Stirlings to Coast Farmers look at clay application rates

Many members of the two grower groups have experienced the transformational productivity benefits of spreading clay on sandy soils, including improved water and nutrient holding capacity, reduced non-wetting expression and increasing soil organic carbon.

For more than 15 years, local growers have been refining their methods of applying and incorporating clay in their paddocks to improve their soil fertility permanently.

SCF project officer Daniel Fay said depending on the methods employed and the amount of clay spread, the costs could be $500 to $1500 per hectare, including incorporation.

"We believe improvements can be implemented by growers and contractors that would significantly improve the clay spreading efficiency," Mr Fay said.

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"For example, a 20 per cent improvement in clay spreading efficiency could save $300/ha for growers applying high volumes."

The project will focus on improving knowledge of the clay quality growers are spreading and calculating and measuring claying rates to optimise the amount distributed per hectare.

Clay pits are selected strategically for the most efficient spreading of the clay in the paddock and once the topsoil and overburden have been removed, growers feel obligated to use the clay in that clay pit because of the dollars invested in uncovering the product.

In some cases, spreading the wrong clay can be detrimental to paddock fertility and be challenging to reverse.

Two grower demonstration sites have been established to measure the benefits obtained from clay spreading, with SCF hosting one on its sandplain soils and Southern Dirt hosting another at Muradup on forest gravel soil.

Installed before seeding this year, the SCF demonstration site at Kojaneerup South had three distinct claying application rates of 350, 270 and 140 tonnes per hectare with incorporation by offset discs to approximately 15 centimetres.

Although this depth is relatively shallow, the target constraint within this paddock was non-wetting soils and the vast majority of the repellent properties lie within the top 15cm of the soil profile.

Clay samples were taken from the clay pit on site and the applied clay had an average clay fraction of 40.4pc, with soil samples from the topsoil of the target paddock revealing the current clay percentage was 1.7pc.

During the early stage of the project, SCF assumed the yield benefits from clay spreading were maximised when the soil reached a 5pc clay content to the incorporated depth.

Mr Fay said all three clay rates should significantly shift the topsoil clay percentage according to the clay calculator developed by Department of Primary Industries and Regional Development (DPIRD) senior research officer David Hall.

He said only the lightest application rate failed to achieve the targeted clay rate of 5pc and the other two rates exceeded the best practice percentage of 5pc

"These calculations highlight the ability to vastly shift the clay percentage in topsoils when using a highly enriched clay source and a relatively shallow incorporation depth," Mr Fay said.

"While the 270t/ha and the 350t/ha ameliorated soils are above the 5pc target, it is unlikely that this over application will result in any adverse effects, given the best practice target rate is often as high as 7pc for soils with a high level of organic matter.

"The results highlight the potential efficiency and productivity savings that can be realised when clay content is known before spreading."

Unlike a lot of other agronomic inputs which can be dialled in on a precision level, productivity in clay spreading is largely dictated by the carrying capacity of the machines, the speed in which they spread clay and the width of the machine.

The fixed nature of the variables involved in clay spreading - clay fraction, topsoil texture, available spreading rates - limit what a grower can control.

Based on that, the fixed variables as well as the target soil constraint need to be considered when choosing incorporation implements and depth.

"For example, to solve a water repellence issue like the trial site at Kojaneerup South, a shallower incorporation method such as an offset disc, or speed tiller can be used," Mr Fay said.

"However, if targeting non wetting, plus eliminating soil erosion and improving soil water holding capacity, then a different implement will be needed to incorporate the clay deeper."

Ultimately, the incorporation depth will dictate the amount of clay a grower needs to spread to reach a 5pc topsoil clay content.

A Plozza plough can incorporate clay to about 30cm, while a deep ripper can incorporate clay to about to 50cm with inclusion plates.

Mr Fay said using the soil parameters and claying rates at Kojaneerup South as an example, there were enormous differences of clay required (t/ha) depending on the depth of incorporation.

"This emphasises the value in having a clear understanding of exactly what you are trying to achieve before taking on a large-scale claying project," he said.

"The calculations show that the application rates needed to achieve the 5pc targeted clay rate vary greatly depending on the incorporation depth.

"The machine used at Kojaneerup South to spread the clay might not be the most efficient choice if you were targeting a deeper incorporation depth, so consideration must also be given to the type of machine suited to the constraints you are targeting."

SCF observed three different methods of applying clay to sandy soils and each method had its specific positives and negatives.

One of the critical project findings so far was that the current estimation of clay spreading rates is highly variable and based mainly on guesswork by growers and contractors.

Anecdotal evidence from local growers would suggest the highest clay rate at the demonstration site will have the highest grain yield, however SCF will measure yield results from the clay treatments over the next two seasons to get a clear picture.

As part of the project, which has been funded by the Grains Research and Development Corporation, a workshop will also be hosted early next year and feature DPIRD soil scientist David Hall, who has been an industry-leading researcher for clay-spreading in the Esperance region for over two decades.

At that workshop, growers and contractors will learn about the local clay types and the hierarchy of clay quality which can be determined by soil testing or visual assessment.