Most of the water repellent soils have clay contents of less than one per cent. Many sands that have 3-4 per cent clay contents and organic carbon levels of less than one per cent do not appear to have these water repellent characteristics.
Clay particles are less than two micron in size and have a greater ‘wettable’ surface area that water will contact. Water infiltration is therefore greater in clay based soil types. Assuming clay can be evenly distributed through the soil the net affect is to increase the specific surface area and lower water repellence.
Clay does not increase the water holding capacity of the soil by changing the matric potential, but allows more of the soil to wet up. The plant roots are therefore able to exploit more of the soil volume for water and nutrients.
If the clay rich subsoil is deeper than 60cm, spreading and incorporation of clay is the only option to increase the clay content of the soil surface. The method involves excavating clay from the subsoil in a pit close to a deep sandy area and spreading it (using a scraper or multi-spreader for example) onto the soil surface. The clay rich subsoil is then incorporated soon after it is applied. The incorporation can be achieved with tines, off-set discs, heavy harrows, or rotary hoe and rotary spader.
Poorly incorporated clay can result in surface sealing and poor root exploration into the subsoil, which when coupled with increased evaporation can often result in crops on clayed paddocks haying off due to the lack of water during grain filling. Heavy clay rich subsoil application rates of 200t/ha or more are difficult to incorporate and more costly to apply given the high volumes that need to be excavated and spread. A rotary spader can help dilute excess clay through more of the profile where heavy rates of clay-rich subsoil are applied.
Clay application has shown to improve productivity by:
- increasing moisture infiltration
- allowing a more even wetting of the soil
- causing a more even germination of weeds
- increasing cation exchange capacity improving retention of nutrients including phosphorus and potassium
- improving soil stability and reducing soil wind erosion
- increasing soil organic carbon
- increasing microbial activity
The aim after clay spreading is to achieve clay content in water repellent topsoil of 3-4 per cent if the soil has an organic carbon content of less than one per cent.
Or, to increase the clay content to 5-6 per cent if the organic carbon content is greater than one per cent.
The subsoil to be spread needs to be tested for clay content, pH and for the presence of nutrients such as potassium and sulphur and for toxic compounds such as salt and toxic concentrations of boron or carbonate or extremes in pH.
The greatest cost in clay spreading is the transport cost, so finding a clay pit as central as possible will be the cheapest option. The advantage is that it can overcome water repellence in the long-term, with claying benefits measured for 15 years or more.
Clay spreading with a heavy duty multi-spreader.
(Source: Stephen Davies)
Clay spreading with a carry grader is a popular method on
the south coast of Western Australia. (Source: David Hall)
The spread, delve, spade, invert publication is a detailed guide on incorporating clay into the surface layers of the soil profile. The publication draws together more than 30 years of research and grower experience and is available at Ground Cover Direct http://www.grdc.com.au/director/events/bookshop