Giacomo Betti-Effect of clay delving on soil water and OC ... · Figure 1. Distribution of (a)...

Effect of clay delving on water and carbon storage: a case study in the South East of South Australia

Giacomo Betti, School of Agriculture Food & Wine, The University of Adelaide, Adelaide South Australia. [email protected]

Giacomo Betti. Effect of clay delving on water and carbon storage: a case study in the South East of South Australia

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Abstract In South Australia, clay delving has become a popular practice for increasing the productivity of contrast texture soils. Clay delving brings to the surface sub-soil clay to be mixed with the sandy top-soil and, unlike clay spreading, it combines the addition of clay material with a ripping-like effect; disrupting the boundary between top-soil sand and subsoil clay. In this case study we evaluate the effects of this soil modification on soil water behaviour and potential effects on soil carbon storage when compared to the original (pre-delving) conditions of the soil. Two sites, near Coonalpyn and Bordertown were studied using a combination of experiments to evaluate soil water behaviour in terms of infiltration, spatial distribution of water infiltrating the soil, root growth and the effect of clay addition on water storage. Random sampling and collection of images of the modified (delved) soil profiles have also been used to estimate the effect on soil carbon storage of clay delving when compared to the original characteristics of the soil. Results from the experiments have shown that the drastic soil modification after clay delving has a clear impact on soil water movements and storage. Deeper and more even water infiltration was found in delved profiles when compared to the control sites (undelved). The effects on water infiltration of clay delving are particularly clear when the soils are in summer dry conditions. The results of water retention of different cores from the site in Coonalpyn have shown that the increase in clay content due to clay delving increases the potential water storage and plant available water. Nevertheless, the variability of the results suggested that other factors, rather than just clay content, have to be taken into account for comprehensive and realistic evaluation of water storage. Carbon storage appears to increase in the top 0.1 m of soil in the delved profiles, probably as a result of greater yields and root growth, and more organic material can be found at depth along the delving lines. Although there are no doubts on the benefits that clay delving can deliver in the amelioration of texture contrast soil, different results were achieved in the sites and the great potential for an increase in productivity of soils with clay delving depends on several factors such as pre-delving soil characteristics and delving methods.


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Acknowledgements The author would like to acknowledge the following people and organisations for making this report possible. Many thanks to:

• South East Natural Resources Management Board (through their Innovation Grants program) –Robert Palamountain

• University of Adelaide • Dr Cameron Grant for his support and supervision • Dr Rebecca Tonkin for being always helpful with her experience in the field • Dr Glenn Bailey for all the information (and suggestions) on clay delving • Mr. Kim Marshall and Mr. Peter Murch for the back hoe service at Coonalpyn and Bordertown

The author would like to personally thank Mr. and Mrs. Groocock, Mr. and Mrs. Edmond-Wilson and Mr. Schilling, the owners of the farms where the experiments were conducted. Without their help and generosity the entire experience would not have been possible. Finally, a special recognition goes to Mr. Roger Groocock; whose sense of observation and creativity gave birth to clay delving in South Australia.

Disclaimer The information contained in this report is proposed as a guide only. It is not intended to be comprehensive, nor does it constitute advice. No responsibilities are accepted for the consequences of the use of this information. You should seek expert advice in order to determine whether application of any of the information provided in this guide would be useful in your circumstances.


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Index 1. Introduction.........................................................................................................................................5 1.1 Texture contrast soils: definition and characteristics........................................................................5 1.2 Soil modification with clay delving.....................................................................................................6 2. Site locations……………. ...................................................................................................................9 3. Effect of clay delving on water and carbon storage..........................................................................10

3.1 Changes in water infiltration in texture contrast soils after delving............................................10

3.1.1 Results...............................................................................................................................12

3.2 Effects of clay addition on soil water storage.............................................................................18

3.2.1 Results...............................................................................................................................18

3.3 Effects of clay delving on plant root growth...............................................................................22

3.3.1 Results............................................................................................................................................22

3.4 Changes in soil carbon content: comparing soil carbon content before and after delving…...25

3.4.1 Results..............................................................................................................................25

4. Conclusions and future work............................................................................................................29 5. References...........................................................................................................................................30 6. Appendix...............................................................................................................................................32


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1 Introduction 1.1 Texture contrast soils: definition and characteristics The Australian Soil Classification (Isbell, 2002), defines texture contrast soil by the presence of a “clear or abrupt textural B horizon” where a strong change in texture between two horizons (normally A and a B2t) occurs within a sharp, abrupt or clear boundary. Even though texture contrast is nowadays a widely used term in Australia, these soils are often described using the term duplex soils, based on the diagnostic criteria proposed by Northcote (1979). In his factual key, Northcote classifies duplex soils as those profiles where the dominant texture class of the subsoil is at least one and a half texture groups finer than the above horizon. Both definitions are often used (and confused) to refer to similar soils but this report will only use the term texture contrast, as this is the definition used by the current Australian Soil Classification. In Australia, texture contrast soils cover around 20% of the total land (Chittleborough, 1992) and are often associated with sodic/saline characteristics in the subsoil (Fig. 1).

Figure 1. Distribution of (a) duplex soils and (b) sodic duplex soils in Australia (Chittleborough, 1992).

Of this area, a large part is farmed. It was estimated that about 0.8 million Hectares (Ha) are cultivated in South Australia (Gardner et al., 1992). In the south-west agricultural area of Western Australia (WA), texture contrast soils occupy 57% of the total area (Tennant et al., 1992) and in Victoria approximately 1 million Ha are cultivated (McGuinness, 1991). Three aspects are usually highlighted by the scientific literature as main factors influencing the productivity of texture contrast soils: general low fertility, limited plant root growth and plant stress associated with soil water behaviour. The limitations associated with texture-contrast soil can be described as the results of their peculiar morphology with a low fertile sand horizon on top of clay rich subsoil. The characteristics of these horizons can be summarised as following: 1) Poor characteristics of the top sand horizon The sandy textures of the A horizons (with clay content as little as 1%) have a great impact on soil water storage in the root zone, giving also their high hydraulic conductivity. Studies of plant available water content (PAW) in sandy surface profiles have shown values far below the values of most textural classes (Hamblin and Tennant, 1987; Tennant et al., 1992). At such levels of PAW, even minor changes in clay content in the top-soil (Tennant et al., 1992) or increased access to subsoil


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water could considerably enhance soil water storage and availability for crops, increasing their productivity (Dracup et al., 1992). Studies have shown that the yield variation in wheat displayed a linear relationship between root depth and water extracted from the subsoil (Belford, Tennant and Dracup cited in Dracup et al., 1992). Another common limitation of texture contrast soils is the uneven wetting front and limited water infiltration through the profile due to the presence of non-wetting sands (Hardie et al., 2011; Imeson et al., 1992; Ritsema and Dekker, 2000). Because of the hydrophobic properties of these sands, water infiltrates the soil profile through particular paths known as finger flows. Finger flows do not depend on soil structure but rather on the hydrophobic behaviour of the sand. As a result of this process, water usually bypasses large area of the profile, leaving areas with dry soil. The limitation caused by non-wetting sands on water infiltration is often the cause of soil erosion due to run-off (Burch et al., 1989; Lemmnitz et al., 2008; Shakesby et al., 1996) 2) Physical limitation of the clay subsoil Compared to soils with gradational profiles, in texture contrast soils the sharp boundary between the two horizons separates areas of the profile with extremely different physical properties. Tennant et al (1992) identify the low permeability of the B horizon as a soil property which has a major impact on the behaviour of texture contrast soils; in fact they describe the problems with texture contrast soils as "permeability contrast" dependent rather than "texture contrast" dependent. In effect, most of the concerns with these soils appear to be consequences of their peculiar hydraulic properties. In fact, water-logging and perched water tables associated with the hydraulic properties of texture contrast soils seem to represent the main constraints to crop production within these soil types. Edwards (1992), summarised these issues as the irony with texture-contrast soils: where root development limited by water-logging during the wet season has a negative impact during the dry seasons when roots only have access to water in the sandy top-soils with their limited water storage.

1.2 Soil modifications with clay delving In South Australia, the most common approach for correcting water repellent sands is via the addition of clay to the sandy topsoils (Cann, 2000; Ward, 1993). Two different approaches are usually used: clay spreading or clay delving (Davenport et al., 2011). With clay spreading, subsoil clay from a different area of the field is transported and spread over the soil surface. Clay spreading was the first successful large scale method for the treatment of water repellent sands in Australia. However the cost of the operation caused by the transport of the clay can be very high, especially when the source is located far from the area to be treated. While clay spreading is primarily associated with the correction of water repellent sandy soils, clay delving is rising in popularity due to additional advantages in the amelioration of sandy top-soils with poor properties. In fact, with clay delving it is possible to achieve deeper mixing of clay through the soil profile and the breaking of hardpans usually present at the top of the B horizon of texture contrast soils. Delvers (Fig.2) are ripper-like machines where the tines were modified in design in order to bring subsoil clay to the surface (Desbiolles et al 1997, cited in Bailey et al. 2010). Most delvers are produced locally (often by the same contractors) and present two to four tines with spacing ranging


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from 0.5m to 1.8 m; with 1.5 m as the average (Eldridge, 2007 and Bailey, unpublished work). To be able to bring clay subsoil to the surface, delving tines are wider than those of standard ripper tines. The maximum delving depth is variable and depends on the design of the machines. Moreover, a common feature of delvers is the opportunity to adjust the tines’ depth continuously using a hydraulic system. This feature is one of the main reasons of the high variation in delving depth across the same paddock.

Figure 2. Delver in action (source: www.precisionag.com.au). Clay delving represents a drastic modification of texture contrast soils and the result of this practice produces soil profiles morphologically very distinct from their original (Fig 3). Clay delving strongly modifies the soil profile, disrupting the interface between A and B horizons and mixing clay subsoil in the top-soil. The new soil profile presents many differences when compared to the original one (undelved); with direct effects on the soil’s physical and chemical characteristics. In particular, the distribution of these properties will be changed both in the vertical and lateral directions. Moreover, the result of clay delving depends on both the characteristics of the texture contrast soil and the way soil modification is operated, producing each time new soil profiles which exhibit a wide range of characteristics.

Figure 3. Soil surface immediately after delving and a delved soil profile (Site A-Coonalpyn, SA)


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The following is a list of authors who have previously studied clay delving and its effects in texture contrast soils. Research Topic Source

Effect of clay delving on crop yield (Bailey, 2009; Bailey et al., 2010; Eldridge, 2007; Fogden, 2010; May,

2006; Rebbeck et al., 2007)

Soil pH changes in the A horizon after delving (Bailey et al., 2010; May, 2006; Tonkin, 2010)

CEC changes in the A horizon after delving (Bailey et al., 2010; May, 2006; Tonkin, 2010)

Comparison of soil chemical characteristics before/after

delving in sites in SE region of South Australia

(Rural Solutions SA personal comm.)

The effect of clay delving and/or clay spreading on non-

wettable sands

(Bailey et al., 2010; Cann, 2000; D.A.F.F et al., 2006; Eldridge, 2007;

Fogden, 2010; Hall et al., 2010; Harper and Gilkes, 2004; May, 2006;

Rebbeck et al., 2007)

The risk of boron toxicity on delved soils (May, 2006)

The effects of delving in soils with saline or sodic B

horizons

(May, 2006)

The quantity of clay subsoil to be mixed to the top-soil (May, 2006)

The effects of delving on vertical distribution of soil

total OC (organic carbon)

(Bailey et al., 2010; May, 2006)

The effects of delver spacing on crop yield (Eldridge, 2007)

The effect of different combinations of delving and

spading on crop yields

(Eldridge, 2007; May, 2006; Tonkin, 2010)

The effect of timing on the mixing of clay after delving (May, 2006)


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2. Site locations The experiments were conducted in two different locations in the South East of South Australia: site A, near Coonalpyn (S35 41 28, E139 53 05); site B, near Bordertown (S36 12 52, E140 42 08). Both sites were classified as Sodosols (Fig.4), according to the Australian Soil Classification (Isbell, 2002), where a sand top profile (with average clay content ranging from 1 to 5%) cover a sodic clay subsoil ( clay content ranging from 25 to 40%). Site A was delved in 2007 with average depth of 0.6 m and with delving tines spaced at 0.9 to 1.0 m. Site B, delved in 2005, had the tine spacing at 1.3 m and depth ranging from 0.4 to 0.5m. Selected soil properties from the three sites were estimated from the original profiles (undelved) and are presented as averaged properties due to the variability across the paddocks. From each horizon undisturbed cores (0.05x0.05m) were collected for estimating bulk density (after being oven dried at 105° C). Sand water repellence was assessed using the water drop penetration test (WDPT), where the average time range for a water drop to penetrate the surface of the sand is established.

a) b) Figure 4.Soil profiles of the experiment sites. a) Site A (Coonalpyn). b) Site B (Bordertown)

SITE A (Coonalpyn) Horizon Depth 9 (m) Bulk density

(g/cm3) Clay (%) Sand (%) WDPT *1

(seconds) Sodicity*

A1 0-0.15 1.586 1.4% 97.3% 380-800 -

A2e 0.10-0.26 1.682 1.1% 98.2% - -

B21t 0.26-0.35 1.732 28.2% 70.8% - Highly Sodic

B22t 0.35-0.60 0.000 41.0% 56.1% - Highly Sodic

SITE B (Bordertown)

Horizon Depth 9 (m) Bulk density (g/cm3)

Clay (%) Sand (%) WDPT (seconds)

Sodicity

A1 0-0.15 1.451 5.1 92.3 1000-2500 -

A2e 0.15-0.30 1.638 1.5 97.5 - -

B21t 0.30-0.45 1.784 37.0 61.5 - Highly Sodic

B22t 0.45-0.70 1.810 43.8 49.6 - Highly Sodic

Table 1. Soil characteristics of site A and B. * Level of sodicity of horizon B2 established with sodicity meter (Rengasamy and

Bourne, 1990)


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3. Effect of clay delving on water and carbon storage To study the effects of clay delving on water and carbon storage, this research was divided into different experiments in order to evaluate: 1) Changes in water infiltration in texture contrast soils after delving. 2) Effects of clay addition on soil water storage. 3) Effects of clay delving on plant root growth. 4) Effects of clay delving on soil carbon storage.

3.1 Changes in water infiltration in texture contrast soils after delving Soil profiles, in dry (summer) and wet (winter) conditions, were impregnated with a blue colour dye following a modification of the method illustrated by Hardie et al. (2011). This research had two main objectives: 1) To study the effect of the addition of clay to the top horizon and its effects on water repellent sands and water infiltration at the soil surface. 2) To study the effect of the breaking of the interface between the A and B horizons on water infiltration at depth. The possibility to increase water infiltration through the B horizon has the potential to reduce the occurrence of seasonal perched water tables (particularly during the wet winter season) and reducing plant stress in the root zone. For this experiment only, a third site, called site C, was chosen in the vicinity of Coonalpyn. Like site A and B, site C was classified as a Sodosol, with a sandy top-soil (clay<2%) and subsoil rich in clay (>25%). The experiments were conducted in dry and wet conditions for sites A and B and for dry conditions only in site C. At each site, two different areas of the paddock were chosen as representative of the original soil profile (undelved) and the modified one (delved). The undelved soils were chosen to be in closest proximity to the delved ones, where soil type was the same and cropping history was the same or similar. Prior to the experiment, the average water infiltration rates were assessed with a single ring infiltrometer method, on random areas of undelved soils and, for the delved ones, along and between the delving lines. Soil profiles were impregnated with a solution of water and Blue Dye FCF, a colourant easily visible and mobile, with low toxicity to the soil (Flury and Fltihler, 1994). Dry treatments were conducted in summer 2012 (site A, B) and January 2013 (site C), after a long period without significant rainfall. Wet treatments were conducted during winter after a period of natural rainfall, or after water were applied artificially in order to produce a wet profile. Both dry and wet treatments were impregnated with 25mm of dye mix on a 1.05x1.05m area over a period of 2 hours. The resulting average application rate was 11.3 mm/hour, with estimated rainfall IFD (Intensity-Frequency-Duration) return period within 1-2 years for all sites (Bureau of Meteorology, 2011) as show in Table 2. The IFD return period refers to statistics on rainfall and represents the frequency at which a rainfall with specific characteristics (intensity and duration) can occur. A custom portable simulator was built in order to produce the low application intensity required for the experiments, using standard drip irrigators on a swing like system and fed by a 12 Volt pump. After impregnations, soils were left for a period of 22-24 hours prior the collection of the profile images.

Site Treatment Approximate dye application rate (mm/h)

Application time (h) IFD (years)

A and B Dry/wet 11.3 2 2-5

C Dry only 11.3 2 2-5

Table 2. Application rates of the blue dye solution and relative IFD.


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At each site, 5 to 6 profile slides every 15-20cm interval were excavated; firstly with the help of a backhoe followed by a manual refinement in order to have a reasonably flat surface. A metal frame (W 0.9m H 0.6m) was positioned over each profile prior to taking the photo. Photos were taken with a digital SLR camera (Nikon D5000 or Canon 650D) in manual and automatic setting at maximum resolution in RAW format. Using different editing software packages, each photo was first corrected for the lense distortion and perspective, the blue stained areas were separated from the rest of the profiles and the images were then converted to a binary format where the black areas represented the portion of dye stained soil and the white areas the untouched ones (dry areas). Finally, the distribution of dye stained soil at 0.02 m depth increments was calculated using Image J

©

64 for each profile slide (Fig. 5). Delved soils were analysed as an average of the entire profile; and after separating the main zones of soil modification (delving line and between delving lines) in order to estimate and compare water movement within the two distinct areas (Fig. 6).

a) b) c)

Figure 5. Pictures of stained profiles (a) were converted in binary images (b) where the black area represented the soil

impregnated by the blue dye. The proportion of stained soil to the depth of 0.6 was then calculated using the software ImageJ®64 (c) .

Figure 6. The extent of stained soil in the delved profiles was also estimated for the delving line and between the delving lines

only, in order to compare the water movement in the two distinct areas of a delved profile


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3.1.1 Results The variability of soil properties and morphologies has probably influenced the outcomes from the 3 sites; so that different results from the dye impregnation experiments were achieved. Results on the effect of clay delving on water infiltration are initially presented for each site individually. Water infiltration in summer dry conditions Site A (Coonalpyn, SA) Results from the experiments in dry conditions (Fig. 9a) have shown a considerable effect of the soil modification on water infiltration. In the undelved soil, the wetting front decreased drastically in the first 0.1m (from 100% to less than 20% of dye stained area) as a result of water infiltrating via preferential flow pathways. Due to the formation of finger flows, the A1 horizon is wetted uniformly only in the first top 0.05m while infiltrating water bypassed large areas of the A2e horizon (which remained largely dry). Once water reached the clay horizon, the sudden change in hydraulic conductivity probably promoted the building up of a water pond, increasing again the wetting front area as shown in Fig. 9a. As a consequence of extremely low conductivity of the clay rich subsoil, water infiltrated through the B21 horizon predominantly via natural cracks in the clay. The results with the delved profiles showed that the addition of clay to the surface reduced the water repellency of the sand and the influence of finger flows was also consequently reduced. The outcome was a deeper and more even waterfront that decreased with depth more gradually when compared to the undelved soil. The first 0.1m was wetted over 80% of the area on average. The proportion of area still dry at the A2e horizon was smaller on average when compared to the undelved soil. As a result, the area dye stained in the delved profiles was on average 35% greater than in the undelved in the top 0.15 m of soil. While water infiltration in the A horizon has changed with clay delving, the area stained in the B horizons at depths greater than 0.35m have not changed significantly between the undelved and the delved soils. However, when the areas along the delving line and between the delving lines were separated and compared (Fig. 9b), water infiltration in those areas acted differently. At surface, both areas have similar water infiltration as a result of to the even clay distribution on the top soil. However, when the water reached the top of the clay horizon in the area between the delving lines the flows were interrupted and water has accumulated similarly to the undelved soil. Along the delving lines, the breaking up of the A/B boundary allowed a greater quantity of water to infiltrate deeper in the soil (11% more on average); with the potential for reducing water ponding significantly. Observation of the images showed that the water, once it reached the interface with the B horizon, followed the pattern created by the soil modification; moving toward the deeper channel in the delving line. Therefore, the comparison of these two areas has shown the major contribution of the delving line to the overall higher and deeper infiltration of the delved profile compared to the undelved. Site B (Bordertown, SA) Compared to the site at Coonalpyn, site B has shown different results and the influence of clay delving on water infiltration was less evident. In dry conditions (Fig. 10a) little differences were found between the delved and undelved soil. In particular, towards the surface the delved profile produced a shallow waterfront similar to the undelved soil. In both soils, while the wetted area accounts on average for more than 80% at 0 to 0.05m depth, between 0.05 to 0.13 m this area has reduced to less than 27%. The wetting front in the B horizon increased with depth in the delved soil compared to the undelved. Comparing the two different areas of the delved profile (Fig. 10b) highlighted again the positive effect of the delving line on the deeper infiltration.


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Site C (Coonalpyn South, SA) Site C was morphologically very similar to site A and the results of the infiltration in dry conditions were similar too. The addition of clay significantly increased the wetting front in the first 0.2 m of topsoil. On the other hand, the wetting front in the undelved profile was much smaller due to preferential flows that bypassed large area of the sand (Figs. 8 and 11a). Unlike sites A and B, the size of the wetting front in the delved profile was larger throughout the profile to a depth of 0.6m. The comparison of the delving line with the area between the lines (Fig. 11b) again shows the even infiltration of water at the soil surface. No differences of wetting front area were found in the top 0.2m between these areas, suggesting an even mixing of clay in the field. At the boundary between sand and the clay subsoil, the delving line (as per sites A and B) allowed more water to infiltrate deeper into the profile. Again, the subsequent increased ability for water to infiltrate at depth (as a result of delving) highlights the benefits of clay delving in reducing seasonal perched water tables.

a) b) c)

Figure 7. Field observation of water flowing through the patterns created by clay clods (resulting from clay delving). a) Site A, delving line in dry conditions. b) Site A, delving line in wet conditions. c) Site B, delving line in dry conditions.

Figure 8. Field observation of the topsoil in the undelved profile at site C: the effect of the water repellent sand produces

preferential flows and large areas of the top 0.1 m of soil remains dry even after 27mm of rain.


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Water infiltration in wet conditions Site A (Coonalpyn, SA) In wet (winter) conditions (Fig. 9c), soil water repellency and the impact of finger flows were reduced as expected. The wetting front in both delved and undelved soils decreased gradually with depth and even though the total area wetted in the delved profile was greater (8% more) than the undelved one, the differences were less significant when compared to the experiment in dry conditions. In the top 0.25m (A horizon) little difference was also found when the delving line and off delving line areas were compared (Fig. 9d). Nonetheless, the breaking up of the top of the B horizon again contributed to a deeper water infiltration along the delving lines, with 16% more wetted soil below 0.27m depth. Again, the delving line had the major impact to the overall results of the delved profile, while the off line area behaves similarly to the undelved soil (Fig. 9c). Site B (Bordertown, SA) In wet conditions (Fig 10 c), similar to site A, water repellency was reduced and the wetting front decreased more gradually with depth. However, in this case the difference in the top 0.1m of soil between delved and undelved areas was more obvious (Fig 10 d). Greater areas of the bleached A2 horizon were wetted in the delved profile, while preferential flows left large dry area of soil in the A2 of the undelved profile. The waterfront in the clay rich subsoil was very similar between delved and undelved. Nevertheless, when looking at the delving line only, more water infiltrating deeper into the soil was a confirmation of the effects of the v-shaped area created by the delver that disrupted the sand/clay boundary.

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3.2 Effects of clay addition on soil water storage The aims of the experiment were: 1) To study the relation between clay content and plant available water. 2) To quantify the amount of clay added in the topsoil after clay delving and estimate the effect on plant available water. To understand the effect of clay addition on soil water storage after delving, random undisturbed soil cores were collected from site A in Coonalpyn. Samples were collected from the surface and up to a depth of 0.6 m from the delved and undelved soil profile. Water retention curves were produced for each sample, using porous pressure plates were different water head (expression of water tension) of 0.01, 0.03, 0.1, 0.5, 1, 10 and 150 m were applied. Plant available water (PAW) was then estimated as the integral function of the water curve between water tensions of 1 m (field capacity) and 150 m (permanent wilting point). Each core was then sieved and the percentage weight of different aggregates was estimated. Total clay content of each sample was estimated using the pipette method.

3.2.1 Results In Figure 12a, the results of PAW are plotted against clay content. As expected, the data showed the positive relation between the addition of clay content and the increase of PAW. PAW estimated from the samples of the sand collected from the undelved soil (clay content between 1 and 2%) ranged between 32 and 35 mm/m. At a clay content of 28% (from the top 10 cm of the clay subsoil), PAW reached its maximum values; ranging from 90 to 120 mm/m. The results also show that the rate of increase in PAW tends to decrease with increasing clay content in the soil. The greatest incremental rate of increase of PAW was found with clay contents between 4 and 6%. On average, this relatively small increase in clay content doubled (or more) the PAW of the sand (between 60 and 70 mm/m) when compare to the original (undelved) conditions of the sand. Note that, although there was found to be a good relationship between clay content and PAW (R

2=0.67), results were still quite variable and predicted PAW was sometime under or overestimated

in comparison to the PAW measured in the laboratory. These results may suggest that PAW is not exclusively dependent on the texture (i.e. clay content); but that other factors such as structure and size of the clay aggregates could play a major role in the ability of the soil to store and release water for plant growth. Moreover, a more linear relation between clay and PAW was in fact found in those samples where the proportion of small clay aggregates was greater than in other samples; suggesting that a better admixing of the subsoil clay could produce greater benefits in terms of water storage. On the other hand, the effect in terms of soil strength and compaction has to be taken into account when high clay content in form of fine aggregates is mixed with the sand that could produce constraint to plant root growth and reduced soil water infiltration. Samples of disturbed soils were collected from sites A and B in order to predict the increased PAW in the top 0.1 m of soil based on the relation PAW/clay content found at site A. In order to consider the variability within the delved profiles, samples were collected at different distances from the delving line: at 0, 0.1, 0.2, 0.3, 0.4 and 0.5 m. From each area, the soil was sieved and clay aggregates separated in six classes of aggregate size (Figs. 12a and 12b) and average clay content estimated (Figs. 13a and 13b). Results showed that in both sites the distribution of clay in the top 0.1 m of soil was generally even. At site A, the proportion in weight of clay aggregates bigger than 2 mm (average clay content= 24%) was within 15 to 23% from the delving line to the area between the delving lines. The total clay content ranged between 7.5 and 10.2%; equal to four to five times the original clay content of the undelved soil (less than 2%). Based on the relationship shown in Figure 14a, the potential new PAW in the topsoil could have changed from the minimum of 35 mm/m of the undelved sand to values in the order of 70 to 85 mm/m (Fig. 14b). At site B, the proportion of aggregates bigger than 2 mm was found to be 8 to 11%. However, due to the greater clay content of the subsoil (>35%), the total estimated clay content in the topsoil ranged


19

between 8.7 to 10.3 %, with similar potential for increased PAW at site A. Nevertheless, in term of clay aggregate size distribution, smaller aggregates were found at site B in comparison to site A. More than 50% of the clay aggregates found at site B were smaller than 4.7 mm and aggregates bigger than 20mm were not found. At site A, a large proportion of the aggregates greater than 2mm were in the size range between 6.7 and 50mm. In terms of PAW, these differences could be translated as a greater potential for site B to increase water storage per unit of increased clay content in comparison to site A. However, finer clay aggregates may lead to problems such as compaction and hard setting; which could explain the different results obtained between the two sites during the infiltration experiment with the blue dye in dry conditions.

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3.3 Effects of clay delving on plant root growth Soil cores were collected during September 2012 in sites A and B, in order to study the effect of clay delving on plant root growth. Root length density (Lv) was chosen as a best estimate of root growth in relation to water content. The aim of the experiment was: 1) To study the effect of clay delving on plant root growth - comparing delved and undelved soils 2) To evaluate the variability of plant root growth in the different areas of a delved profile (from the centre of the delved line to the area between the delving lines) Soil cores were collected in random areas of sites A and B; both from the delved soils and from the undelved ones. Cores from the delved soils were collected in groups of ten placed perpendicularly to the delved lines, from the centre of the line to the area between the lines (Fig. 15). Cores were taken to a depth of 0.6 m, based on the maximum depth of soil modification. Every core was then divided in 6 soil samples representing 0.1m increments of depth. Each sample was washed and roots were separated using a 1mm sieve. The clean roots were then scanned using a high definition scanner and root length density calculated using the WinRizo software. At the moment of collection, site A was growing barley while site B was under pasture. At site A it was not possible to collect roots of barley growing in an undelved area so roots were collected from an area covered with annual wild grass adjacent to the barley crop.

3.3.1 Results Figures 16a and 16b show the average root length density (cm of root length/cm

3 of soil) found in

each area of the delved profile, from centre line to between the delving lines. Surprisingly, root length density was variable but no significant differences between the areas were found. Root length densities along the delving lines were in fact similar to the other areas, and roots were well established in the clay subsoil of the areas untouched by the delver tines. One of the possible explanations could be due to the particularly wet winter which occurred in 2012, and which probably has masked the differences between centre line and the area between the delving lines. However, when comparing root growth in the delved and undelved profiles, the effect of the soil modification becomes clear. In Figs. 16c and 16d, the average root length density at different depth within the delved and undelved profiles in sites A and B are compared. The main outcome is a deeper and more even growth in the delved profiles than the undelved ones. Root density in the undelved profiles was mainly concentrated (>50%) in the top 0.1 m of soil and decreased drastically after that. On the other hand, more roots developed at depth in the delved profiles. In particular more roots were found in the clay subsoil of the delved profiles in comparison with the undelved profiles. In terms of plant growth, a greater root density at depth translates to greater access to the clay rich subsoil and its higher PAW than the sandy topsoil.


23

Figure 15. Method of sampling collection for the analysis of plant root growth. Soil cores were taken in groups of ten, perpendicularly to the delved lines in order to characterise the variability of the modified soil profile.

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3.4 Changes in soil carbon content: comparing soil carbon content before and after delving Random samples were collected from the delved and undelved soils in sites A and B, and total organic carbon percentage (KgOC/Kgsoil) was estimated using the Walkley-Black method. The aim of the experiment was: 1) To evaluate the effect of clay delving on the organic carbon (OC) when compared to the undelved soil. 2) To evaluate the variability of OC content in the delved profile as function of the distance from the centre of delving line. Random samples were collected from the control (undelved) sites in order to estimate the average OC content in the horizons A1 (top sand), A2 (also called E, bleached sand horizon) and B21 (top clay horizon). As per the study of the variability in clay content in delved soils, random samples were collected from the topsoil (0.1 m) at different distances from the delving line (0, 0.1, 0.2, 0.3, 0.4 and 0.5 m). In order to study the variability of OC content at depths greater than 0.1 m (where the morphology of a delved profile is much more variable), soil pits were excavated for collection of profile pictures and targeted sampling.

3.4.1 Results Changes in OC content in the top 0.1 m of soil before and after delving In both sites A and B, a higher OC content was found in the top 0.1 m of soil when compared to the undelved soils (Fig 18a). In site A, the sand from the A1 horizon had an average OC content of 0.98% while the average of the A1 in the delved area was 1.48% (Table 3), an increase in OC of 150%. Similar results were found in site B where OC increased from 1.0% in the undelved to 1.40% in the delved topsoil, an increase in OC of 140% (Table 4). Note that site B was delved in 2005, two years before site A in 2007. In terms of spatial variability of OC content in the topsoil based on the distance from the delving line, little difference was found at both sites. OC content in the delved soils of sites A and B ranged from 1.3 to 1.6% (Fig 17). Moreover, only site B showed the higher values at the vicinity of the delving line, in relation with the higher clay content found in that area (Fig. 13b). In fact, unlike site A, only site B showed a linear relationship between clay content and OC. As previously noted for water storage potential, site B showed a greater presence of smaller clay aggregates that could explain the good relationship between OC and clay content. Variability of OC in the delved profile at depths greater than 0.1 m From different soil profiles excavated in site A, targeted samples were taken in order to estimate OC content and to relate the results to the different areas (identified by different colour) of a delved profile (Appendix 2). The main challenge using standard methods for the estimate of OC content in delved profiles comes from their extreme variability of physical and chemical properties, in particular at depths greater than 0.1-0.2 m, where the distribution of the clay aggregates is highly uneven. The results of this sampling method are not meant to evaluate the average OC at depth in delved profile but to be used as a guide (to some extent) to describe the consequences of this variability. Samples taken from the areas of the delving lines had an exceptional variability of OC content (Fig 18b) as a consequence of the morphological variability of the profiles. The values of OC content at 0.2 m in the delving line were ranging from as little as 0.7% to 1.4%. Even in the area between the lines, where little soil modification was found at depth greater than 0.2 m (i.e. horizon A2 is almost untouched) the values of OC were ranging from less than 0.2% to 1.6%.


26

Regarding this high variability, it should be noted that the OC content in some areas of the A2 bleached horizon in the delved profiles (untouched by the delver) were sometimes much higher than the average OC content of the A2 horizon of the undelved soil. At 0.3 m depth, most of the samples from the area between the delving lines were collected from the clay horizon B21 with values in line with the undelved profile (average OC content 0.36 %). In the delved lines, due to the presence of the V-shaped modification of the subsoil clay, the variability of OC was obviously much greater, with values ranging from 0.4 to 1.4%. These results showed how little changes in the area of sampling can affect the results of OC at depths greater than 0.1-0.2 m in delved profiles, in particular along the delving line. This condition has to be taken into account during sampling for OC content; by increasing as much as possible the number of sample in order to compensate for the extreme variability and to avoid over- or under-estimations.

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A1 0.98% 1.48%

A2e 0.19% 0.75-1.4% 0.48-1.6%

B21t 0.36% 0.43-1.4% 0.36%

Horizon Undelved Delved (average)

A1 1% 1.40%

A2e <0.1% -

B21t 0.70% -

Table 4. Average OC content in the delved and undelved soil at

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4. Conclusions and future work This study showed the great potential of clay delving in the correction of texture contrast soil in terms of water behaviours and for carbon storage. The experiments on water infiltration using a dye tracer have shown how the addition of new clay in the top-soil has reduced significantly (in dry conditions) the negative effects of preferential water flows due to the presence of non -wetting sands. As a result, a more even and deeper water infiltration in the top horizon was achieved. In wet conditions, the impact of water repellency was reduced due to the higher soil moisture content and results showed little difference in the upper part of delved and undelved profiles. In general, greater and more uniform water infiltration is achieved in the delving lines, where the boundary between the A and B horizons were disrupted, in both dry and wet conditions. These results, in addition with the increase PAW obtained with the addition of clay, showed the benefits of clay delving in reducing the limitations in terms of soil water properties typically related to texture contrast soil. One of the outcomes found in this study was more even and deeper plant root growth, with plants able to access more water in greater areas of the modified profiles. In both sites A and B, OC content was increased significantly in the top 0.1 m of soil with little variability in the different areas of the delved soil. However, extreme variability of OC content was found in delved profiles at depths deeper than 0.1 m, due to the great variability of the chemical and physical properties of this soil. These characteristics have to be taken into account when sampling clay-delved soils for OC content evaluations. This study represents a first attempt for characterising the changes in soil water behaviour in texture contrast soils following clay delving. Although the study produced positive and encouraging results, the variability of the characteristics of delved soils (due to initial characteristics and clay delved method used) can produce highly variable results and more studies at different locations are needed. Future works should also concentrate on the way clay has been added and mixed to the soil. The results suggested that the increases in PAW and soil OC may be influenced by the size and type of clay aggregates and not just by the amount of clay added to the soil.


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5. References

Bailey, G. 2009. Clay made to pay. Stock Journal. Bailey, G., B. Hughes, R. Tonkin, R. Dowie, and N. Watkins. 2010. Gross soil modification of duplex

soild through delving and spading p. 4 Proc. world congress of soil science 2010, brisbane2010.

Burch, G.J., I.D. Moore, and J. Burns. 1989. Soil hydrophobic effects on infiltration and catchment

runoff. Hydrological Processes 3:211-222. Bureau of Meteorology, A.G. 2011. Rainfal IDF data system. Available at

http://www.bom.gov.au/hydro/has/cdirswebx/cdirswebx.shtml. Cann, M.A. 2000. Clay spreading on water repellent sands in the south east of South Australia--

promoting sustainable agriculture. Journal of Hydrology 231-232:333-341. Chittleborough, D. 1992. Formation and pedology of duplex soils. Australian Journal of

Experimental Agriculture 32:815-825. D.A.F.F, Australian Gov., and Advisory Board of Agriculture (ABA). 2006. Delving Clay- Best

Practice- Report and horizons classifications. Davenport, D., B. Hughes, S. Davies, and Hall D. 2011. Spread, delve, spade, invert: a best practice

guide to the addition of clay to sandy soils. Dracup, M., R. Belford, and P. Gregory. 1992. Constraints to root growth of wheat and lupin crops in

duplex soils. Australian Journal of Experimental Agriculture 32:947-961. Edwards, I. 1992. Farming duplex soils: a farmer's perspective. Australian Journal of Experimental

Agriculture 32:811-814. Eldridge, R. 2007. Clay delving at Parilla, Parilla SA. Flury, M., and H. Fltihler. 1994. Brilliant Blue FCF as a Dye Tracer for Solute Transport Studies—A

Toxicological Overview. J Environ Qual 23. Fogden, A. 2010. Increase yield with clay. Stock Journal. Gardner, W.K., R.G. Fawcett, G.R. Steed, J.E. Pratley, D.M. Whitfield, and R.H. Van. 1992. Crop

production on duplex soils in south-eastern Australia. Australian Journal of Experimental Agriculture 32:915-927.

Hall, D.J.M., H.R. Jones, W.L. Crabtree, and T.L. Daniels. 2010. Claying and deep ripping can

increase crop yields and profits on water repellent sands with marginal fertility in southern Western Australia.(Report). Australian Journal of Soil Research 48:178(110).

Hamblin, A., and D. Tennant. 1987. Root length density and water uptake in cereals and grain

legumes: how well are they correlated. Australian Journal of Agricultural Research 38:513-527.

Hardie, M.A., W.E. Cotching, R.B. Doyle, G. Holz, S. Lisson, and K. Mattern. 2011. Effect of

antecedent soil moisture on preferential flow in a texture-contrast soil. Journal of Hydrology 398:191-201.


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Harper, R.J., and R.J. Gilkes. 2004. The effects of clay and sand additions on the strength of sandy topsoils. Soil Research 42:39-44.

Imeson, A.C., J.M. Verstraten, E.J. van Mulligen, and J. Sevink. 1992. The effects of fire and water

repellency on infiltration and runoff under Mediterranean type forest. CATENA 19:345-361. Isbell, R. 2002. The Australian Soil Classification : Revised Edition, pp. 153 p., In E. Corporation.,

(ed.) Australian Soil and Land Survey Handbooks, 4. CSIRO PUBLISHING, Melbourne. Lemmnitz, C., M. Kuhnert, O. Bens, A. Güntner, B. Merz, and R.F. Hüttl. 2008. Spatial and temporal

variations of actual soil water repellency and their influence on surface runoff. Hydrological Processes 22:1976-1984.

May, R. 2006. Clay spreading and delving on Eyre Peninsula : a broadacre clay application manual

for farmers, contractors and advisors / field work and report compiled by Rachel May ; editors: David Davenport ... [et al.] South Australian Research and Development Institute, Rural Solutions SA, Grains Research and Development Corporation (Australia), [Adelaide].

McGuinness, S. 1991. Soil Structure Assessment Kit:a guide to assessing the structure of red duplex

soi. Government of Victoria, Bendigo. Northcote, K.H. 1979. A factual key for the recognition of Australian soils / by K.H. Northcote. 4th

ed. Rellim Technical Publications, Adelaide, S. Aust. Rebbeck, M., C. Lynch, P.T. Hayman, and V.O. Sadras. 2007. Delving of sandy surfaced soils

reduces frost damage in wheat crops. Australian Journal of Agricultural Research 58:105-112.

Rengasamy, P., and J. Bourne. 1990. Managing sodic, acidic and saline soils Cooperative Research

Centre for Soil & Land Management, Glen Osmond, South Australia. Ritsema, C.J., and L.W. Dekker. 2000. Preferential flow in water repellent sandy soils: principles and

modeling implications. Journal of Hydrology 231–232:308-319. Shakesby, R.A., D.J. Boakes, C.d.O.A. Coelho, A.J.B. Gonçalves, and R.P.D. Walsh. 1996. Limiting

the soil degradational impacts of wildfire in pine and eucalyptus forests in Portugal: A comparison of alternative post-fire management practices. Applied Geography 16:337-355.

Tennant, D., G. Scholz, J. Dixon, and B. Purdie. 1992. Physical and chemical characteristics of duplex

soils and their distribution in the south-west of Western Australia. Australian Journal of Experimental Agriculture 32:827-843.

Tonkin, R. 2010. Soil modification trials at Karoonda, pp. 6, Vol. 7, Winter Edition ed. Rural

Solutions SA, Australian Government. Ward, P.R. 1993. Generation of water repellence in sands, and its amelioration by clay addition. PhD,

University of Adelaide, Department of Soil Science.


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Appendix 1: Examples of delved and undelved profiles impregnated with blue dye tracer

a) b)

c) d)

SITE A, impregnation in wet winter conditions: undelved (a, b) and delved (c, d) profiles

e) f)

g) h)

SITE C, impregnation in dry summer conditions: undelved (d, e) and delved (g, h) profiles


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Appendix 2: examples of soil profiles from site A and OC content in different areas of the profile

a) b)

c) d) Figure a: average OC content in the top three horizons of the undelved soil. Figure b, c and d: three pictures from the delved soil show the high variability of OC content across the profile of these soils.

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