The Australian Cotton Industry and four decades of deep drainage research: a review

The Australian cotton industry and governments have funded research into the deep-drainage component of the soil–water balance for several decades. Cotton is dominantly grown in the northern Murray–Darling and Fitzroy Basins, using furrow irrigation on cracking clays. Previously, it was held that furrow irrigation on cracking clays was inherently efficient and there was little deep drainage. This has been shown to be simplistic and generally incorrect. This paper reviews global and northern Australian deep-drainage studies in irrigation, generally at point- or paddock-scale, and the consequences of deep drainage. For furrow-irrigated fields in Australia, key findings are as follows. (i) Deep drainage varies considerably depending on soil properties and irrigation management, and is not necessarily ‘very small’. Historically, values of 100–250 mm year–1 were typical, with 3–900 mm year–1 observed, until water shortage in the 2000s and continued research and extension focussed attention on water-use efficiency (WUE). (ii) More recently, values of 50–100 mm year–1 have been observed, with no deep drainage in drier years; these levels are lower than global values. (iii) Optimisation (flow rate, field length, cut-off time) of furrow irrigation can at least halve deep drainage. (iv) Cotton is grown on soils with a wide range in texture, sodicity and structure. (v) Deep drainage is moderately to strongly related to total rainfall plus irrigation, as it is globally. (vi) A leaching fraction, to avoid salt build-up in the soil profile, is only needed for irrigation where more saline water is used. Drainage from rainfall often provides an adequate leaching fraction. (vii) Near-saturated conditions occur for at least 2–6 m under irrigated fields, whereas profiles are dry under native vegetation in the same landscapes. (viii) Deep drainage leachate is typically saline and not a source of good quality groundwater recharge. Large losses of nitrate also occur in deep drainage. The consequences of deep drainage for groundwater and salinity are different where underlying groundwater can be used for pumping (fresh water, high yield; e.g. Condamine alluvia) and where it cannot (saline water or low yield; e.g. Border Rivers alluvia). Continuing improvements in WUE are needed to ensure long-term sustainability of irrigated cropping industries. Globally there is great potential for increased production using existing water supplies, given deep drainage of 10–25% of water delivered to fields and WUE of <50%. Future research priorities are to further characterise water movement through the unsaturated zone and the consequences of deep drainage.

The effect of sodicity on cotton: Does soil chemistry or soil physical condition have the greater role?

Soil sodicity is widespread in the cracking clays used for irrigated cotton (Gossypium hirsutum L.) production in Australia and worldwide and sometimes produces nutrient imbalances and poor plant growth. It is not known whether these problems are due primarily to soil physical or to soil chemical constraints. We investigated this question by growing cotton to maturity in a glasshouse in large samples of a Grey Vertosol in which the exchangeable sodium percentage (ESP) was adjusted to 2, 13, 19, or 24. A soil-stabilising agent, anionic polyacrylamide (PAM), was added to half the pots and stabilised soil aggregation at all ESPs. Comparison of the effect of ESP on cotton in the pots with and without PAM showed that, up to ESP of 19, the soil physical effects of sodicity were mainly responsible for poor cotton performance and its ability to accumulate potassium. At ESP >19, PAM amendment did not significantly improve lint yield, indicating that soil chemical constraints, high plant sodium concentrations (>0.2%), and marginal plant manganese concentrations limited plant performance. Further research into commercial methods of amelioration of poor physical condition is warranted rather than application of more fertiliser.

Influence of cracking clays on satellite observed and model simulated soil moisture

Vertisols are clay soils that are common in the monsoonal and dry warm regions of the world. A defining feature of these soils is the development of shrinking cracks during dry periods, the effects of which are not described in land surface models nor considered in the surface soil moisture estimation from passive microwave satellite observations. To investigate the influence of this process we compared the soil moisture (θ in m3 m−3) from AMSR-E observations and the Community Land Model (CLM) simulations over vertisols across mainland Australia. Both products agree reasonably well during wet seasons. However, during dry periods, AMSR-E θ falls below values for surrounding non-clays, while CLM simulations are higher. The impacts of soil property used in the AMSR-E algorithm, vegetation density and rainfall patterns were investigated, but do not explain the observed θ patterns. Analysis of the retrieval model suggests that the most likely reason for the low AMSR-E θ is the increase in soil porosity and surface roughness through cracking. CLM does not consider the behavior of cracking clay, including the further loss of moisture from soil and extremely high infiltration rates that would occur when cracks develop. Analyses show that the corresponding water fluxes can be different when cracks occur and therefore modeled evaporation, surface temperature, surface runoff and groundwater recharge should be interpreted with caution. Introducing temporally dynamic roughness and soil porosity into retrieval algorithms and adding a "cracking clay" module into models, respectively, may improve the representation of vertisol hydrology.

On the nature of soil aggregate coalescence in an irrigated swelling clay

Aggregate coalescence in irrigated cracking clays constrains crop yields, yet little is known about it or how it can be managed. A measure of coalescence is introduced to separate the effects of natural aggregate-bed densification from those of age-hardening; this measure, χ, comprises a ratio of the net change in (tensile or penetrometer) strength, Y, that occurs in relation to the corresponding net change in dry bulk density, ρb, as follows: χ = ΔY/Δρb. A laboratory study was conducted to illustrate the variation in χ for a virgin and cultivated cracking clay exposed to 16 weekly cycles of wetting and draining. Penetrometer resistance and tensile strength at –100 kPa, plus bulk density and other physical and chemical properties, were measured throughout the experiment. The cultivated soil rapidly became denser and stronger, it developed larger aggregates, and its water-uptake rate in the air-dry state was significantly greater than that for the virgin soil. The &chi; values suggested that age-hardening processes constituted a greater component of coalescence in the cultivated soil than it did in the virgin one, and this was thought to be mediated by the large differences in the content and composition of organic matter in the two soils.

Structural decline of soils, assessment and prevention

Two extreme textural types of cultivated surface soils are mainly considered here, non-shrinking red-brown earths and highly shrinking cracking clays. Total porosity is used to assess the structural status of the former. Values are compared with the highest and lowest values found in the field. For the latter, the criterion used is the porosity of dry aggregates or clods. Values here are taken from the literature. To find out why inter-particle bonding in soil aggregates is insufficient to stop structural decline, a scheme has been developed which includes a modified version of Emerson's (1967) classification of soil aggregates. Slaking is carefully assessed. The bulk density of a cube made from soil at 'field capacity' is measured as well as testing another for dispersion. Class 3 is now divided into 3a and 3b, according to the degree of dispersion of remoulded soil in water. Also apart from soils which disperse spontaneously from dry, classes 1 and 2, the dispersion of all soils is assessed after remoulding at 'field capacity'. It has been found that the red-brown earth site which had the best visual structure also had the largest total porosity and aggregates in class 4. At the worst site, aggregates were in class 3a and the porosity had been reduced to that of the soil cube. For cracking clays, porosity is appreciably higher where the aggregates are in class 4 rather than class 3a. Water content/dispersion curves are presented for the clays showing the extent of the increase in OD apparently associated with the presence of carbonate. Dispersion of sheared, class 3a soil immersed in water is only an outward sign of the structural damage caused when the soil is sheared too wet. If the soil is dried instead, porosity is still lost. Mechanisms are suggested by which the structure of class 3a clay soils are improved by adding carbonate. The slumping of red-brown earths and the use of surface dressings of gypsum to prevent severe dispersion after cultivation wet are discussed. The structural stability of aggregates in the other five classes is briefly considered. Classes 1 and 2 require an ameliorant to be added, the rest pose few problems.

Winter cereal production on the Darling Downs dash a comparison of reduced tillage practices

Five experiments, 1 of which was continued over 3 years on the same site, were established on non-sloping Darling Downs cracking clays to compare conventional, reduced and zero tillage systems of fallowing for annual wheat production. Average values for soil water storage efficiency (percentage of fallow rainfall stored) were 14.0% for stubble burnt and conventional cultivation with tined implements (TI); 19.8% for stubble retained and conventional cultivation with tined implements (T2); 25.3% for stubble retained and zero tillage with chemical control of fallow weed growth (T3); 21.1% for stubble retained with no tillage but chemical weed control until early March, followed by cultivations with tined implements until sowing (T4); and 21.1% for stubble retained and fallow cultivations with a sweep plough (T6). Nitrogen mineralisation during fallow periods was measured over 3 seasons at the final site. No major treatment differences occurred. A small mean grain yield advantage of 4.6% to T3 over T1 was established in those seasons when improved fallow water storage was obtained with T3. The lack of yield improvement by reduced tillage treatments (T4, T5 and T6) over T1 is attributed largely to above-average crop period rainfall in those seasons when the treatments had resulted in improved presowing water.

Estimates of effective rooting depth for predicting available water capacity of burdekin soils, Queensland

Estimates of rooting depth are necessary parameters in predicting available water capacity (AWC) of soils. In a recently assembled database for the Burdekin River Irrigation Area, no single criterion, commonly used to estimate rooting depth, was available for all sites. Therefore a number of methods of estimating rooting depth which give interchangeable results were required. This paper compares eight methods of estimating rooting depth within three AWC models and compares the outcome with field determinations. Soil properties used to estimate rooting depth were laboratory-based (two chloride methods, electrical conductivity and pH), morphological (carbonate and mottling) and two fixed depths (0.9 and 1.0 m). For all soils tested, the laboratory-based methods used within one AWC model (based on regression equations by using -1500 kPa water retained) resulted in predicted AWC values not significantly different (P< 0.05) from field measurements. The suitability of mottling was limited to cracking clays and sodic duplex soils and other rooting depth methods had varying applicability depending on soil type. This work shows that a range of rooting depth methods can be used to predict AWC of Burdekin soils. The results should have application to soils of other areas.