in CWV, was as effective in harvesting rainfall as a tilled long fallow (~11 months) with stubble incorporation, as in CW. Drainage under cotton was generally in the order CW ≥ CWV > CC = CV, all of which were considerably greater than drainage during fallows. Except for very wet and dry winters, drainage under wheat rotation crops was greater than that under vetch. During wet winters, saturated soil in the 0–0.6 m depth of treatments under fallow resulted in more drainage than in the drier, cropped plots. No definitive conclusions could be made with respect to the effects of cropping systems on salt and nutrient leaching. Leachate contained less nitrate-nitrogen, magnesium, and potassium, but leachate electrical conductivity was ~6 times higher than infiltrated water. The greater salinity of the leachate may pose a risk to groundwater resources.

Potential contribution by cotton roots to soil carbon stocks in irrigated Vertosols

The well-documented decline in soil organic carbon (SOC) stocks in Australian cotton (Gossypium hirsutum L.) growing Vertosols has been primarily analysed in terms of inputs from above-ground crop residues, with addition to soil C by root materials being little studied. Potential contribution by cotton roots to soil carbon stocks was evaluated between 2002 and 2008 in 2 ongoing long-term experiments near Narrabri, north-western New South Wales. Experiment 1 consisted of cotton monoculture sown either after conventional tillage or on permanent beds, and a cotton–wheat (Triticum aestivum L.) rotation on permanent beds; Experiment 2 consisted of 4 cotton-based rotation systems sown on permanent beds: cotton monoculture, cotton–vetch (Vicia villosa Roth.), cotton–wheat, and cotton–wheat–vetch. Roundup-Ready™ (genetically modified) cotton varieties were sown until 2005, and Bollgard™ II-Roundup Ready™-Flex™ varieties thereafter. Root growth in the surface 0.10 m was measured with the core-break method using 0.10-m-diameter cores. A subsample of these cores was used to evaluate relative root length and root C concentrations. Root growth in the 0.10–1.0 m depth was measured at 0.10-m depth intervals with a ‘Bartz’ BTC-2 minirhizotron video microscope and I-CAP image capture system (‘minirhizotron’). The video camera was inserted into clear, plastic acrylic minirhizotron tubes (50-mm-diameter) installed within each plot, 30° from the vertical. Root images were captured 4–5 times each season in 2 orientations, left and right side of each tube, adjacent to a furrow, at each time of measurement and the images analysed to estimate selected root growth indices. The indices evaluated were the length and number of live roots at each time of measurement, number of roots which changed length, number and length of roots which died (i.e. disappeared between times of measurement), new roots initiated between times of measurement, and net change in root numbers and length. These measurements were used to derive root C turnover between times of measurements, root C added to soil through intra-seasonal root death, C in roots remaining at end of season, and the sum of the last 2 indices: root C potentially available for addition to soil C stocks. Total seasonal cotton root C potentially available for addition to soil C stocks ranged between ~50 and 400 g/m2 (0.5 and 4 t/ha), with intra-seasonal root death contributing 25–70%. These values are ~10–60% of that contributed by above-ground crop residues. As soil organic carbon in irrigated Vertosols can range between 40 and 60 t/ha, it is unlikely that cotton roots will contribute significantly to soil carbon stocks in irrigated cotton farming systems. Seasonal root C was reduced by cotton monoculture, stress caused by high insect numbers, and sowing Bollgard II varieties; and increased by sowing non-Bollgard II varieties and wheat rotation crops. Permanent beds increased root C but leguminous rotation crops did not. Climatic factors such as cumulative day-degrees and seasonal rainfall were positively related to seasonal root C. Root C turnover was, in general, highest during later vegetative/early reproductive growth. Large variations in root C turnover and seasonal C indices occurred due to a combination of environmental, management and climatic factors.

Residual effects of cotton-based crop rotations on soil properties of irrigated Vertosols in central-western and north-western New South Wales

The residual effects of cotton (Gossypium hirsutum L.) based crop rotations on soil physical and chemical properties were evaluated in 2 irrigated on-farm experiments located at Warren (1999–2001) in the central-west and Merah North (2000–05) in the north-west of New South Wales. The soils in both sites were grey, self-mulching Vertosols. The rotations sown at Warren from 1993 to 1998 were: (1) continuous cotton (cotton sown every year); (2) long-fallow cotton (cotton alternating with a bare fallow); (3) cotton–high input wheat (Tricticum aestivum L.), in which wheat was sown at a rate of 100 kg/ha and fertilised with 180 kg/ha of urea; (4) cotton–low input wheat, in which wheat was sown at a rate of 40 kg/ha and did not receive any N fertiliser; and (5) cotton–green manured field pea (Pisum sativum L.). At Merah North the rotations sown from 1993 to 2000 were: (1) continuous cotton; (2) long-fallow cotton; (3) cotton–green manured faba bean (Vicia faba L.) until 1999 when sorghum was sown during the 1999–2000 growing season; (4) cotton–dolichos (Lablab purpureus L.)–green manured faba bean from 1993 to 1994 followed by cotton–unfertilised wheat in which wheat was sown at a rate of 50–70 kg/ha thereafter; (5) cotton–dolichos; and (6) cotton–fertilised dolichos with P and K removed by cotton replaced as fertiliser. Soil was sampled to a depth of 0.6 m at 0.15-m increments and analysed for pH (in 0.01 m CaCl2), EC1 : 5, ESP, specific volume, nitrate-N, organic C (SOC), plastic limit, and dispersion. Residual effects of rotation history were reflected in subsoil specific volume at both sites, and nitrate-N in the surface 0.3 m and SOC in the 0–0.6 m depth at Warren. In general, higher values of specific volume occurred where cotton–wheat rotations, and in particular, fertilised wheat, had been sown. At Merah North, subsoil specific volume in ex-long-fallow cotton was similar to that in the cotton–wheat rotation. At Warren, ex-continuous cotton had lowest subsoil specific volume, the ex-cotton–high input wheat rotation and ex-long fallow cotton had greater SOC sequestration, and the ex-cotton–high input wheat rotation had higher nitrate-N. These differences mirrored those present when the rotation treatments were in place. Residual effects of crop rotations are more likely to occur where the residues of the rotation crops are relatively recalcitrant or where cropping intensity is lower.

Soil properties, black root-rot incidence, yield, and greenhouse gas emissions in irrigated cotton cropping systems sown in a Vertosol with subsoil sodicity

Comparative studies of soil quality and energy use in two- and three-crop rotations in irrigated cotton (Gossypium hirsutum L.) based cropping systems under varying stubble management practices in Australian Vertosols are sparse. Our primary objective was to quantify selected soil quality indices (salinity, sodicity, exchangeable cations, nitrate-N, pH), crop yields, and greenhouse gas emissions in four irrigated cotton-based cropping systems sown on permanent beds in a Vertosol with subsoil sodicity near Narrabri in north-western New South Wales. A secondary objective was to evaluate the efficacy of sowing vetch in rotation with cotton over a long period on the incidence of black root-rot in cotton seedlings. Results presented in this report pertain to the period June 2005–May 2011. The experimental treatments were: cotton–cotton; cotton–vetch (Vicia benghalensis L.); cotton–wheat (Triticum aestivum L.), where wheat stubble was incorporated; and cotton–wheat–vetch, where wheat stubble was retained as in-situ mulch. Vetch was terminated during or just before flowering by a combination of mowing and contact herbicides, and the residues were retained as in-situ mulch. Soil pH, electrical conductivity (EC1 : 5), Cl–, NO3–-N, exchangeable cations, exchangeable sodium percentage (ESP), electrochemical stability index (= EC1 : 5/ESP), and EC1 : 5/ESC (exchangeable sodium concentration) were evaluated in samples taken from the 0–1.2 m depth before sowing cotton during late September or early October of each year. Incidence of black root-rot was assessed 6 weeks after sowing cotton. Compared with sowing cotton every year, including wheat in cotton-based cropping systems improved cotton yield and reduced soil quality decline, emissions of carbon dioxide equivalents (CO2-e) per unit area, and CO2-e emissions per unit of cotton yield. Including vetch in the rotation was of negligible benefit in terms of yield and CO2-e emissions per unit of yield. The rate of soil quality decline was unaffected by including vetch in a cotton–wheat rotation but was accelerated when included in a cotton–cotton sequence. Among all cropping systems, soil quality was best with cotton–wheat and cotton–wheat–vetch but poorest with cotton–vetch. Although CO2-e emissions associated with growing 1 ha of cotton could be reduced by 9% by growing vetch because of substituting fixed atmospheric N for N fertiliser derived from fossil fuels, this advantage was partly negated by the emissions from farming operations associated with growing a vetch crop. Relative to a two-crop rotation (one cotton–one rotation crop), negligible benefits in terms of yield, soil quality, greenhouse gas emissions, and black root-rot control accrued from a three-crop rotation (one cotton–two rotation crops). Incidence of black root-rot increased as the number of cotton crops sown increased. In addition to the cropping systems, soil quality indices and yield were significantly influenced by irrigation water quality and climate.

Short-term variations in chemical properties of Vertisols as affected by amounts, carbon/nitrogen ratio, and nutrient concentration of crop residues

Abstract Wheat (Triticum aestivum L.), faba bean (Vicia faba L.), and soybean (Glycine max L.) are sown in rotation with cotton (Gossypium hirsutum L.) on eastern Australian Vertisols. Several long‐term studies, where soil was sampled annually, have reported that rotation crops affected only soil structure and soil nitrogen (N). The long time interval between sampling times in the field studies may have resulted in changes in soil quality remaining undetected. The objectives of this study were to (1) determine if mineralization of crop residues in Vertisols previously sown with cotton resulted in short‐term changes in soil properties, and (2) describe their nature. Short‐term changes in properties of Vertisols were studied in two laboratory experiments by adding milled residues to a sodic, heavy clay soil (635 g/kg clay, exchangeable sodium percentage, ESP, of 12) and a nonsodic medium clay soil (540 g/kg clay, ESP of 2). The crop materials used were vegetative and seed material from faba bean, vegetative material from wheat at anthesis (“green wheat”), cotton, wheat straw (“mature wheat”), and soybean stubble, which were added to the soils at five rates: 0% (untreated control), 2%, 5%, 10%, and 20% of the air‐dried weight of the soil. Soils were analyzed for pH (in CaCl2), EC1:5, exchangeable calcium (Ca), magnesium (Mg), potassium (K), and sodium (Na) and were measured with an atomic absorption spectrophotometer after washing with aqueous alcohol and aqueous glycerol to remove soluble salts followed by extraction with alcoholic 1 M NH4Cl at a pH of 8.5. The sodicity indices ESP and EC1:5/exchangeable Na were then calculated. Most soil improvement, viz. distinct increases in exchangeable K and lower sodicity, occurred with green wheat residues, due to its high K concentration and low C:N ratio. Variation in soil changes between crop residues was directly related to the amounts added, their C:N ratios, and nutrient concentrations. The primary cause of the inability of rotations to cause extensive soil changes appears to be the small amounts and low mineral content of crop residues returned to the soil. Significant improvements of soil quality in Vertisols sown with cotton‐based farming systems are more likely if large amounts of green cereal crop residues are added.

Physical and Chemical Properties of Soil Near Cracks in Irrigated Vertisols Sown with Cotton-Wheat Rotations

Properties of soil adjacent to cracks and in bulk soil were compared in furrow irrigated Vertisols sown to intensively tilled cotton (Gossypium hirsutumL.) followed by (fb.) minimum-tilled wheat (Triticum aestivumL.) in a 2-year rotation; minimum-tilled cotton fb. and minimum-tilled wheat in a 2-year rotation; and a perennial pasture in NW New South Wales, Australia. A backhoe pit was dug at right-angles to cracks, and a 50 mm thick layer of soil sampled from the exposed crack walls and from adjacent bulk soil in 0.3 m depth increments to a depth of 0.9 m. Soil properties evaluated were: aggregate stability (dispersion index), soil resilience to structural destruction (as geometric mean diameter of aggregates formed after puddling and drying of soil), electrical conductivity (EC1:5), exchange able cations, EC/exchangeable Na ratio, CEC, CaCO equivalent, pH, and organic C1:53. Intensive tillage resulted in soil adjacent to cracks having lower exchangeable Ca, iCEC, and CaCO3 ksequivalent, and higher organi...

Soil organic carbon concentrations and storage in irrigated cotton cropping systems sown on permanent beds in a Vertosol with restricted subsoil drainage

Abstract. Long-term studies of soil organic carbon dynamics in two- and three-crop rotations in irrigated cotton (Gossypium hirsutum L.) based cropping systems under varying stubble management practices in Australian Vertosols are relatively few. Our objective was to quantify soil organic carbon dynamics during a 9-year period in four irrigated, cotton-based cropping systems sown on permanent beds in a Vertosol with restricted subsoil drainage near Narrabri in north-western New South Wales, Australia. The experimental treatments were: cotton–cotton (CC); cotton–vetch (Vicia villosa Roth. in 2002–06, Vicia benghalensis L. in 2007–11) (CV); cotton–wheat (Triticum aestivum L.), where wheat stubble was incorporated (CW); and cotton–wheat–vetch, where wheat stubble was retained as in-situ mulch (CWV). Vetch was terminated during or just before flowering by a combination of mowing and contact herbicides, and the residues were retained as in situ mulch. Estimates of carbon sequestered by above- and below-ground biomass inputs were in the order CWV >> CW = CV > CC. Carbon concentrations in the 0–1.2 m depth and carbon storage in the 0–0.3 and 0–1.2 m depths were similar among all cropping systems. Net carbon sequestration rates did not differ among cropping systems and did not change significantly with time in the 0–0.3 m depth, but net losses occurred in the 0–1.2 m depth. The discrepancy between measured and estimated values of sequestered carbon suggests that either the value of 5% used to estimate carbon sequestration from biomass inputs was an overestimate for this site, or post-sequestration losses may have been high. The latter has not been investigated in Australian Vertosols. Future research efforts should identify the cause and quantify the magnitude of these losses of organic carbon from soil.

An integrated mechanical and chemical method for managing prostrate cover crops on permanent beds

Cover crops in minimum or no-tilled systems are usually killed by applying one or more herbicides, thus significantly increasing costs. Applying herbicides at lower rates with mechanical interventions that do not disturb or bury cover crop residues can, however, reduce costs. Our objective was to develop a management system with the above-mentioned features for prostrate cover crops on permanent beds in an irrigated Vertisol. The implement developed consisted of a toolbar to which were attached spring-loaded pairs of parallel coulter discs, one set of nozzles between the individual coulter discs that directed a contact herbicide to the bed surfaces to kill the cover crop and a second set of nozzles located to direct the cheaper glyphosate to the furrow to kill weeds. The management system killed a prostrate cover crop with less trafficking, reduced the use of more toxic herbicides, carbon footprint, labor and risk to operators. Maximum depth of compaction was more but average increase was less than that with the boom sprayer control.