Claying to ameliorate soil water repellence

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Claying to ameliorate soil water repellence involves the application and incorporation of clay-rich subsoil into water repellent topsoil to overcome the repellence. Addition of clay-textured soil provides the longest-term solution to soil water repellence. The high expense of claying will usually need a few seasons to recover costs and beware of poor incorporation of clay-rich subsoil which can hinder productivity gains. It is best-suited to higher rainfall environments with higher yield potential and can benefit both crop and pasture production.

Claying is the process by which clay-rich subsoil is applied and incorporated into topsoil for the long-term amelioration of soil water repellence.

Addition and incorporation of more clay into repellent topsoil increases the soil surface area and 'masks' and 'dilutes' the hydrophobic organic matter that causes the repellence to such an extent that water infiltration is no longer inhibited.

Methods for applying clay

Clay is typically applied to water repellent sands by excavating and spreading clay-rich subsoil or by lifting clay subsoil from beneath the sand using a delver. The method used to apply clay to the sandy topsoil depends on the depth of sand over clay and suitability of the clay-rich subsoil for claying.

  • For deep sands with 60cm or more sand over clay, the clay is too deep for delving so excavation from a pit and spreading is used to apply clay.
  • For deep sandy duplex soils where there is 30-60cm of sand over clay, then delving can be used where large tynes penetrate and lift the clay to the soil surface.
  • For shallow sandy duplex soils where there is less than 30cm of sand over clay, rotary spaders, some deep rippers or shallow delving can be used to lift clay. This should be approached with caution as the benefits obtained may not be large and there is a risk of bringing up too much clay to the surface. In general repellent shallow sandy duplex soils will wet up more quickly than deeper sands because they can more readily wet up from underneath due to slow infiltration into and perching of water on the clay B horizon.

Depth to clay and samples for assessment can be obtained using auger or post-hole diggers. It is possible to correlate these observations with spatial data such as yield, biomass or EM38 maps for mapping of broader soil type areas. Ground penetrating radar is particularly useful for measuring depth of sand over contrasting layers such as clay or gravel.

Suitability of subsoil for claying

For claying to be cost effective the clay-rich subsoil needs to contain a reasonable quantity of clay. In general the aim is to increase the clay content in the repellent topsoil to 3-7%. As a rule-of-thumb a topsoil clay content of 3-4% clay is usually adequate to overcome soil water repellence in soil that has less than 1% organic carbon. In soil with organic carbon levels greater than 1% the clay content may need to be as high as 5-7% clay to fully overcome the water repellence. Gravelly topsoils with high organic carbon will not need such high subsoil rates as the effective soil volume is reduced by the gravel.

Typically the subsoils used for claying contain 30-40% clay but this can go as low as 20% or has high as 50%. Knowing the clay content and depth of incorporation is critical to knowing what rate of clay to apply to achieve the targeted clay content. Incorporating 100t/ha of clay-rich subsoil that has a clay content of 30% into the top 10cm will increase the clay content of the topsoil by 3-4%. Clay content can be estimated using hand texturing where moist soil is formed into a ribbon between the thumb and forefinger. A continuous unbroken soil ribbon of approximately 75mm is indicative of a clay content around 30%.

Clay types or mineralogy can also be important but in general terms clay aggregates that are dispersive and readily break-down in water are good for claying as the clay can be more easily spread throughout the soil. Subsoil that slakes when wet is essential for clay spreading. Slaking is the process by which soil aggregates disintegrate and crumble when they are saturated with water. It can be tested by looking at the behaviour of dry aggregates submerged in water, slaking will occur almost immediately. Dispersive subsoils are typically higher in kaolinite clay and this is a slower process by which the clay particles become suspended in the water which can be seen when the water takes on a 'cloudy' or 'milky' appearance around the slaked clay. Non dispersive clays take longer to improve repellent soils.

Clay-rich subsoil can be high in nutrients and may have a high pH but neutral and more acidic subsoils also occur. Importantly the subsoil can have high potassium content which can be of significant benefit to crops. A few clay subsoils contain lime which will significantly increase pH but the content of lime varies with depth. Subsoils may also contain toxic concentrations of boron and salt which can be detrimental to crop and pasture growth in the short to medium term until they are leached deeper into the profile. Trial work with alkaline clays has been found it inhibits lupin growth in the first year of incorporation, but typically not beyond. For this reason subsoil should be tested for clay content, pH, electrical conductivity and nutrients before it is used for spreading.

Excavation of clay subsoil

Because of the high volumes and high cost of transporting clay-rich subsoil good sources of clay-rich subsoil need to be found near the site where clay is being spread. It is most economical to use clay excavated from the same paddock being top-dressed with clay. In Western Australia, the clay subsoil is typically found below a layer of sand and sometimes gravel. This overburden needs to be removed then rippers, carry graders, bulldozers or scrapers are used to excavate the clay. In situations where the subsoil is in large aggregates the clay may need to be broken up in the pit prior to spreading.

Spreading of clay subsoil

There are several methods growers are using to spread the clay-rich subsoil in claying operations. Carry graders are used to both excavate the clay then to spread it in strips across the paddock. These strips are then 'smudged' with iron bars to spread the clay and can also worked with tynes to breakdown clods and spread the clay. This method can be used to spread high rates of clay but trying to spread the clay into an even layer across the surface is critical.

Alternatively clay-rich subsoil can be spread using heavy duty multispreaders. The subsoil needs to be fairly well broken up without very large aggregates. Multispreaders tend to spread the clay fairly evenly often with multiple passes and overlap required to get to the required rate. This means that smudging is not necessary and is a small cost saving. Typically this approach is used to apply more moderate rates of clay-rich subsoil typically between 75-150 t/ha.

Considerable care is advised to minimise risks of subsoil compaction when spreading by avoiding, where possible, spreading when the subsoil is wet and more prone to compaction. Very dense subsoils have been formed by clay spreading with heavy graders in wet summers. Incorporation of deep deep ripping after claying can remove much of the compaction and this benefit can be maintained if a controlled traffic system is also implemented which will protect the investment in claying and optimise the benefits.

Delving clay subsoil

Clay delving involves the use of large delving tynes that can penetrate into subsoil clay layer and lift clay to the surface. There are many variations of delving tyne design and the tynes can be up to 2.5m long and 10-18cm wide and are typically 80-100cm apart. Because of their large size and operating depth, delvers typically only have 3-4 tynes, generally they are pulled with tractors with 400 horsepower or more and tracked machines are preferable due to better traction. Typically the tynes operate at an angle of 45 degrees. The optimal designs are wider at the tip and become narrower towards the top which helps the 'blocks' of clay subsoil, which may be quite sticky, come off the tynes as they reach the surface. Having a shallow concave shape or raised edges on the face of the tyne helps hold the clay on the tyne so that it can be lifted right up to the soil surface.

Delving has several particular advantages:

  • Delvers have a deep ripping effect creating loose pathways for rapid root growth.
  • Seams of sand can fall into the clay B-horizon as a consequence of delving that can provide improved root access deeper into the subsoil.
  • Some growers have tried to place gypsum down behind the delving tynes to improve the structure and root access of sodic subsoils.

Problems with delving are:

  • Depth to clay and therefore rate of clay lifted varies across a paddock. In areas where insufficient clay is brought to the surface there may be a deep ripping benefit but this will not be as long lasting as claying benefits.
  • The soil can be left very soft, rough and difficult to traffic. The incorporation process will help level the soil and rolling or working along controlled traffic lines can help with trafficability.
  • It can be difficult to break up, spread and mix the large clods of clay brought to the surface. Rotary spaders are proving useful tools for effectively doing this otherwise delved clay subsoil is broken up with smudge bars, cultivators, rollers, heavy harrows or prickle chains.

Incorporation of clay subsoil

Achieving effective incorporation is critical for successful claying. Poorly incorporated clay can ultimately hinder crop establishment and limit crop productivity by increasing the risk of haying off. The type and extent of incorporation required is dependent on the amount of clay-rich subsoil that has been spread on the soil, the existing clay content of the topsoil and the depth of repellent topsoil.

Traditionally incorporation has been conducted with offset discs and tyned implements such as scarifiers, cultivators or heavy harrows. These are adequate for moderate rates of clay up but can be insufficient at higher subsoil application rates of 250t/ha or more. Rotary hoes and rotary spaders are now being used to better incorporate higher rates of applied subsoil. Rotary spaders can effectively incorporate clay to depths of 25-30cm. This depth of incorporation may be excessive for clay applied at more moderate rates because the clay concentration at the surface may become too diluted and not fully effective at overcoming the soil water repellence.

Table 1 Suggested equipment and incorporation depths for different rates of applied clay-rich subsoil
Application rate of clay-rich subsoil Suggested incorporation equipment Incorporation depth
150t/ha or less Offset discs; tyned cultivator with wide shares; one-way plough 10-15cm
150-250t/ha Rotary hoe; disc plough with large discs; rotary spader 15-20cm
More than 250t/ha Rotary spader 20-30cm

Benefits of clay spreading on water repellent soils

There are many benefits that can arise from effective clay spreading, these include:

  • long term amelioration of soil water repellence - benefits have been observed for up to 30 years
  • improved water and nutrient holding capacity in the surface soil layers
  • improved crop and pasture establishment resulting in better ground cover
  • reduced wind erosion risk s clay binds and stabilises the surface of sandy topsoils that are susceptible to wind erosion and can result in the development of some soil aggregates
  • improved pasture utilisation as pastures can be grazed harder without risk of wind erosion
  • build up of soil carbon because the clay protects the organic matter from degradation and when coupled with higher inputs due to improved productivity organic carbon levels can increase, although this will take many years
  • reduced frost risk can occur in some cases because the change in soil colour by claying and higher topsoil moisture content.

Productivity responses to clay spreading

Crop yield increases from effectively implemented claying typically range from 20-100% or more. Nil or negative responses can occur where high rates of clay-rich subsoil are applied and incorporation is inadequate. Yield benefits have been shown to last for more than eight years in several long-term trials conducted in WA.

Higher productivity gains in response to clay spreading are more likely when:

  • sands are deep (>60cm) but have reasonable plant available water holding capacity
  • soils are highly repellent
  • cation exchange capacity is low (CEC <3)
  • potassium is marginal or deficient (<50 ppm).

Problems that may reduce or negate productivity benefits includes:

  • use of a subsoil which does not slake and disperse can reduce its effectiveness as it remains in aggregates and does not get thoroughly mixed throughout the repellent topsoil so it will take longer to be effective
  • insufficient incorporation and too much clay can result in surface crusting, increased runoff, increased evaporation, poor establishment and poor subsoil root development
  • claying operations, such as loaded carry graders, can cause severe subsoil compaction and this can reduce crop yields and may need to be removed using deep ripping and ideally sustained using a controlled traffic farming system
  • induced nutrient deficiencies and toxicities can occur due to application of moderate to high rates of alkaline subsoil which can induced micronutrient deficiencies such as manganese deficiency in lupins, these subsoils can also contain toxic levels of boron, salts, carbonates and bicarbonates. The impact of these toxicities such as salt and boron can decline over time as they leach deeper into the profile
  • presence of other constraints such as subsoil acidity or compaction or soils with inherently low yield potential due to poor water holding capacity can limit the size of any benefits.
Acknowledgement

Soil water repellence research is supported by DAFWA and Grains Research and Development Corporation through DAW00204 Delivering agronomic strategies for water repellent soils in Western Australia.

Contact information

Giacomo Betti
+61 (0)8 9956 8554
Jeremy Lemon
+61 (0)8 9892 8413
+61 (0)8 9083 1142