Changing the physical properties of texture-contrast soils by clay delving

Abstract

Texture-contrast soils are important in Australian agriculture but they are known for their low chemical and physical fertility, and their consequent low productivity. Clay delving, a soil modification that combines mixing of clay-textured subsoil into the topsoil with deep ripping, is widely practiced on texture-contrast soils in agricultural regions across southern Australia. Success in terms of crop productivity has been mixed but there is a general consensus that clay delving increases yields, at least in the short term. A review of the available literature reveals that the practice of clay delving is based primarily on trial-and-error experience reported in the so-called ‘grey’ literature, which focusses mainly on chemical fertility and largely ignores the role of soil physical properties and their effects on plant available water. Until we understand how delving influences soil physical properties, this practice will remain more in the realm of art than in science. The present study set out to first characterise the changes in physical properties caused by delving and then to evaluate how these changes influence infiltration, water redistribution in the soil profile, and ultimately soil water availability and root growth. Chapter 1 reviews the literature on texture contrast soils and outlines their primary limitations for agricultural production. The practice of clay delving is then reviewed and the major gaps in our understanding of this practice are identified. A set of hypotheses is presented, which form the basis for the experimental work outlined in subsequent chapters of this thesis. The work outlined in Chapter 2 is based on the hypothesis that clay delving strongly modifies the soil profile, disrupting the interface between A and B horizons and mixing subsoil with the topsoil. The morphology of the new soil profiles differ greatly from the originals (un-delved) with direct effects on soil physical characteristics. In particular, the distribution of these properties is changed both in the vertical and lateral directions. To address this hypothesis, I characterised the physical changes in a typical texture-contrast soil five years after delving, and found indeed that the extensive morphological disruption produced an entirely new soil profile with different soil physical characteristics from the original texture-contrast soil. On a small (profile) scale, bulk density in the delved profiles was highly variable and ranged between that for the A-horizon sand (1360 kg m⁻³) and that for the subsoil clay (<1800 kg m⁻³). Because of the great variability in composition there was no correlation between bulk density and average clay content of the soil. As might be expected the regions having large clods of subsoil (mainly below 0.25 m) had greater mean clay content than regions containing smaller clods (mainly above 0.25 m). The saturated hydraulic conductivity, Ks, in the upper part of the delved profile was significantly reduced (several orders of magnitude) and variability was greater. The abrupt reduction in Ks at the A/B boundary in the un-delved soil became more gradual (and variable) in the delved profiles. Mean soil resistance to penetration was inversely related to soil water content and directly related to clay content in the disturbed zone immediately above the delving line (although the effect diminished with lateral distance between delving lines (off line). Using the IWC concept (taking into account all limiting soil physical factors), high soil resistance was shown to be the single greatest factor limiting soil water availability; where delving reduced soil resistance, available water increased. At a larger (field-transect) scale, results were consistent with the small (profile) scale findings. Furthermore (based on aggregate size and clay content distributions) an average of nearly 500 t ha⁻¹ of clay was brought up into the top 0.1 m by delving – this significantly exceeded current recommendations for clay spreading (300 t ha⁻¹). Water repellence in the top 0.1 m was significantly reduced in delved soil (on the delve line and off line) and this significantly increased infiltration rates and reduced the time to reach steady state infiltration. Field penetrometer measurements showed delving significantly reduced soil resistance in the top 0.45 m (especially in the V-shaped zone) but the effect diminished with distance from the delve line. Visual images of the soil profiles confirmed what was found by directly measuring (laboriously) aggregate size and clay content distributions and suggested delving could increase available water in the root zone by between 12 and 23 mm. Chapter 3 was based on the hypothesis that inserting ‘new’ clay from the subsoil into the sandy topsoil will decrease surface water repellence and significantly increase the wettability of the entire soil profile. In addition, disrupting the hard A / B horizon interface will allow water that would otherwise pond at the horizon boundary to move significantly deeper in the soil profile. To evaluate this hypothesis, I applied a blue-dye solution (using a rainfall simulator) to the surface of delved and un-delved soils then photographed the dye-stained soil profiles and conducted a digital analysis of the images. Under relatively dry conditions I found that delving significantly increased the wettability of the topsoil and that under wetter conditions water moved to greater depths in the profile. These findings were published as: Betti G, Grant C, Churchman G, Murray R 2015. Increased profile wettability in texture-contrast soils from clay delving: case studies in South Australia. Soil Research 53:125- 136. https://doi.org/10.1071/SR14133. Chapter 4 was based on the hypothesis that the modification of the soil, in particular, the disruption of the A/B boundary caused by delving contributes to deeper plant root growth and enhances root distribution in the soil profile, especially immediately below the delving line. To evaluate this hypothesis, I collected soil core samples down the entire soil profile of three delved and un-delved soils and collected root samples. I then measured total root length, root length density (RLD) and root mean diameter (RMD), and although the results were highly variable, RLD in the delved soils was significantly greater than that in the un-delved soils; the effects were particularly evident at the A / B horizon boundaries. At two of the three sites examined, the mean root length density (RLD) was significantly greater (and more uniformly distributed) down the profile in the delved soils compared with the un-delved controls. Furthermore, there were no significant differences between RLDs directly under the delving line and the regions between the lines. This suggested the benefits of delving are manifest laterally well beyond the delving lines and indicated that optimum tine-spacing could be evaluated by measuring RLDs as a function of distance from the delving line – the absolute maximum distance at a site would be that where the RLDs differ significantly from those directly under the delve line. At all three sites, roots were significantly thinner (as measured by root mean diameter, RMD) in the delved soils relative to the un-delved controls (both directly under the delve line as well as laterally). This is consistent with the root-thickening effect brought on by high soil strength typically found in un-delved soils, particularly in the subsoil. Chapter 5 was based on the hypothesis that when mixing clay-rich material with sandy soil by delving, the physical characteristics of the mixture are not influenced exclusively by the changes in clay content but also by the size of the clay-rich aggregates incorporated in the sand. While the average clay content of the topsoil increases by delving, many of the clay-rich clods and aggregates remain discrete entities rather than becoming mixed intimately with the sand. This hypothesis was tested in the laboratory by mixing different sizes and amounts of subsoil with sand and measuring soil physical properties relevant to plant-available water. The work demonstrated that both the mean clay content and the size of the subsoil clods significantly influenced the physical properties and the plant available water of the delved soils. The classical measure of plant available water, PAW, increased in mixtures containing more subsoil clay, particularly when smaller aggregates were used (< 6 mm). However, when the potential physical restrictions on PAW were taken into account using the integral water capacity, the benefits of adding clay reached a peak at ~40% incorporation, 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). This indicates that excessive post-delving cultivation may not be warranted and may explain some of the variability found in crop yields after delving. This work was published as: Betti G, Grant CD, Murray RS, Churchman GJ (2016) Size of subsoil clods affects soil-water availability in sand-clay mixtures. Soil Research 54:276-290. doi.org/10.1071/SR15115. The final Chapter 6 summarises the principal findings of the work and makes recommendations for future research based upon questions raised by each experiment.