The current status of nitrogen fertiliser use efficiency and future research directions for the Australian cotton industry

Fifty years of sustained investment in research and development has left the Australian cotton industry well placed to manage nitrogen (N) fertiliser. The average production in the Australian cotton industry today is greater than two tonnes of lint per hectare due to improved plant genetics and crop management. However, this average yield is well below the yield that would be expected from the amount of N fertiliser used. It is clear from the recent studies that across all growing regions, conversion of fertiliser N into lint is not uniformly occurring at application rates greater than 200–240 kg·hm−2 of N. This indicates that factors other than N availability are limiting yield, and that the observed nitrogen fertiliser use efficiency (NFUE) values may be caused by subsoil constraints such as sodicity and compaction. There is a need to investigate the impact of subsoil constraints on yield and NFUE. Gains in NFUE will be made through improved N fertiliser application timing, better targeting the amount of fertiliser applied for the expected yield, and improved soil N management. There is also a need to improve the ability and confidence of growers to estimate the contribution of soil N mineralisation to the crop N budget. Many Australian studies including data that could theoretically be collated in a meta-analysis suggest relative NFUE values as a function of irrigation technique; however, with the extensive list of uncontrolled variables and few studies using non-furrow irrigation, this would be a poor substitute for a single field-based study directly measuring their efficacies. In irrigated cotton, a re-examination of optimal NFUE is due because of the availability of new varieties and the potential management and long-term soil resilience implications of the continued removal of mineralised soil N suggested by high NFUE values. NFUE critical limits still need to be derived for dryland systems.

The current status of nitrogen fertiliser use efficiency and future research directions for the Australian cotton industry
MACDONALD Ben C. T., LATIMER James O., SCHWENKE Graeme D., NACHIMUTHU Gunasekhar and BAIRD Jonathan C.
Journal of Cotton Research. 2018; 1:15.
https://doi.org/10.1186/s42397-018-0015-9

PLANT RESPONSES TO LATE SEASON WATER DEFICITS IN ACALA COTTON CULTIVARS

Abstract                                                                       Back to Table of contents

Continued development of irrigation strategies that minimize crop yield losses, while increasing water use efficiency are needed in many semi-arid and arid cotton producing areas world wide.  Controlled deficit irrigation (CDI) incorporates the knowledge of crop physiology and phenology to identify specific plant growth stages in which water deficits have a minimal impact on crop yield and quality.  Previous research in California and other irrigated agricultural regions have documented the severe yield impact when moderate early season water stress is allowed to accumulate in cotton (Gossypium hirsutum L.).  Conversely, similar late season water stress following plant cutout has had a minimal impact on crop yield and quality.  This paper is a report of recent studies that were undertaken in an effort to apply the concepts of CDI for cotton (Gossypium hirsutum L.) and suggest approaches to farm water managers which enable a greater understanding of deficit irrigation strategies.  Studies conducted in the San Joaquin Valley of California from 1991 to 1993 have consistently demonstrated that high yields can be obtained although late season water deficits in cotton were present.  The optimum timing of the final in-season irrigation for cotton was shown to be dependent on the cultivar.  The determinant plant types tended to have more significant yield reductions as water stress is increased following plant cutout, while the indeterminate types were found to be less sensitive to the timing of late season water stress.  The timing of late season water directly alters the stress accumulated in the crop thereby impacting late season boll retention, boll maturation and crop yield.

Conclusions 

The monitoring of cotton plant performance characteristics following periods of induced late season water stress can assist in developing deficit irrigation management strategies.  To date, very little information is available regarding modern cotton cultivators and their tolerance to late season water stress.  By varying the degree and timing of water stress accumulation, we can begin to recognize both general trends for timing the final irrigation and select varieties that suit an individual field managers needs with respect to timing the final irrigation.    Moderate plant water stress induced late in the season, can reduce consumptive water use without severely impacting on crop yield or quality.  The decision of when to time the final irrigation for cotton, is dependent upon the variety and the degree of water deficits.  Delayed scheduling of late season water is preferred for more indeterminate plant types resulting from their improved tolerance to water deficits.  Shorter season, more determinant plant types, experienced significant yield reductions when moderate late season water stress was allowed to build.  The preferred irrigation strategies for determinant varieties would therefore favor the deliver of available water supplies prior to the development of moderate water stress levels (-21 bars).

The pressure chamber can be an effective tool in evaluating the intensity and duration of cotton water stress.  Generally, plants performed well with highest yields obtained when LWP readings were not allowed to exceed -23 bars.  Significant impacts on plant growth and fruit retention were observed when LWP readings were allowed to reach the wilting point of -30 bars.  At these high water stress levels, decreases in transpiration rate and photosynthesis are likely causes of delayed fruit set and hence the production of unharvestable late season bolls.  The production of these late season bolls, although not equivalent to the lost production of lower fruiting positions, does demonstrate a resiliency of cotton to partially recover from severe water stress levels.

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DETERMINING NON-EROSIVE VELOCITIES FOR DESIGN OF CONTOUR BANKS

Abstract                                                                         Back to Table of contents

One farming system found to give good control of erosion on sloping agricultural land is the use of agronomic measures in conjunction with contour banks.  Contour banks constructed with a constant gradient result in layouts with interbank areas of uneven width.  This leads to inefficient operation of machinery.  For more parallel workbays, gradients along contour banks must vary.  Limits to the gradients, both upper and lower, need to be defined to prevent damage.  Water was run down five contour banks, of 0.1, 0.3, 0.6, 1 and 3% gradient respectively, at five rates, increasing to 450 l/s, on a cracking clay soil when in a fine tilth state and again after a crop of forage sorghum had been grown and grazed.  Unacceptable soil loss occurred from all channels except that on 0.1% gradient, where soil was redistributed over the channel floor.  No net deposition of sediment occurred in any of the contour bank channels studied here.  Soil shear strength increased with depth and time since wetting.  This influenced sediment concentrations which, for a given flow rate, decreased with time.  It the stream power of flows exceeded about 0.003 g m3 /cc s3 then excessive soil movement occurred.  The stream power of design flows should be kept below that when designing contour banks for cracking clay soils.

Conclusions

Unacceptable scouring of contour bank channels occurred for all flows where the channel gradient was 1% or greater.  No net loss of soil occurred for a contour bank with a channel gradient of 0.1%.  For channels with gradients between 0.1% and 1%, increasing discharges resulted in increasing soil loss.

When designing contour banks on dark cracking clay soils, the stream power of design discharges should be kept below 0.003 g m3 / cc s3.  This design criteria is limited to this soil type.  Similar criteria should be developed for other soil types.

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VIRTUAL COTTON: A NEW TOOL FOR RESEARCH, MANAGEMENT AND TRAINING

Abstract                                                                         Back to Table of contents

“Virtual plants” are computer abstractions which contain sufficient spatial information for realistic images of successive growth stages to be generated.  Virtual plants are assembled by software which interprets rules of growth worked out from measurements of real plants.  The software keeps track of the shapes and 3-dimensional positions of plant parts over time, calculating geometric and topological properties as well as numbers of squares, bolls and other parts.  Output can be numeric as well as in the form of schematic plant maps or realistic images.  Time-lapse photography can be simulated by sequential display of images.  The spatial information may be used to calculate such things as how leaf shape and internode length affect canopy penetration by light and insecticides. Interactions between the arrangements of fruiting points and meristems, locations of damage and distances between pest feeding sites may be taken into account in predicting compensatory growth and in defining thresholds for application of pesticides.  Simulated performance of hypothetical plant architectures could suggest target forms for plant breeders and genetic engineers.  Decision making and training of farm managers should be improved by using dynamic plant maps and realistic images to visualise the likely results of alternative management actions.  Personal computers able to run this software should be available within about four years.

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STRUCTURE REPAIR WITH WET/DRY CYCLES IN A COTTON SOIL

Abstract                                                                         Back to Table of contents

Eighty percent of irrigated cotton in Australia is grown under the management system of retained beds.  In this system wheel traffic is restricted to furrows and beds are only lightly cultivated.  Furrow compaction becomes a problem when hills need replacing either due to poor alignment of beds or when wheels compact the “shoulders” of beds.  As the soils have high clay contents and contain swell/shrink clay minerals, compaction can be repaired by wet/dry cycles.

A laboratory and field trial was undertaken to assess soil structure repair with repeated wet/dry cycles.  In the laboratory, intact soil cores (0.3 m dia. x 0.5 m high) from the furrows of a Vertisol were subject to wet/dry cycles.  Wetting was by rainfall and flooding, and drying was by evaporation.  Structure repair was assessed by measuring the changes in four soil physical properties:  total water infiltration (at each wetting event), torsional shear strength, water infiltration by disc permeameter and the digitising of soil cracks from photographs of the soil surface.

Seven flood wet/dry cycles gave an almost 14% increase in total water infiltration relative to the amount of water added to the soil at the first wetting occasion.  Total infiltration dropped significantly, though, after nine flood wet/dry cycles and this was associated with a 50% reduction in surface cracks, as fine cracks developed with increased wet/dry cycles. Rain wetting increased percent crack (per unit surface soil area) up to nine wet/dry cycles, but was not associated with significant or large increases in total water infiltration. Infiltration from the disc permeameter was almost double for flood wetting than rain wetting, reaching a maximum for flood wetting after five wet/dry cycles, that was maintained to after nine wet/dries.  Rain wetting gave slightly lower shear strength of the soil surface than flood wetting, with both wetting types giving the maximum decrease in shear strength after five wet/dry cycles with only a small further reduction to nine.

The study demonstrated soil structure repair solely by wetting and subsequent water evaporation.  Flood wetting is more effective in repairing the soil to depth than rain wetting as demonstrated by greater water infiltration into the flood-wetted soil with repeated wetting/drying.  Rain-wetting gave a coarser surface structure with no related increase in infiltration; the large surface cracks rapidly closing with wetting, so forming a surface seal.

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SOIL MANAGEMENT OPTIONS FOR COTTON-BASED FARMING SYSTEMS IN SWELLING AND NON-SWELLING SOILS

Abstract                                                                         Back to Table of contents

The yield of cotton lint in Australia has increased greatly over the last 15 years.  This improvement is due partly to the development of soil management systems that are based on the objective measurement of soil structure in individual fields.  Average yields now exceed those of other major producers elsewhere in the world.  Grey swelling clays (Vertisols) dominate, but hard-setting red duplex soils with non-swelling surfaces (Alfisols) are also important for cotton in some areas.  On these soils mechanical compaction and instability in water are soil structural problems that can cause major yield declines if managed incorrectly.  Most Australian cotton is grown on ridges and is furrow irrigated.

Available methods for overcoming natural and man-induced soil structural problems include shrinkage crack formation (created by drought-stressed rotation crops; particularly useful for Vertisols), biopore formation and organic matter accumulation (due to decomposing roots, and soil fauna such as earthworms and ants; particularly useful for the topsoil of Alfisols), low draft deep tillage, and the use of gypsum and lime.  Extra water and nitrogen fertiliser can be used to obtain high yields on degraded soils, but such an approach tends to be inefficient and may cause off-site pollution.  Where the soil has favourable conditions for cotton root growth and water movement, ‘controlled traffic – reduced tillage’ systems are recommended to minimise costs.

In the future it is necessary to provide soil structure assessment procedures that are more objective and many of the procedures for improving and living with degraded soils need to be refined. Also, better farm machinery should be developed for controlled traffic systems under cotton; vital are engineering and soil mechanics inputs to optimise axle loadings, and tyre and tool dimensions and configurations, on soils with different water contents and pre-stress conditions.

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THE OPTIMISATION OF SOIL STRUCTURE FOR COTTON PRODUCTION

Abstract                                                                         Back to Table of contents

In cropping soils, optimising soil structure means minimising soil structure degradation and emphasising natural processes of structure formation.  Man-made soil structure degradation (compaction and shear stresses) has been shown to reduce green boll numbers in cotton up to 50% and cotton lint by 35%.

The Australian Cotton Industry is now, actively minimising the problem of soil structure degradation and providing the basis for optimising soil structure with the adoption of “retained-bed” systems.  In this form of controlled traffic, tractor and equipment wheels are restricted to specific furrows and the beds are only lightly cultivated.  Beds have been retained for up to seven years, which differs dramatically from practices of the 1960s and 1970s where beds were removed after harvest, the soil cultivated and the beds reformed for the next season.

Heavy, cracking clays (Vertisols) account for 84% of the irrigated cotton area.  The nature of these soils ensures that a retained bed system minimises structure degradation and allows natural processes of structure formation to be optimised.  The high water holding capacity of these soils causes them to remain in a plastic (mouldable) state for long periods of time.  However, the presence of swell-shrink clay minerals ensures cracking on drying, and with repeated wet/dry cycles the maintenance or re-formation of suitable soil structure for cropping.  Non-cracking soils (Alfisols) can also benefit with the retained bed system. Reducing mechanical disturbance increases organic matter (from break crops) and increases biological activity, leading to less surface crusting and an increase in the number of continuous macropores from the surface to depth.

Monitoring physical improvements in soil structure under cotton continues at the field, glasshouse and laboratory levels.  Measures include aeration, shrinkage, strength, water infiltration and image analysis. Studies cover several cotton regions and different bed systems and soils.  In new research the potential risk of structure degradation in cropping soils and the potential of these soils for self repair are being investigated.  These classifications will be based on the soil’s plastic limit, inherent properties that affect workability and repair potential (cations, clay content, etc.) and the dynamic forces of irrigation and weather.

Conclusions

Almost two decades ago soil structure degradation was recognised as a significant potential threat to the productivity of Australian cotton.  The problem has been intensively researched, leading to practical management strategies to not only prevent and control structure degradation but also to optimise the soil physical environment for reduced-cost cotton production.  Retained beds and minimum tillage of dry soil together with rotation crops will ensure optimum soil structure conditions that maximise natural structure formation and maintenance with minimum effort.  Structure degradation can not be eradicated, but current practices ensure its control and prevents it from being a problem.

Future research will continue to emphasise the optimisation of natural structure formation. However, research currently underway in the Department of Primary Industries (Queensland), is aiming to forecast the potential risk of structure degradation in soils and their potential for self repair.  It is important to know if some soils are more prone to degradation than others, so they can be managed with maximum care.  Alternatively, soils with a high self-repair potential can be cropped more frequently as structure regeneration will be achieved through wet/dry cycles especially with rotation crops.  Additionally, determination of the inherent physical/chemical properties that lead to greater strength or resilience may lead to the use of soil ameliorants, akin to fertiliser inputs to optimise chemical response.

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COMPACTION IN COTTON BEDS – MEASUREMENTS, MODELLING AND MANAGEMENT

Abstract                                                                         Back to Table of contents

We used field measurements and computer modelling of soil compaction to investigate management options for trafficking cotton beds.

In the field, we measured the influence of vehicles on vertical stresses in the soil and the consequent changes in soil physical properties. These effects were investigated for several different vehicles and also for different furrow and bed layouts. The impact of vehicles on soil deformation were modelled using computer simulations (finite element models). The models were tested by simulating the known soil behaviour measured in the field. The models furthered our understanding of compaction processes.

The information from the field and the models, run on “what if” scenarios, were then used to investigate compaction management options such as: wider versus narrower tyres in furrows; different furrow shapes; influence of bed width; influence of repeated wheelings; influence of initial soil moisture content.

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MANAGEMENT OF LABLAB PURPUREUS L. RESIDUES IN COTTON-BASED FARMING SYSTEMS AND CONSEQUENT EFFECTS ON PROPERTIES OF A TYPIC PELLUSTERT

Abstract                                                                         Back to Table of contents

The effects of mulching or incorporating residues of dolichos (Lablab purpureus L.), sown in rotation with cotton (Gossypium hirsutum L.), in broad (1.5 m wide) beds on soil properties of a Vertisol was studied in Northern New South Wales, Australia. Soil was sampled from the 0-0.10 m (bed surface) and 0.20-0.30 m (below bed) depths of edges and centres of broad beds during January 1993. Soil properties monitored were particulate, mineralized and total organic matter, dispersion index, plastic limit, geometric mean diameter (GMD) of soil aggregates formed after puddling and drying at 40˚C (soil reactivity), soil aggregate density, exchangeable cations, nitrate-N and electrical conductivity of 1:5 soil:water suspension. Residue management had no significant effect on soil organic matter fractions, although coarse (2 mm – 212 µm), fine (212-53 µm) and total soil organic matter contents on bed surfaces were greater than that below beds, and coarse particulate organic matter at the edges of beds was greater than that at the centres. Compared with mulching, incorporating dolichos residues resulted in a significantly lower dispersion index. Mulching also resulted in higher values of dispersion index below beds when compared with bed surfaces. Plastic limit at the centres of beds, was significantly lower than that in the edges. Smallest GMD of soil aggregates occurred in the centre of mulched beds. Greatest values of soil aggregate density occurred at soil water contents ≤ 0.10 kg kg-1 below beds when dolichos residues were mulched. Where dolichos residues were incorporated, at soil water contents ≤ 0.10 kg kg-1 aggregate densities in the soil surface were lower in bed centres when compared with those at the edges of beds. Greatest exchangeable K, and lowest exchangeable Na and exchangeable sodium percentage (ESP) occurred where dolichos residues were incorporated. In comparison with mulching, exchangeable Mg was higher and exchangeable Ca lower below beds with residue incorporation. Nitrate-N on bed surfaces was higher than that below beds with mulching. Mulching improved only friability of surface soil in bed centres, whereas surface and sub-surface indices of soil physical and chemical fertility were improved by incorporating dolichos residues.

Conclusions

Soil reactivity in the of centre bed surfaces was the only soil property to be improved by mulching of dolichos residues whereas incorporating dolichos residues improved indices of soil quality such as aggregate stability, exchangeable cations and ESP (Doran et al., 1994) in the surface and at 0.2-0.3 m. Good soil quality can, therefore, be maintained at this site by incorporating residues of dolichos sown in rotation with cotton.

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THE PRINCIPLES OF COTTON WATER RELATIONS AND THEIR APPLICATION IN MANAGEMENT

Abstract                                                                         Back to Table of contents

The keys to understanding the principles of water relations that are specific to cotton are found in: (i) the ecology of the wild ancestors of cotton  (xerophytic shrubs), (ii) the basic pattern of development (the orderly and regular production of mainstem nodes, lateral fruiting branches and fruiting sites, and the progression at each fruiting site from floral bud through to open boll or shed fruiting form), (iii) relative sensitivity of these developmental processes, and the growth physiological processes, to water stress. The biological and agronomic responses to variation in water supply are reviewed and interpreted in light of this understanding.  Cotton is well adapted physiologically for both rain grown and irrigated production, and economically for both production on plantations and small holdings.  The relative importance of rain grown and irrigated cotton on a global scale are considered.  The management requirements of each in respect to optimising use of water discussed.  Effects of excess water (water logging) are as important as water deficits.

Technological developments relevant to water relations can be broadly classified as software (rules of thumb, indices, plant mapping, osmotic adaptation, computer models and decision support systems) and hardware (neutron probes, pressure chambers, infra-red thermometry, drip irrigation, lateral shift and centre pivot irrigation). Their use in management and research applications is discussed.  Management should aim to optimise the use of limited water resources, be it rainfall or irrigation supply, by maximising returns per unit input and minimising environmental impact.  Management decisions have to be made at policy, strategic and tactical levels and appropriate software and hardware selected. There are specific important challenges and opportunities: management of limited irrigation water supplies; environmental impact of irrigation on salinity, and pesticide and nutrient pollution; the use of urban domestic wastes and saline drainage water; interaction of water with other factors; and risk analysis.

Conclusions

Modern cultivated cotton species have inherited from their wild relatives attributes that enable them to survive long periods of drought and develop rapidly when water is available, enabling the crop to make full use of variable rainfall or respond well to irrigation.  These attributes are the indeterminate habit and the relative sensitivity of physiological processes to water deficits.  The former confers a flexible morphological and reproductive development and the latter determines priorities for assimilates.  Ancestral sensitivity to the putative wet and dry “signals” from the environment imposes special constraints on management of the cotton crop in relation to water.  Research is needed to see if signals from roots in drying soils found in other crops occur in cotton, and to determine their role in the response of the crop to water deficits, particularly boll shedding, priorities for growth, and the balance of vegetative and reproductive growth.

In order to maximise returns from limited rainfall and irrigation water supplies, research and management should concentrate on improving WUE in its various aspects.  Opportunities exist to improve WUE with:

  • new irrigation technology, or better use of existing technology, in order to reduce application, conveyancing and drainage losses and reduce the E component of ET;
  • use of hardware and software to monitor crop progress for tactical optimisation of irrigation scheduling;
  • identify by using simulation software, and then adopt, robust management strategies for irrigated and rainfed crops that will minimise risk and use maximise return from limited irrigation supplies and rainfall;
  • soil surface management technology to retain rainfall and reduce runoff and soil evaporation;
  • pursuit of genetic improvement of agronomic WUE by improved gas exchange and/or partition of assimilates.

If we are successful in raising WUE we will reap the benefits of higher returns from a limiting resource, with the added potential to reduce contamination of rivers and groundwater and reduce the risk of salinisation.

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