The clay that killed
GroundCover™ Issue: 109 | 03 Mar 2014
John ‘Chompy’ Masters has been using clay spreading and delving since the mid-1990s on his 2430-hectare cereal and sheep property near Wharminda, on the Eyre Peninsula in South Australia.
However, John’s experiences with claying have been mixed. Early spreading gave visible improvements in crop performance following the clay incorporation, but one paddock has not responded. John spread the paddock five years ago because it had a sand hill in the middle and his aim was to make it easier to work the paddock in one piece.
There was no clay at a suitable depth for delving so he spread the paddock at 80 tonnes/ha, followed by incorporation with double railway iron smudge bars, cultivation with wide points and discing with an offset bar.
The first wheat crop after claying averaged 3.2t/ha, which was in line with his expectations.
However, in the 2013 season, John says the subsequent crop of wheat “just looked terrible”. Plants in clayed parts of the paddock had pale green leaves that he assumed was manganese deficiency. Before he had time to address the deficiency, the patches were dead.
“I had expected that manganese deficiency could knock back a plant’s development, but I never thought that it would kill it,” he says.
“The line where the clay spreading started was obvious – it was as if it had been sprayed with glyphosate.”
By harvest, John decided there were too few surviving plants to justify reaping the clayed area at all.
While John has not yet pinpointed the exact cause, he suspects the wet 2013 winter may have exacerbated a manganese deficiency in the clay soil.
“Because it was such a wet year, the roots didn’t have to dig down deep into the sand to get their moisture – they just sucked it all up from the top six inches,” John says.
His cousin, South Australian Research and Development Institute farming systems specialist at Minnipa, Linden Masters, says manganese is one element that can be tied up quickly.
“When you put additional calcium into the soil, the cation exchange alters and the clay hangs onto the manganese and it’s not available to the plant. In those sands there’s only minimum manganese so if the clay hangs onto it and it’s not available to the plant, it can create a huge issue,” Linden says.
John says the key message from his experience is to know your soils.
“It just depends on what you dig up, sometimes you dig up terrible stuff.
“Bad clay is something I learnt to recognise. With hindsight, the bad pits had typical sodic ‘domes’, and the clay was yellow in colour, and often contained some light limestone rubble.
“In contrast, the best results came from pits that contained red or orange clay and these pits were usually located in the middle of a good flat. If the topsoil was good, usually the subsoil was also good.”
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