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Soil Testing A Must Before Mouldboard Ploughing

17 October 2011

AUSTRALIA - Deep soil testing before mouldboard ploughing or spading is critical, new Western Australian research has revealed.

The trials, funded by the Grains Research and Development Corporation (GRDC) and Department of Agriculture and Food (WA), have been conducted this year at Badgingarra by DAFWA researcher Craig Scanlan.

They are the first formal trials in WA to investigate the effect of soil amelioration – used by growers to address non-wetting soils - on crop nutrition.

Dr Scanlan said one of the important findings from the work so far was that pH, phosphorus, sulphur and potassium levels in the soil profile varied from site to site.

This meant that growers who planned to lime paddocks intended for mouldboard ploughing or spading should first soil test to 30 or 40cm below the soil surface.

“Soil testing to this depth is necessary because pH levels down the soil profile, as well as nutrient levels, vary from site to site, and soil inversion techniques bring subsoil to the surface,” Dr Scanlan said.

“Currently only a small proportion of growers test soils to this depth, but it is necessary to provide the information they need to help them decide whether to apply lime before or after mouldboard ploughing or spading,” he said.

“If soil testing reveals the subsoil to be more acidic than the topsoil, growers should apply lime after mouldboard ploughing or spading.

“But when pH levels are low down the whole soil profile, they will get better results liming before mouldboard ploughing or spading, to help achieve better lime incorporation.

“After mouldboard ploughing or spading, but before seeding, growers should ideally go back and sample soils a second time to a depth of 20cm to help them assess soil nutrition requirements - particularly for phosphorus and potassium.

“This is because soil inversion can bring soil to the surface which is very low in these nutrients, and growers may need to apply higher rates of phosphorus and potassium at seeding time to supply enough nutrients to achieve good plant establishment.

“For example, at our trial in Badgingarra, the phosphorus levels were high down to 30cm, so there was no change in management required for phosphorus fertiliser after mouldboard ploughing or spading.

“But at other sites, subsoil phosphorus levels were very low, meaning that higher rates of phosphorus needed to be used after these treatments.

“If growers do not have enough time to do nutrient testing between mouldboard ploughing or spading, and seeding, they need to make sure they test for nutrient levels, as well as pH, when soil testing the first time.”

Dr Scanlan said this year’s trial work also revealed the importance of separating seed from nitrogen fertiliser after mouldboard ploughing or spading, to avoid nitrogen fertiliser toxicity.

“Plant counts two weeks after seeding, on sites where these techniques were used, suggested nitrogen toxicity had occurred,” he said.

“We found that soil moisture was 50 per cent lower in the soil surface layer (0 to 10cm) after mouldboard ploughing and spading, and believe this contributed to fertiliser toxicity.”

Dr Scanlan said that, surprisingly, soil moisture levels measured four weeks after seeding were no different in untreated control plots, compared with plots where banded wetting agents were applied.

“But while soil surface moisture levels were lower where mouldboard ploughing and spading was conducted, these treatments resulted in moisture levels 50 per cent higher than the control 10 to 20cm below the surface,” he said.

Dr Scanlan said the trials also revealed that mouldboard ploughing reduced organic carbon levels at the soil surface, but increased levels at 20 to 30cm below the surface.

“This means that the crop will not access nitrogen mineralised from this source during early growth, but may have better access to it late in the season,” he said.

TheCropSite News Desk

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