Probing nutrient release under no-till

Photo of Dr Jeff Schoeanu

Growers adopting no-till seeding are often concerned about its impact on the recycling of nutrients from crop residues. Emma Leonard spoke to Canadian researcher Dr Jeff Schoenau about his work on the subject

[Photo (left) by Emma Leonard: Canadian researcher Dr Jeff Schoenau demonstrates the reusable PRS™- probes that "trap" available nutrients on a resin membrane.]

Studies by Canadian researcher Dr Jeff Schoenau in the province of Saskatchewan show that tillage may not be essential for effective short-term recycling of nutrients from crop residues or from the soil organic matter. Dr Schoenau discussed his studies, which took place in a climate of semi-arid to sub-humid climate with 300 to 400 millimetres of annual rainfall, at the South Australian No-Till Farmers Association"s annual conference.

A wheat crop that yields two tonnes of grain per hectare returns about 4t/ha of organic matter to the soil as stubble and roots. The stubble alone contains about 18 kilograms per hectare of nitrogen, 2kg/ha of phosphorus, 30 kg/ha of potassium and 6kg/ha of sulphur, as well as trace elements.

Over the long term, the increased surface residue cover was shown to protect the soil from nutrient losses caused by soil erosion.

In a wheat-fallow and cereal pea rotation, no significant differences in available nitrogen supply were recorded in years of average or below-average precipitation. However, in wet years, lower nitrogen availability was recorded in the no-till fallow part of the rotation. In the same study, higher supply rates of available phosphorus were recorded near the soil surface under the no-till treatments.

"Both of these results are considered to relate to increased soil moisture under the no-till system," explains Dr Schoenau. "Greater water retention under no-till treatments resulted in the soil being saturated for longer and more nitrogen was lost due to de-nitrification, while the higher soil moisture improved phosphorus availability by allowing greater movement of phosphate to root surfaces."

Over the long term, an increase in the soil"s ability to release available nitrogen to the crop was recorded after several years of no-till. This was most pronounced when the crop rotation included legumes, fallow was reduced and fertiliser was applied at recommended rates.

By using Plant Root Simulator (PRS)™-probes, Dr Schoenau was able to take nutrient measurements throughout the year without disturbing the soil or growing plants. These reusable probes contain an ion exchange, resin-based membrane. The membrane can "trap" available nutrients from moist soil, irrespective of soil type.

"These probes mimic the action of nutrient uptake by roots, so they give us a very clear picture of changes in nutrient availability and how this may impact on the crop."

For more information: Dr Jeff Schoenau,

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