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Tillage increases P accessibility

Tillage can redistribute phosphorus (P) within the top 10 cm of the soil. P often becomes concentrated at the soil surface, especially in no-till paddocks. Getting P deeper into the soil is important in dry seasons when the active root zones are deeper.

Trials tested the response of wheat to P in soils with high Colwell P levels at Cowra, NSW in 2016. The trials compared various P fertiliser rates, application techniques and tillage practices.

The trials help understand:

  • the extent of ┬átopsoil P stratification within central NSW
  • how P stratification in topsoil alters critical P concentration in wheat and canola
  • if tillage (using disc, tyne, knifepoint press wheels) can improve P stratification to 10 cm
  • if tillage increases crop access to residual P from previous seasons; and
  • why there can be significant grain yield responses to P in soils with high Colwell P (29 mg/kg at 10 cm).

2016 was a very wet season, ideal for plant roots to take up residual P stratified at the soil surface. Despite it being unlikely for stratified P to limit crop growth, the trials showed:

  • tillage increased the Colwell P levels at depths of 4cm – 12.5cm compared to no-till soils
  • tillage increased P uptake, slightly above the uptake in no-till trials after Crop Growth Stage 31
  • a higher P uptake resulted in a higher N uptake
  • Grain yield was slightly higher on the cultivated trial plot (5.741 t/ha) than on the no-till trial plot (5.526 t/ha).

The GRDC, AgGrow Agronomy, FarmLink, Southern Farming Systems and MacKillop Farm Management Group support this project.



Factors affecting critical P values and its responsiveness in different soil types

FACT SHEET: Strategic tillage


Ehsan Tavakkoli, Col McMaster, Mark Conyers, NSW DPI

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