AJOUTER DE L'ARGILE AUX SOLS SABLEUX? Addition of clay to sandy soils.

Un livre très intéressant sur une expérience australienne: apporter de l'argile aux sols sableux. Deux métodes:
-épandre de l'argile sur le sable de surface
-remonter l'argile située en profondeur sous le sable.

Spread, delve, spade, invert a best practice guide to the addition of clay to sandy soils

GCTV19: Clay spreading research uncovers keys to ... - YouTube

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INTRODUCTION

Follow the rules to improve sandy soils with clay

Photo:  Engin à dent pour remonter de l'argile du sol-mère.

Photo: Aspect de la surface après passage d'un engin à dents.

Across Western and South Australia there are many millions of hectares of deep sand or sand over clay-rich subsoils that are used for agricultural production. However, these sandy soils present a range of challenges due to their poor water holding capacity, inherent low fertility, extremes of pH, low levels of microbial activity and vulnerability to wind erosion. In addition, many sandy soils are non-wetting, which causes uneven germination resulting in poor weed control, low levels of soil cover and reduced productivity . Sands and loamy sand soils have less than five per cent clay content. As clay protects organic material from decomposition these soils are also low in organic carbon. Raising the clay content changes the soil texture class, which increases the capacity for the soil to store water (Table 1 page 6), nutrients and soil organic carbon. Experience has found that it is feasible and profitable to raise the percentage of clay in the soil to above five per cent. For example, adding 200t/ha of soil containing 30 per cent clay would raise the clay content in the topsoil from 0.5 to about five per cent, if incorporated to 10cm. Provided that appropriate methods are followed, remediating sandy soils with clay-rich subsoil can result in substantial yield improvements. Trials in WA and SA have reported yield improvements of 20 to 130 per cent across cereal, lupin and canola crops in the years following clay additions. However, achieving the correct rate of clay addition and understanding the chemical nature of the clay-rich subsoil to be used is vital, if the process is to be successful. The depth to clay-rich subsoil determines which method of clay addition is most appropriate. Deep sands can only be addressed through spreading of clay-rich subsoil excavated from a pit, spread across the soil surface and then incorporated. Sand over clay soils (Duplex soils, Figure 1) offer the potential to provide a source of clay that can be incorporated in the upper sandy horizons. Clay at between 30 and 60cm can be brought to the surface by delving. Where clay is less than 30cm below the surface a rotary spader or possibly a mouldboard plough can be used to lift and incorporate the clay in the topsoil. Adding clay is relatively expensive and time consuming and if done incorrectly can result in negative effects that are difficult to reverse. Consequently, detailed planning of each stage of the process and following best practice is essential. Drawing together over 30 years of research and grower experience, this publication answers the key questions that must be addressed for a successful claying program to be achieved.

Photo: Extraction d'argile.

Photo: Epandage d'argile.

Case study one Spader man a pioneer Delving plus spading to mix clay has boosted grain yield dramatically on Roger Groocock’s farm in southeast South Australia. Felicity Pritchard

Roger Groocock’s entire property has been delved or clay spread to eliminate non-wetting properties. Improved incorporation using a spader is the latest development in his on-going soil improvement program.

FARM DETAILS Grower: Roger Groocock Location: Bordertown, South Australia Average annual rainfall: 450mm Farm size: 1400ha, 950ha cropped area and 450ha pasture. Enterprises: mixed farming with sheep Soil types: bleached loamy sand over a brown mottled clay Area clayed: the whole farm has been treated one way or the other; now just fine tuning small areas in paddocks or spading previously spread areas. Claying history Clay spreading began in the early 1990s and delving began on more shallow sands in 1993. Clays were mixed with a home-built ‘Alabama’ machine that was replaced with a spader in 2005. Spading is continuing on the property.

Roger Groocock is one of Australia’s clay spreading pioneers. In the early 1990s, Roger and his peers in the Bordertown Landcare group did all they could to overcome the scourge of non-wetting sands that were afflicting their farms. Capeweed and silvergrass dominated pastures on deep sands, while crop rotation options were unsustainable on the shallow sands, the ‘good country’. While minimum tillage and no-till with full stubble retention were tested, these did nothing to alleviate water repellence. Eventually they hit the jackpot by spreading clay over sand, after learning of this techniques success on Clem Obst’s farm. “We recognised the benefits as soon as we did the clay spreading,” says Roger, who was group leader at the time. Clay was not spread on paddocks with shallower sands, as these were considered most productive and ideal for subclover pasture. However, once continuous cropping was adopted, water repellence became an ever-increasing issue. This was due to waxes, created from the breakdown of extra organic matter produced by cereals, coating the sand particles. Wax levels after lucerne, phalaris and annual ryegrass were quite bad as well. Realising water repellence was actually worsening with cropping, the group decided to test ripping-up the clay-rich subsoil.

The idea of delving the clay-rich subsoil was initiated by former CSIRO soil scientist Dr Bob Fawcett, after he visited a soil pit on Roger’s property. Armed with this idea and information gleaned from a water repellence workshop Roger attended in Western Australia in 1993, trials were undertaken using a trench digger to 60cm depth and 1.2m spacing. They found water repellence did not return on the delved shallow sands. Funding for the Landcare project (Operation Finetune) enabled the first clay delver to be built by University of South Australia in 1994. “Very quickly, contractors and farmers recognised the opportunity to amend soils by delving. Within three years, it really took-off in this district,” says Roger. In 1997, Roger decided more spreading was needed on his farm. He bought a Lehman scraper to extend the area of clay spreading further down the sand hills. The expense was covered by the uplift in productivity brought about by clay spreading. Roger began contract clay spreading for an additional income stream. Roger’s entire property has now been delved or clay spread, with some paddocks spread twice where light rates were used. Clay spreading took place on sands deeper than 40cm, while delving was used for shallow sands over clay. He has since sold his Lehman scraper but Roger still owns a small delver. In early days, nearly all his country was delved with the small delver. In the last five years, Roger has used contractors to delve in areas with clay too deep for his small delver.

Deep incorporation Roger built his own clay mixing ‘Alabama’ machine, in 1993. This created furrows about 25cm deep using V-shaped shares to mix both the spread and delved clay. A spray nozzle, mounted 30cm behind each shank and 30cm above the soil, was added to spray trace elements into the furrow. After levelling the trace elements would be located between 7.5 and 20cm deep. Liquid copper, zinc and manganese sulphate were applied at 2.5kg/ha of each element to the whole paddock. A 4.5m wide railway line was dragged behind the ‘Alabama’ to level soil. Roger continues to push the boundaries of soil manipulation. In 2007, he was awarded a Churchill Fellowship and travelled to the Netherlands to research spaders. He had learnt of their existence from his Dutch farm worker, who said they were better than the ‘Alabama’ machine. These digging machines thoroughly mix soil to incorporate clay or other materials to a depth of about 30cm. He was so impressed with the results that he imported the first spader into Australia, to replace the ‘Alabama’ clay mixer. Farmax, a producer of spaders, then offered Roger the Australian agency for their machines. Roger is in the process of spading all his delved and clay spread paddocks. After three years experience, he finds thebenefits are clear. Grain and pasture yields after thorough deep mixing have increased by 70 to 80 per cent on delved paddocks. Before delving wheat yields were about 2.2t/ha now they average 3.8/ha. On clay spread areas following spading yields have doubled from 1.8 to 3.6t/ha. These paddocks can now support a more intensive crop rotation. Roger has been fortunate, not all growers achieve such startling results from clay spreading. According to Roger, it is essential to know details of the soil profile before embarking on clay spreading or delving. “With experience, we found we can use any clay on our farm. Some farmers have had trouble with high magnesium, chloride, carbonates or low pH in their clay,” says Roger.

Types and rates The top 30cm of Roger’s clay-rich subsoil are sodic, however he considers these fine for spreading as the sodicity is readily ameliorated with gypsum. The subsoil below that layer has a higher calcium carbonate content, so rates are reduced. The clay-rich subsoil’s pH (in water) ranges from 6.8 to 9.7. After delving or spreading clay, the topsoil pH is increased from about pH5.5 to ph6.5 to 7.5. Reducing topsoil acidity benefits crop growth and nutrient availability. In the district most clay spreading and delving is undertaken after harvest. However, Roger has delved mostly in spring and spread clay in autumn, the optimal time. The spader allows more flexibility in the timing of these operations, although summer is not ideal due to the risk of erosion.

Liquid trace elements are sprayed on the surface before incorporating the clay-rich subsoil with the spader. In the early days, Roger engaged contractors to spread on sandhills 200 to 250t/ha of clay-rich subsoil, which comprised of 35 to 40 per cent clay. This was fully incorporated to 10cm. The clay was removed from strategic points to create dams or water courses in low lying areas. Finding his clays were ‘friendly’, free from major toxic elements, Roger decided to test heavier rates with deeper incorporation. Rates were doubled (400 to 500t/ha) as was the incorporation depth (20cm) resulting in the same clay percentage as the lower rate but distributed through a large volume of soil. “About five per cent clay is adequate for long-term amelioration of water repellence. So, provided we have no more than five per cent clay, we are unlikely to create problems; the deeper we have mixed the clay, the lower the haying-off effect has been,”

Method Roger now believes it is only worthwhile delving soil where the clay-rich subsoil is less than 30cm from the surface. When delving with a 1 to 1.5m tyne spacing, he has found his soil is disturbed up to 90cm deep depending on depth to clay. Clay clods (football-sized or bigger) are brought to the surface. Delved soil is initially left to dry, allowing clods to break down with weathering over summer. The ground is then levelled (smudged) with a scarifier or levelling bars. Trace elements are now sprayed before incorporating the clay-rich subsoil with the spader. Spading is carried out at between 35 and 55 degrees to the delver lines to improve clay mixing throughout the tyne inter-row area. The rotation on delved soil is canola, wheat then barley. He believes canola provides greatest root penetration of the deep ripped layer once hardpans are removed. Pulses have been difficult in the past, but on delved and spaded soil, beans can be profitable. Balansa clover is sometimes used in lieu of a pulse. Roger is always looking for a better rotation, for example he has tried growing fodder rape as a summer forage. The addition of nitrogen through legumes is a major benefit of Roger’s rotation.

On clay spread paddocks, Roger grows two cereal crops followed by long-term legume-based pasture for livestock. His aim is to raise soil organic matter levels. After spading old pastures, he returns to two years of crop then spades in all straw. While spading works to 40cm depth, it generally mixes soils and straw to 30cm. Three years in ten, a spring fodder rape crop has been sown pre-delving and used for finishing fat lambs through to March, then the area has been smudged and clay incorporated ready for a winter crop.

Benefits A number of benefits have come from claying his country, these combine to help improve yield. Root penetration has increased on delved paddocks, providing entry points for roots to access previously unavailable soil moisture, increasing water use efficiency. Sandy topsoils now have better structure, trafficability, and more nutrient and water holding capacity. Soil erosion from slopes is also lower. Weeds now germinate on the first rain providing better control with knockdown herbicides. Post-seeding weed control is better too, while less frost damage is another improvement. Roger has seen a significant improvement due to better nutrition. The introduction of clay to the topsoil helps retain the applied nutrients in the root zone. In nine out of 10 years Roger has covered the costs of delving and spreading after the first year. However, he knows owning his own machinery means his costs are less than the contract rate, for example $120/ha for delving. Roger remains an advocate of clay spreading and delving. His persistence, ingenuity and enthusiasm have led to a major change in practices and attitudes of farming sandy soils in the Mallee and beyond. “The interaction between soil scientists and farmers has been pivotal to the success of this soil amelioration process.” 

More details: Roger Groocock, 08 8754 6025 or 0427 546 025, grooks@internode.on.net

Photo: L'extraction d'argile permet de créer une retenue d'eau.