Robert S. Murray

The effect of soil compaction on growth and P uptake by Trifolium subterraneum: interactions with mycorrhizal colonisation

The effects of vesicular-arbuscular mycorrhizal (VAM) colonisation on phosphorus (P) uptake and growth of clover (Trifolium subterraneum L.) in response to soil compaction were studied in three pot experiments. P uptake and growth of the plants decreased as the bulk density of the soil increased from 1.0 to 1.6 Mg m-3. The strongest effects of soil compaction on P uptake and plant growth were observed at the highest P application (60 mg kg-1 soil). The main observation of this study was that at low P application (15 mg kg-1 soil), P uptake and shoot dry weight of the plants colonised by Glomus intraradices were greater than those of non-mycorrhizal plants at similar levels of compaction of the soil. However, the mycorrhizal growth response decreased proportionately as soil compaction was increased. Decreased total P uptake and shoot dry weight of mycorrhizal clover in compacted soil were attributed to the reduction in the root length. Soil compaction had no significant effect on the percentage of root length colonised. However, total root length colonised was lower (6.6 m pot-1) in highly compacted soil than in slightly compacted soil (27.8 m pot-1). The oxygen content of the soil atmosphere measured shortly before the plants were harvested varied from 0.18 m3m-3 in slightly compacted soil (1.0 Mg m-3) to 0.10 m3m-3 in highly compacted soil (1.6 Mg m-3).

Assessing effects of aerobic and anaerobic conditions on phosphorus sorption and retention capacity of water treatment residuals.

Water treatment residuals (WTRs) are the by-products of drinking water clarification processes, whereby chemical flocculants such as alum or ferric chloride are added to raw water to remove suspended clay particles, organic matter and other materials and impurities. Previous studies have identified a strong phosphorus (P) fixing capacity of WTRs which has led to experimentation with their use as P-sorbing materials for controlling P discharges from agricultural and forestry land. However, the P-fixing capacity of WTRs and its capacity to retain sorbed P under anaerobic conditions have yet to be fully demonstrated, which is an issue that must be addressed for WTR field applications. This study therefore examined the capacity of WTRs to retain sorbed P and sorb further additional P from aqueous solution under both aerobic and anaerobic conditions. An innovative, low cost apparatus was constructed and successfully used to rapidly establish anoxic conditions in anaerobic treatments. The results showed that even in treatments with initial solution P concentrations set at 100 mg l(-1), soluble reactive P concentrations rapidly fell to negligible levels (due to sorption by WTRs), while total P (i.e. dissolved + particulate and colloidal P) was less than 3 mg l(-1). This equated to an added P retention rate of >98% regardless of anaerobic or aerobic status, indicating that WTRs are able to sorb and retain P in both aerobic and anaerobic conditions.

Effects of pH on mycorrhizal colonisation and nutrient uptake in cowpea under conditions that minimise confounding effects of elevated available aluminium

Effects of the arbuscular mycorrhizal (AM) fungi Gigaspora margarita and Glomus etunicatum on the growth of cowpea (Vigna unguiculata L. Walp.) were assessed at low pH by growing plants, with and without AM inoculation, individually in pots containing a mixture of sand and soil adjusted to pH 4.7, 4.9 or 5.2 at the start of the experiment, and with soluble aluminium (Al) concentrations at a sub-toxic level for the plant. Cowpea grew poorly in the absence of AM colonisation, particularly at pH 4.7. Growth was enhanced both by increasing the pH and by inoculating with the AM fungi, with plant responses greater with inoculation. The relative growth improvement by mycorrhizas (mycorrhizal growth response) was highest at pH 4.7, and decreased as the pH increased, although effects were not always significant. Gi. margarita was much more effective than G. etunicatum. There were differential effects of the two fungi on uptake of mineral elements. Plants inoculated with Gi. margarita took up a range of elements, including P and Zn as well as Al, to a much greater extent than those inoculated with G. etunicatum, regardless of medium pH. The effectiveness of Gi. margarita in increasing plant growth was closely correlated with colonised root length.

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 χ 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 stability of sodic soils in sugarcane production as influenced by gypsum and molasses

The aim of this work was to determine whether molasses, a by-product of sugar manufacture, alone or combined with gypsum, could improve the structural stability of sodic soils used for sugarcane production. A Burdekin sandy clay loam with an exchangeable sodium percentage (ESP) of 7.9, and a Proserpine loamy sand with an ESP of 18.8 were incubated with molasses (0 and 10 t/ha) and gypsum (0 and 10 t/ha) for 12 weeks, during which time they were leached 5 times with water (0.5 pore volumes each time). In the Burdekin soil, molasses and gypsum, either alone or combined, decreased spontaneous clay dispersion from 2.6 to 250 μm was 31% in the control and 71% with molasses + gypsum. Electrical conductivity (EC 1:5) was 0.1 and 1.9 dS/m, pH1:5 in water was 7.7 and 7.1, and ESP was 4.1 and 0.2 in the control and molasses + gypsum treatments respectively. In the Proserpine soil, the amounts of dispersible clay were much less than in the Burdekin soil. The effects of molasses and gypsum in decreasing spontaneous and mechanical clay dispersion were similar to those in the Burdekin soil, but less pronounced. Molasses and gypsum, either alone or combined, improved the structural stability of both soils by decreasing dispersion and/or slaking. An implication of this work is that molasses may be a useful ameliorant for sodic soils, either alone or combined with gypsum.

Establishing woody perennials on hostile soils in arid and semi-arid regions – A review

Background and aimsWoody perennials can be difficult to establish on harsh soils in arid and semi-arid regions. Historically, technological advances have focussed on methods to improve transplanting and direct-seeding but the available information on these advances remains fragmented and the edaphic factors have been largely ignored. This review explores the literature on plant establishment and identifies soil properties that limit plant response in harsh environments.ConclusionsWe reveal that some woody perennials are particularly well-adapted to dry conditions and can also help reclaim degraded landscapes. Furthermore, the environmental and phenological factors that limit the success of direct seeding are well understood but the edaphic factors are not. For example, seedbed preparation and subsoil amelioration before seeding have not been evaluated in dry regions. Seed-priming and seed-placement are also poorly understood, as is the tolerance of woody perennials to different salt types in waterlogged soils of extreme pH and high soil strength. The reason why woody perennials can penetrate strong, hard soils is not obvious from the literature. They apparently cannot exert root growth pressures of the same magnitude as domesticated plants, so they must be able to exploit soil biopores and cracks more efficiently. Other gaps in our understanding of the soil factors that limit woody perennial establishment on hostile soils are identified.

Increased profile wettability in texture-contrast soils from clay delving: case studies in South Australia

Clay delving is becoming a popular practice to increase productivity of texture-contrast soils in southern Australia. The practice brings subsoil clay to the surface to be mixed with the sandy topsoil, and unlike clay spreading, it combines the addition of hydrophilic material with a ripping effect that disrupts the sharp boundary between the sandy topsoil and clayey subsoil. Our objective was to evaluate the magnitude of effects caused by delving on the spatial distribution of water through the profile for three Sodosols (Stagnic Solonetz soils) in the south-east of South Australia. We also wished to evaluate the extent to which clay delving might reduce water ponding at the A–B horizon interface. We wetted both delved and undelved texture-contrast soils with a Brilliant Blue dye solution under initially dry and wet conditions (to evaluate the effect of antecedent water content), and then took digital images of the stained profiles for quantitative comparison of the wetted areas. The stained soil profiles indicated that clay delving reduced preferential water flow (finger flow) and resulted in deeper and more uniform wetting of the A horizon, particularly under initially dry conditions. Under wet conditions (where water repellence was largely overcome), finger flow was significantly reduced regardless of delving but it still occurred to varying degrees depending on site characteristics. Delving significantly reduced ponding of water at the A–B horizon boundary and allowed greater penetration into the B horizon. At all sites, greater effects occurred directly on the delving lines and diminished with distance, implying that closer spacing of delving tines would increase uniformity of wetting throughout the profile. The effectiveness of delving on profile wetting was highly variable across the three sites, indicating that the outcome depends inter alia on the intrinsic soil characteristics and the delving equipment used in the field.

Effects of subsoil amendments on soil physical properties, crop response, and soil water quality in a dry year

In southern Australia the ability of field crops to extract soil moisture and nutrients from depth depends on the physical and chemical properties of the subsoil. In texture-contrast soils accumulation of water and nutrients in the E or A2 horizon, immediately above a clay B horizon of much lower hydraulic conductivity (herein called the interface), may generate lateral flows and enhanced nutrient and solute transfer to water bodies. Evidence that deep-ripping with addition of subsoil nutrients can increase crop productivity in regions having hostile, alkaline subsoils led to experiments to test whether this response was related to an increase in the use of water and nutrients in the subsoil. Our study measured the effects of deep-ripping with and without amendments on soil physical and chemical properties of the A and upper B horizons of 2 South Australian soils. Deep-ripping and deep-placement of nutrients increased grain harvest weight even in an exceptionally dry season. The greater yield was accompanied by significantly lower field-penetration resistance to 0.35–0.50 m depth, which we hypothesise enabled the crop to better access stored soil water and deep placed nutrients in the subsoil. Residual effects from deep-ripping were minimal after 4 growing seasons; therefore, ripping will need to be practiced at regular intervals to maintain treatment effects. The ripping and nutrient amendments had no significant effect on exchangeable sodium percentage, electrical conductivity, and readily extractable phosphorus and nitrate-nitrogen, despite changes in these soil properties between spring and harvest sampling.

Size of subsoil clods affects soil-water availability in sand–clay mixtures

Clay delving in strongly texture-contrast soils brings up subsoil clay in clumps ranging from large clods to tiny aggregates depending on the equipment used and the extent of secondary cultivation. Clay delving usually increases crop yields but not universally; this has generated questions about best management practices. It was postulated that the size distribution of the subsoil clumps created by delving might influence soil-water availability (and hence crop yield) because, although the clay increases water retention in the root-zone, it can also cause poor soil aeration, high soil strength and greatly reduced hydraulic conductivity. We prepared laboratory mixtures of sand and clay-rich subsoil in amounts considered practical (10% and 20% by weight) and excessive (40% and 60% by weight) with different subsoil clod sizes (<2, 6, 20 and 45 mm), for which we measured water retention, soil resistance, and saturated hydraulic conductivity. We calculated soil water availability by traditional means (plant-available water, PAW) and by the integral water capacity (IWC). We found that PAW increased with subsoil clay, particularly when smaller aggregates were used (≤6 mm). However, when the potential restrictions on PAW were taken into account, the benefits of adding clay reached a peak at ~40%, beyond which IWC declined towards that of pure subsoil clay. Furthermore, the smaller the aggregates the less effective they were at increasing IWC, particularly in the practical range of application rates (<20% by weight). We conclude that excessive post-delving cultivation may not be warranted and may explain some of the variability found in crop yields after delving.

Comparison of the penetration of primary and lateral roots of pea and different tree seedlings growing in hard soils

Establishment and survival of tree seedlings in hard soils depends on production of deep root systems. This study evaluated the primary and lateral roots of an annual crop and several tree species growing in soils of varying strength. We grew peas and acacias by direct seeding, plus three eucalypts by direct seeding and transplanting, and measured various root characteristics. At all levels of soil compaction, the primary roots of acacia were thicker and they elongated faster than did those of the eucalypts. However, lateral roots of transplanted eucalypts elongated faster than their primary roots, and the rate of root elongation was negatively correlated with soil penetration resistance, especially for Eucalyptus camaldulensis. The primary root diameter of all plants increased with increasing penetration resistance, but acacia roots continued to elongate faster than pea roots. Pea plants produced most of their roots in the top 5 cm, whereas tree roots were more uniformly distributed with depth. Although not statistically significant at P = 0.05, the relative rate of root elongation in very hard soil correlated modestly (P = 0.11) with the maximum root growth pressure of four tree species. These variations in root growth behaviour can be related to the intrinsic variability of root characteristics for each plant species and the natural abundance of each species in different environments.