Duplex Soils Research Articles

Factors controlling gully development: Comparing continuous and discontinuous gullies

ABSTRACTGully erosion is a degradation process affecting soils in many parts of the World. Despite the complexity of a series of collective factors across different spatial scales, previous research has not yet explicitly quantified factor dominance between different sized gullies. This factorial analysis quantifies the differences in factor dominance between continuous gullies (cgs) and discontinuous gullies (dgs). First, gullies (totaling 5273 ha) visible from SPOT 5 imagery were mapped for a catchment (nearly 5000 km2) located in the Eastern Cape Province of South Africa. Eleven important factors were integrated into a geographical information system including topographical variables, parent material‐soil associations and land use–cover interactions. These were utilized in a zonal approach in order to determine the extent factors differ between cgs and dgs. Factors leading to the development of cgs are gentle footslopes in zones of saturation along drainage paths with a large contributing area, erodible duplex soils derived from mudstones and poor vegetation cover due to overgrazing. Compared to cgs conditions, more dgs occur on rolling slopes where the surface becomes less frequently saturated with a smaller contributing area, soils are more stable and shallow. Factorial analysis further illustrates that differences in factor dominance between the two groups of gullies is most apparent for soil factors. A combination of overgrazing and susceptible mudstones proves to be key factors that consistently determine the development of cgs and dgs. Copyright © 2011 John Wiley & Sons, Ltd.

Hillslope runoff and erosion on duplex soils in grazing lands in semi-arid central Queensland. III. USLE erodibility (K factors) and cover - soil loss relationships

Measured Universal Soil Loss Equation (USLE) soil erodibility (K) values are not available for soils in grazing lands in northern Australia. The K values extrapolated from croplands are used in national and river-basin scale assessments of hillslope erosion, using an assumption that the cover factor (C) equals 0.45 for undisturbed (uncultivated) bare soil. Thus, the K needed for input into the models is the measured K for undisturbed soil (KU) divided by 0.45. Runoff and erosion data were available for 7 years on 12 hillslope plots with cover of 10–80%, with and without grazing, with and without tree canopy cover, on a variety of soils according to various soil classification systems. Soils were grouped into those derived from sandstone (SS), mudstone (MS), and eroded mudstone (MSe). These data were used to determine USLE KU, K, and C factor–cover relationships. Methods used to fit the parameters affected the results; minimising the sum of squares of errors in soil losses gave better results than fitting an exponential equation. The USLE LS (length–slope) factor explained the increase in measured average annual soil loss with slope, for plots with low cover. Erodibility (K) was 0.042 for SS and MS soils, irrespective of Australian Soil Classification (Chromosol, Kandosol, Rudosol, Sodosol, Tenosol); K was 0.062 for exposed, decomposing mudstone (MSe Leptic Rudosol). The measured K factor for SS and MS soils was close to that used in catchment-wide soil loss estimation for the site (0.039). This indicates that the method used for estimating K from soil properties (derived from cultivated soils) gave a reasonable estimate of K for the main duplex soils at the study site, as long as the correction for undisturbed soil is used in deriving K from measured data and in applying the USLE model. A 20% increase in K (0.050) for SS and MS soils may be warranted for heavy grazing by cattle. The C factor–cover relationship was different from the standard revised USLE (RUSLE) relationship, requiring a greater exponent (‘bcov’) of 0.075, rather than the default for cropland of 0.035. Increasing cover is therefore more effective at the site than suggested by the USLE. Parameters of USLE were also derived for bedload, allowing suspended load to be calculated by subtracting bedload from total soil loss.

Hillslope runoff and erosion on duplex soils in grazing lands in semi-arid central Queensland. II. Simple models for suspended and bedload sediment

The use of simple models of soil erosion which represent the main effects of management in grazing lands in northern Australia is limited by a lack of measured parameter values. In particular, parameters are needed for erosion models (sediment concentration v. cover equations) used in daily soil-water balance models. For this research, we specifically avoided equations that use rainfall and runoff rates (e.g. peak flow), as current daily models are limited in their ability to estimate these rates. The resulting models will therefore give poor estimates of soil losses for individual events, but should give good estimates of long-term average erosion and management influences. Runoff and erosion data were available for 7 years on 12 hillslope plots with cover of 10–80%, with and without grazing, with and without tree canopy cover, on a variety of soils according to various soil classification systems. Soils were grouped into those derived from sandstone (SS), mudstone (MS), and eroded mudstone (MSe). These data were used to determine two parameters, i.e. (i) efficiency of entrainment for bare soil and (ii) a cover factor, for simple models of bedload and suspended sediment concentrations. Methods used to fit parameters affected the results; optimising to obtain the minimum sum of squares of errors in soil losses gave better results than fitting an exponential equation to sediment concentration–cover data. The use of a linear slope factor in the sediment concentration models was confirmed with data from plots with slopes 4–8%. Parameters for the bedload sediment concentration model were the same for SS, MS, and MSe soils. Parameters for the suspended sediment concentration model were the same for SS and MS soils, but the MSe soil had a greater efficiency of entrainment for bare soil (about double). The sediment concentration–cover relationships and fitted cover factors were different for suspended and bedload sediment. Thus, the resulting modelled proportion of sediment as suspended load changed with cover, from ~0.3 for bare soil to 0.9 at 80% cover, mimicking the measured data. The cover factor was lower than published values for cultivated soils, indicating less reduction in sediment concentration with greater cover. A compilation of parameter values for the sediment concentration model from published and unpublished sources in grazing and cropping lands is provided.

Hillslope runoff and erosion on duplex soils in grazing lands in semi-arid central Queensland. I. Influences of cover, slope, and soil

Many soils in semi-arid grazing lands develop low pasture cover or bare areas (scalds) under heavy grazing and have a low tolerance to soil erosion, due to low total water-holding capacity and concentration of nutrients in the soil surface. Runoff and erosion was measured for 7 years on 12 hillslope plots with cover (pasture plus litter) ranging from 10 to 80%, slopes from 4 to 8%, with and without grazing, with and without tree canopy cover, on a variety of soils. Soils were grouped into those derived from sandstone (SS), mudstone (MS), and eroded mudstone (MSe). One plot with low cover had a grass filter at the outlet. Runoff was strongly influenced by surface cover and was high with low cover (200–300 mm/year or 30–50% of rainfall). Runoff averaged 35 mm/year or 5.9% of rainfall with >50% cover. All soils fitted the same runoff–cover relationship. The grass filter had no effect on runoff and suspended load, but did reduce bedload. Grass pasture cover and tree litter cover were equally effective in controlling runoff and erosion. Total, bedload, and suspended load sediment concentrations increased linearly with slope in the range 4–8% for plots with low cover, and decreased exponentially with greater cover. Total and bedload sediment–cover relationships were similar for SS, MS, and MSe. However, plots on MSe had higher suspended sediment losses and thus slightly higher total soil losses. For all soils, erosion resulted in low sediment concentrations due to the hard-set surface soil, but total soil losses were high due to the large volumes of runoff generated. Concentration–cover relationships were different for bedload and suspended sediment. Consequently, suspended sediment was 20–40% of total soil loss for bare soil, and increased with cover to about 80% with cover >80%. The proportion of suspended sediment for bare soil was similar to the proportion of dispersed silt plus clay in the surface soil. About 90% of suspended sediment was fine-sized (<0.053 mm). Bedload was mainly coarse and fine sands, which were enriched compared with the surface soil. Grazing in semi-arid pastures should be managed to maintain >50% ground cover to avoid excessive runoff and soil erosion, and degradation of soil productivity, and to maintain good off-site water quality.

Salinity dynamics and the potential for improvement of waterlogged and saline land in a Mediterranean climate using permanent raised beds

Waterlogging and salinity in the Mediterranean climate of Western Australia (WA) have long been recognized as major constraints to the production of agricultural crops and pastures. While raised beds (RB) prevent waterlogging, the impact of RB on salinity is not known. Their impact on salinity was studied at 3 sites in the Wheatbelt of WA. The salinity dynamics was captured through frequent soil sampling. Two small trials with different mulches were also conducted to investigate possibilities to alter salinity dynamics in other ways. Each of the sites responded differently to the RB. At Cunderdin, being the driest site, little impact of the precipitation on the movement of salt was found, and no difference between the RB and Control. The deeper salinity was found to be more associated with the presence of the shallow saline groundwater. At Woodanilling, a high-rainfall site, salinity changes were governed by precipitation, and the RB greatly reduced waterlogging in the top 10 cm of the soil surface and improved productivity. Little differences were found between RB and Control with respect to salinity due to the incorporation of clayey subsoil. At North Stirlings with medium rainfall, the RB were very effective in reducing waterlogging, improving productivity as well as reducing salinity, following the disruption of the gravelly subsoil layer. Applying a sand mulch made a bare salt scald reasonably productive again. The dry-soil mulch process is also thought to be responsible for the low salt concentrations near the soil surface at Cunderdin. Maintaining the physical properties of this topsoil is crucial for sustaining a productive system. It is also proposed that the salinity–waterlogging interactions are not relevant to the dry-land salinity conditions in WA as the soil surface was either waterlogged or saline but not both at the same time. Our findings have narrowed down the benefits of permanent RB to the waterlogged medium-to-high-rainfall area of WA in regards to waterlogging and loamy gravelly duplex soils in regards to waterlogging and salinity control. As growers in a changing climate (physical and economical) become interested in cropping the waterlogged and saline areas of the valley floors in WA, RB will make that a realistic option. This would equally apply to other parts of the world where waterlogging and salinity are major constraints provided due consideration is given to soil properties of the top soil, particularly the soil texture.

Advances in soil physics: Application in irrigation and dryland crop production

This is the third soil physics review to be published in South African Journal of Plant and Soil. In the previous reviews the focus was broad and covered almost every aspect of the subject, providing a comprehensive list of contributions in soil physics. For the 25th year anniversary celebration of South African Journal of Plant and Soil, I have chosen to narrow the scope and focus on advances in soil physics in relation to irrigation and dryland agriculture. From a bio-physical viewpoint, South African researchers have made a major contribution to the body of scientific knowledge about irrigation and its application, expressed mainly in the form of irrigation or crop models such as PUTU, SWB and BEWAB. Attention was also given to modern ways of irrigation scheduling based on continuous soil water monitoring. Several irrigation scheduling service providers have adapted their businesses accordingly, with the result that South Africa is probably the leading country in Africa with respect to soil water monitoring and associated communication technology. In contrast, the review has shown that at farm and irrigation scheme level, salt management requires urgent attention. This is necessary as a precautionary measure to protect our natural resources. In the second part of the review the contribution of soil physics in relation to tillage practices is explored, and in particular how these have modified the field water balance components in order to enhance yield and rain water productivity. Based on the results of field experiments, new relationships were established, viz. rainfall and maize yield; water storage and yield; runoff and surface coverage by crop residue mulches; tillage depth, texture and yield relationships. Lastly, the review also showed how the water balance on clay and duplex soils in semi-arid zones can be modified through in-field rainwater harvesting to increase their rain water productivity. This technology has enhanced the livelihoods of many communal families who have applied the technique in their homesteads.

A grazier survey of the long-term productivity of leucaena (Leucaena leucocephala)-grass pastures in Queensland

Leucaena leucocephala subsp. glabrata (leucaena)-grass pastures are productive, perennial and long-lived (>40 years). However, little is known about changes in the productivity of these pastures as they age even though they are grazed intensively and are rarely fertilised. A postal survey of beef cattle producers in Queensland who grow leucaena pastures was conducted. The questionnaire gathered information regarding: property location; extent and age of leucaena pastures; soil type; leucaena and grass establishment methodology; grazing and fertiliser management; and grazier perceptions of changes over time in leucaena productivity, grass growth and ground cover, prevalence of undesirable grasses and weeds, and livestock productivity. Graziers were asked to report on both young (≤10 years old) and aging (>10 years old) pastures under their management. Eighty-eight graziers responded describing 124 leucaena paddocks covering 11 750 ha. The survey results described the typical physical and management characteristics of leucaena pastures in Queensland. Graziers reported a decline in leucaena productivity in 58% of aging pastures, and declines in grass growth (32%) and livestock productivity (42%) associated with declining leucaena growth. Leucaena decline was greater in soil types of marginal initial fertility, particularly brigalow clay, soft wood scrub, downs and duplex soils. Maintenance fertiliser was not applied to most (98%) leucaena pastures surveyed despite significant amounts of nutrient removal, particularly phosphorus and sulphur, occurring over prolonged periods of moderate to high grazing pressure. It is predicted that large areas of leucaena pasture will continue to suffer soil nutrient depletion under current management practices. Research is needed to develop ameliorative actions to reinvigorate pasture productivity.

Advances in pedology in South Africa

The ability of soil to serve as a unique buffer of water, organic carbon, nutrients, pH, redox and temperature is recognised in environmental research. A detailed study on hardpan carbonate and dorbank horizons in the commonly occurring ‘heuweltjies’ of the arid Namaqualand coastal region revealed a ‘sepiolitic’ hardpan carbonate centre through ‘sepiolitic’ / ‘petrosepiolitic’ (hardened) to the dorbank horizon on the edges. Footprints of ferrolysis manifests in various properties of duplex and plinthic soils of the semi-arid eastern Free State as (1) acidity, (2) colour, (3) Fe-Mn, (4) mottles and concretions, (5) CEC and (6) abrupt textural differentiation. Soils with melanic A horizons cover 2% of South Africa and are confined to regions with annual rainfall between 550 and 800 mm (aridity index of 0.2–0.5). The understanding of soil formation is applied to divide a land type into soilscapes with a more homogeneous soil distribution pattern. The quantification of the water regime of South African soils showed that ‘freely drained’ soils may have significant periods of drainable water.

Accelerated testing of nutrient release rates from fertiliser granules

A method is developed for estimation of accelerated nutrient release rates into a soil from particulate fertiliser formulations, by carrying out release studies at elevated temperatures. This can provide time-saving targetting of samples to be subsequently used in field trials. The rate of total phosphorus leaching into water from a standard commercial fertiliser product embedded in grey, deep, sandy duplex soil was monitored using inductively coupled plasma-optical emission spectroscopy. The time dependence of phosphate concentration at a given temperature was found to obey simple first-order kinetics. These data fitted an Arrhenius relation within acceptable limits, enabling release rates at ambient field temperatures to be estimated significantly faster than by field experiment. A release rate calculated for the accelerated test, i.e. (1.2 ± 0.1) × 10−3/min, when compared with a more standard-type pot-trial leaching rate for the corresponding temperature of 2.3 × 10−4/min, gives a direct correlation of ~1 : 1 when corrected for concentration differences.

Catchment management in semi-arid area of central South Africa: Strategy for improving water productivity

In the semi-arid part of central South Africa, population growth and industrial development are the driving forces for an increased demand for water. This accentuates the need for wise decisions by catchment management agencies (CMAs), especially in water-scarce semi-arid areas. These decisions become more and more complex as the range of demands widens over the spectrum of water consumers, i.e. municipal, industrial, irrigation and rain-fed farming. A study was conducted in the Upper Modder River catchment, which is situated in the semi-arid area of central South Africa, where crop production in the catchment using conventional production technique is currently not suitable due to marginal and erratic rainfall. Moreover, the area is characterised by low precipitation use efficiency because of large runoff and evaporation losses on clay and duplex soils. A labour intensive in-field rainwater harvesting (IRWH) technique recently introduced into a part of the basin occupied by communal farmers has been shown to increase maize and sunflower yields by 30 to 50% compared to conventional tillage, making it a feasible option for the subsistence farmers in the catchment. The area of land suitable for the IRWH located in the communal land is estimated to be 23 000 ha. Two catchment management options presented in this paper are:• Option1: allowing the IRWH suitable land in the communal farming area to remain under grassland and utilising the runoff downstream for irrigating maize• Option 2: utilising the IRWH suitable land for maize production in the basin, using the IRWH techniqueResults showed that the expected maize production from Option 2 was higher than from Option 1. A financial analysis also showed that gross margin of option, expressed as R/ m3 of rainwater utilised, was estimated to be between 0.0234 to 0.0254 under Option 1 and 0.0354 for Option 2. This clearly shows that use of rainwater where it falls has high socio-economic benefits for the communal farmers who are currently struggling to achieve sustainable livelihoods.

Distribution and Speciation of Nutrient Elements around Micropores

In Australia a class of soils known as duplex soils covers approximately 20% of the continent. Their defining characteristic is a sharp texture contrast between the A (or E) and B horizon. The upper B horizon at the point of contact with the E horizon is often highly sodic and of such a high strength that root growth and proliferation, water conductivity, aeration, water storage, and water uptake are restricted. Roots growing in these soils rely on channels created by previous roots or cracks arising from shrink–swell forces associated with seasonal wetting and drying. Although the characteristics of rhizospheres compared with the soil matrix are well documented there is a paucity of knowledge about how long these changes persist after roots decay. This knowledge is fundamental to our understanding of root growth in duplex soils in which plants rely on pore networks formed by previous plants to proliferate in the subsoil. In this study we investigated the heterogeneous chemistry of micropores in situ using synchrotron‐based μ‐x‐ray fluorescence spectroscopy (XRF), μ‐x‐ray absorption near edge structure spectroscopy (XANES), and extended μ‐x‐ray absorption fine structure spectroscopy (EXAFS). The distribution maps of Ca, Mn, Fe, Cu, and Zn at micrometer resolution were collected using μ‐XRF. Subsequently, specific locations with higher concentrations (hot spots) of Mn, Fe, Cu, or Zn were selected and XANES and EXAFS spectra were collected to study the speciation of these elements around the micropore compared with the soil matrix. The μ‐XRF maps showed that Mn was depleted around one of the micropores studied but accumulated around another micropore. Copper and Zn accumulated around the micropores, whereas Ca was predominantly inside micropores. There was no difference between matrix and micropore surface with respect to the distribution of Fe. Around micropores Mn was present in reduced form (Mn II) and Fe was in its oxidized form (Fe III). Manganese and Cu were present in the form of phosphates, Fe as Fe oxides, and Zn as Zn phosphates and adsorbed Zn.

Root biomass distribution and soil properties of an open woodland on a duplex soil

Data on the distribution of root biomass are critical to understanding the ecophysiology of vegetation communities. This is particularly true when models are applied to describe ecohydrology and vegetation function. However, there is a paucity of such information across continental Australia. We quantified vertical and horizontal root biomass distribution in a woodland dominated by Angophora bakeri and Eucalyptus sclerophylla on the Cumberland Plains near Richmond, New South Wales. The site was characterised by a duplex (texture contrast) soil with the A horizon (to 70 cm) consisting of loamy sand and the B horizon (to > 10 m) consisting of sandy clay. The topsoil had a smaller bulk density, a smaller water holding capacity but a larger organic component and a larger hydraulic conductivity in comparison to the subsoil. Root biomass was sampled to 1.5 m depth and declined through the soil profile. Whilst total biomass in the B horizon was relatively small, its contribution to the function of the trees was highly significant. Coarse roots accounted for approximately 82% of the root mass recovered. Lateral distribution of fine roots was generally even but coarse roots were more likely to occur closer to tree stems. Variation in tree diameter explained 75% of the variation in total below-ground biomass. The trench method suggested the belowground biomass was 6.03 ± 1.21 kg m−2 but this method created bias towards sampling close to tree stems. We found that approximately 68% of root material was within a 2 m radius of tree stems and this made up 54% of the total number of samples but in reality, only approximately 5 to 10% of the site is within a 2 m radius of tree stems. Based on these proportions, our recalculated belowground biomass was 2.93 ± 0.59 kg m−2. These measurements provide valuable data for modeling of ecosystem water use and productivity.

A procedure for mapping the depth to the texture contrast horizon of duplex soils in south-western Australia using ground penetrating radar, GPS and kriging

A procedure for the rapid and accurate mapping of the depth to the texture contrast horizon of duplex soils was trialled. Data were collected using ground-penetrating radar with a 250 MHz antenna at 4 sites in the Esperance region of Western Australia. The contrast at the B horizon was identified using a ‘picking’ process similar to seismic surveys. The GPS data were integrated and depth to B horizon maps produced by kriging. The results were related to core data taken for sites and showed that GPR can provide accurate and detailed subsurface maps with ±0.1 m accuracy for B horizon depth. The influence of B horizon material composition on GPR response was also investigated using amplitude mapping. The spacing between GPR transects can affect map quality, particularly if the lateral variation in the B horizon depth is not adequately sampled by the line spacing selected. With further integration of the data collection and post-processing procedures, this would prove to be a useful tool for farmers and natural resource managers.

2-Dimensional soil and vadose-zone representation using an EM38 and EM34 and a laterally constrained inversion model

The network of prior streams and palaeochannels common across the Riverine Plains of the Murray–Darling Basin act as conduits for the redistribution of water and soluble salts beneath the root-zone. To improve scientific understanding of these hydrological processes there is the need to better represent and map the connectivity and spatial extent of these physiographic and stratigraphic features. Groundbased electromagnetic (EM) instruments, which measure bulk soil electrical conductivity (σa), have been used widely to map their areal distribution across the landscape. However, methods to resolve their location with depth have rarely been attempted. In this paper we employ a 1-D inversion algorithm with 2-D smoothness constraints to predict the true electrical conductivity (σ) at discrete depth increments using EM data. The EM data we use include the root-zone measuring EM38 and the deeper sensing EM34. We collected EM38 data in the vertical (EM38v) and horizontal (EM38h) dipole modes and EM34 data in the horizontal mode and coil spacing of 10, 20, and 40 m (respectively, EM34-10, EM34-20, and EM34-40). In order to compare and contrast the value of the various EM data we carried out multiple inversions using different combinations, which include: independent inversions of (i) EM38 (root-zone) and (ii) EM34 data (vadose-zone), and in combination using (iii) EM38v, EM38h, and EM34-10 (near-surface), and (iv) all 5 EM datasets (regolith) available. The general patterns of σ are shown to compare favourably with the known pedoderms, physiographic, and stratigraphic features and soil particle size fractions collected from calibration cores drilled across the lower Macquarie Valley study area. In general we find that the EM38 assists in resolving root-zone variability, specifically duplex soil profiles and physiographic features such as prior streams, while the use of the EM34 assists in resolving the stratigraphic nature of the vadose-zone and specifically the likely location of palaeochannels and subsurface anomalies that may indicate the location of good quality groundwater and/or clay aquitards. In this case, our potential to use σ to predict clay content is limited by the non-linearity of the cumulative functions. In order to improve on the non-linearity of our inversion we need to develop a full solution of the forward problem.

Waterponding: Reclamation technique for scalded duplex soils in western New South Wales rangelands

Summary Building on previous trials initiated in the 1960s, a demonstration programme involving 18 landholders was established at Nyngan, New South Wales Australia; in the mid‐1980s to refine ‘waterponding’ techniques used to rehabilitate scalded claypans. The waterponding technique involves building horseshoe shaped banks (about 240 m in length) to create ponds of about 0.4 ha each. Each pond retains up to 10 cm of water after rain which leaches soluble salts from the scald surface. This improves the remaining soil structure, inducing surface cracking, better water penetration and allows entrapment of wind‐blown seed. Consequently, niches are formed for the germination of this (and any sown) seed and recovery of a range of chenopod native pasture species occurs on the sites, which can be supplemented by direct seeding.What started as a project continues now as a standard rangeland rehabilitation process for reclaiming bare, scalded semi‐arid areas of New South Wales and turning them back into biodiverse and productive rangelands. Since 1985, further modifications have been made to the method and the ongoing programme has surveyed, marked out and built approximately 56 700 waterponds within the Marra Creek waterponding district.

Herbage accumulation, botanical composition, and nutritive value of five pasture types for dairy production in southern Australia

Four pasture treatments were compared with a perennial ryegrass (Lolium perenne) and white clover (Trifolium repens) mixture, the commonly sown pasture type, at 3 sites in south-western Victoria, to determine the extent to which total annual herbage accumulation and seasonal growth pattern could be manipulated in non-irrigated dairy systems. The pasture treatments were: (1) short-term winter-active (STW), based on Italian ryegrass (Lolium multiflorum); (2) long-term winter-active (LTW), based on Mediterranean tall fescue (Festuca arundinaceae) or cool-season active perennial ryegrass; (3) long-term summer-active (LTS), based on Continental tall fescue; and (4) perennial ryegrass pasture receiving moderately high nitrogen (N) inputs in winter and spring (total of 210 kg N/ha.year) to increase herbage accumulation during the ryegrass growing season (RHN). The perennial ryegrass and white clover mixture served as the control treatment. The 3 sites were Heytesbury (heavy clay soil), Terang (duplex soil), and Naringal (light soil). All pasture treatments were grazed by dairy cows. Herbage accumulation rate, botanical composition, and nutritive value of all pasture treatments were measured for 3 years. The winter-active pasture types had little effect on the overall seasonal distribution of forage supply or total annual yield compared with the control treatment. By contrast, the long-term summer-active pasture type significantly increased herbage accumulation during summer compared with all other pasture types (mean of an additional 1.3 t DM/ha during summer compared with the control treatment across all sites and years). However, this came at the cost of lower winter production (mean 0.8 t DM/ha). The LTS pasture also produced less dry matter than all other treatments in the first year after sowing at 2 of the 3 sites, reflecting the slower establishment of tall fescue. Ryegrass receiving high N inputs grew more total herbage on an annual basis than LTS. Mean total annual herbage accumulation (kg DM/ha.year) over the 3 years and 3 sites was in the order: RHN (14400) > LTS (13760) = Control (13170) > STW (12450) = LTW (12170) (P < 0.01). The results demonstrated that improvements over the industry ‘standard’ perennial ryegrass/white clover pasture management system in total herbage accumulation and the seasonal pattern of pasture growth are possible using existing pasture technology (mixtures of different species and cultivars, and N fertiliser inputs). However, significant interactions among sites, years and pasture treatments for several variables reinforced the need to understand limitations to plant growth on a site-by-site basis, to select plants that match the environmental conditions, and to then manage them appropriately to reach their growth potential.

Up-scaling of rain-water harvesting for crop production in the communal lands of the Modder River basin in South Africa: Comparing upstream and downstream scenarios

The study area is the Upper and Middle Modder River basin situated in a semi-arid area of central South Africa. This is an important catchment because of the relatively large nearby towns of Bloemfontein, Botshabelo and Thaba Nchu. Crop production in the basin using conventional production techniques is currently not suitable due to marginal and erratic rainfall, and high evaporative demand, as well as low precipitation use efficiency on the clay and duplex soils caused by large runoff and evaporation losses. A labour-intensive in-field rain-water harvesting (IRWH) technique for crop production recently introduced into a part of the basin occupied by communal farmers has been shown to increase maize and sunflower yields by 30 to 50% compared to conventional tillage, making crop production utilising this technique a feasible proposition for these farmers. The area of land suitable for the IRWH in the basin is estimated to be 80 667 ha, of which 15 000 ha is located in the communal land. The two catchment management options compared in this paper are: • Allowing the 80 667 ha to remain under grassland and utilising the runoff downstream for irrigating maize • Utilising the 80 667 ha for maize production in the basin using the IRWH technique. Results showed that the expected maize production from the options shown above were 23 040t and 137 134t respectively. The large unproductive water losses during storage and conveyance to downstream use points are probably the main reason for this large difference in production. An economic analysis, which enabled the grazing benefit to be included in the first option, shows that the gross margin of this option, expressed as R/m3 of rain water utilized, could be expected to be between 0.0234 to 0.0254 under current conditions, of which irrigation contributed about 25% or less. The comparable value for the IRWH option was 0.0354. The second option is clearly shown to be the most preferable, with high socio-economic benefits for the communal farmers who are currently struggling to achieve sustainable livelihoods. Water SA Vol. 32(2) 2006: pp.223-228

Field measurement of lupin belowground nitrogen accumulation and recovery in the subsequent cereal-soil system in a semi-arid Mediterranean-type climate

In situ 15N-labelling was used to provide a quantitative assessment of the total contribution of lupin (Lupinus angustifolius) to below-ground (BG) N accumulation during a growing season under field conditions, and to directly trace the fate of the lupin BG N in the next season, including quantifying the N benefit from lupin to a following wheat (Triticum aestivum) crop. The experiments were conducted at two sites, both experiencing a semi-arid Mediterranean-type climate in the wheat-growing region of Western Australia but with differing soil types, a deep sand (Moora) and a sand-over-clay shallow duplex soil (East Beverley, EB). Lupin shoot and root dry matter and total plant N accumulation, proportional dependence on nitrogen fixation and grain yield were greater at the deep sand site than the duplex soil site, although there was a similar proportion of shoot N to estimated total BG N at both sites. The proportion of total plant BG N decreased from the vegetative stage (42–51%) to peak biomass (25–39%) and maturity (23–34%). From 56–67% of BG N on the deep sand and 74–86% on the duplex soil was not recovered in coarse roots (>2 mm) or as soluble N, but was present in the insoluble organic N fraction. There was evidence for cycling of lupin root-derived N into soil microbial biomass and soluble organic N during lupin growth (by the late vegetative stage), but no evidence for leaching of legume derived BG N during the lupin season. Estimates of fixed N input BG were at least four times greater if based on total lupin BG N rather than on N recovered in coarse roots (>2 mm). There were no apparent losses of lupin BG N during the summer fallow period subsequent to lupin harvest at either site. Also, immediately prior to sowing of wheat there were similar proportions of lupin BG N in the inorganic (20–25%) and microbial biomass (6–9%) pools at both sites, with the majority of BG N detected in the <2 mm fraction of the soil column. However, the proportion of residual lupin BG N estimated to benefit the aboveground wheat biomass was relatively low, 10% on the deep sand and only 3% on the shallow duplex. Some (14%) residual lupin BG N was leached as nitrate to 1 m on the deep sand compared to 8% of residual lupin BG N leached to the clay layer (0.3 m) on the shallow duplex. About 27% of the residual lupin BG N on the deep sand at Moora had apparently mineralised by the end of the succeeding wheat season (i.e. recovered either in the wheat shoots, as inorganic N in the soil profile or as leached nitrate) compared to only 12% at EB. There was an unaccounted for large loss of residual lupin BG N (50%) from the duplex soil at EB during the wheat season, postulated to be chiefly via denitrification. At both sites after the wheat season a substantial proportion (32–55%) of legume derived BG N was still present as residual insoluble organic N, considered to be an important contribution to structural and nutritional long-term sustainability of these soils.

Increased Concentration of Molybdenum in Sown Wheat Seed Decreases Grain Yield Responses to Applied Molybdenum Fertilizer in Naturally Acidic Sandplain Soils

ABSTRACT Spring wheat (Triticum aestivum L.) is the major crop in southwestern Australia where 75% of the 18 million hectares comprise sandy duplex and deep sandy soils, including uniform yellow sandplain soils. Some of the sandplain soils in the lower rainfall (< 350 mm annual average) eastern region are naturally very acidic (soil pH, as measured in 1:5 soil:0.01 M calcium chloride, 3.7–4.5) in soil horizons explored by wheat roots so molybdenum (Mo) deficiency and aluminium (Al) toxicity adversely affects grain production of wheat. Liming is not an economic option to ameliorate Mo deficiency and Al toxicity in these soils because uneconomical large amounts are required. However, despite Al toxicity, applying Mo fertilizer produces profitable grain yield. The fertilizer also increases Mo concentration in grain, and if this grain was used to sow the next crop, it may reduce the amount of Mo fertilizer required by the subsequent crop. To test this hypothesis we grew wheat in an experiment on naturally acidic sandplain soil (pH 4.5) when either 0 or 160 g/ha fertilizer Mo was applied. The grain harvested at the end of the growing season had Mo concentrations of 0.07 mg/kg when no Mo was applied (low Mo seed) and 0.27 mg/kg when Mo was applied (high Mo seed). In two further field experiments on naturally acidic sandplain soil (pH 4.3 and 4.4) we sowed low and high Mo seed of the same size (36.4 ± 0.2 mg per seed) when 4 rates of Mo fertilizer (0, 35, 70, and 140 g/ha Mo) was applied to soil. Grain yield responses to the Mo fertilizer were 59% for low Mo seed and 55 g/ha fertilizer Mo was required to produce 90% of the maximum grain yield. Corresponding values for high Mo seed were 15% response and 15 g/ha fertilizer Mo. Rather than sowing wheat seed harvested from acidic soils to sow wheat crops on the acidic sandplain soils, we instead recommend seed harvested from alkaline soils with larger concentrations of Mo in the seed be used reducing the rate of fertilizer Mo required for that crop. The concentration of Mo in the youngest emerged leaf blades (YEB) that was related to 90% of the maximum grain yield (critical prognostic tissue test value for grain production) was about 0.08–0.09 mg/kg at tillering (Gs24) and at emergence of wheat heads (Gs59).

Management to increase the depth of soft soil improves soil conditions and grapevine performance in an irrigated vineyard

Grapevine ( Vitis vinifera L.) growth is limited by the hard-setting red duplex soils that occur in many major viticultural regions, including the Goulburn Valley, Victoria. Water often ponds on the soil surface and dense subsoils restrict drainage. An experiment was conducted to develop soil management that improved water infiltration, root growth and grapevine performance. The experiment was conducted from 1995 to 1998 in a vineyard block of Chardonnay on Ramsey rootstock planted at Rosbercon Vineyard, Picola in 1972. Three soil managements were compared over 3 years testing for improved soil physical properties, root growth and grapevine performance. Microjets irrigated the vine-line soil where randomised treatments of grown ryegrass ( Lolium multiforum) (Autumn/Winter) or straw mulch (15 t/ha year round), gypsum application (12 t/ha) and slow (5 mm/h) or fast wetting (15 mm/h) were applied. Ryegrass growth maintained macropores (0.35 cm 3/cm 3 > 30 μm diameter) 33 months after tillage, kept the soil soft (<1 MPa) to a greater depth (134 mm) and had higher concentrations of grapevine roots (5.05 cm/cm 3 at 0–100 mm depth) compared with those of straw mulch (0.31 cm 3/cm 3 macropores, 73 mm depth soft soil and 3.35 cm/cm 3 root length density). Gypsum maintained the surface soil soft for a greater depth (120 mm) and improved water penetration to 250 mm depth compared with no gypsum application (87 mm depth soft soil). Ryegrass and gypsum improved water penetration to 450 mm depth compared with ryegrass and no gypsum application. Grapevine vegetative growth (pruning weight), reproductive performance (yield and bunch weight) and root growth were each directly dependent on the depth of soft soil (depth of soil less than 1 and/or 2 MPa). Grape yield and bunches per grapevine were also each directly dependent on the volumetric water content at field capacity of the surface soil. These results suggest that, to alleviate limitations of hard-setting red duplex soils to grapevine growth and performance, management should increase coagulation of clay particles and stabilisation of aggregates in order to maximize the depth of soft soil (<2 MPa). Moreover, the key soil parameter identified in this study that is most representative as an indicator of quality soil for winegrape production is depth of soft soil (<2 MPa) 24 h after irrigation.