WEST Australian research into removing subsoil constraints has confirmed that the biggest improvements in wheat yields occur in wetter years or in higher rainfall areas, and on sandy soils.
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The research was funded by the Grains Research and Development Corporation (GRDC) as part of its national subsoil constraints initiative, and conducted by the Department of Agriculture and Food Western Australia (DAFWA).
Using data from a computer crop simulation model, researchers have mapped where removing a subsoil constraint is most effective in the WA Wheatbelt.
DAFWA researcher Stephen Davies said the crop simulation model APSIM (Agricultural Production Systems Simulator) and climate data from the past 50 years were used to analyse the impact of removing a ‘pan-type’ subsoil constraint, occurring 20-40cm in the soil profile, on wheat yields at 27 locations.
‘Pan-type’ constraints occur in a distinct layer, which makes ameliorating them more viable, because removing the constraint gives roots ready access to the unconstrained soil below.
Analysis was conducted on two soil types – a loamy sandy soil and a heavier textured loamy duplex profile.
The research investigated the effect of amelioration practices - such as deep ripping or liming - on wheat yields in the first year after the treatment for various seasons, locations and soil types.
Dr Davies said the biggest yield responses to the removal of subsoil constraints, including compaction and subsoil acidity, occurred in seasons and locations with higher rainfall, and with higher yield potential.
“Removing constraints in these circumstances improved root access to subsoil water and nutrients,” he said.
“Better yield responses occurred in seasons with wetter finishes, which reduced the risk of haying off which can be exacerbated by crops growing more biomass after a constraint has been removed.
“In drier seasons, faster water use caused by constraint removal can result in all the water being used before grain filling is complete.
“The marked variation in profitability of amelioration in different seasons suggests we might get better returns from these practices if we could use them in years with higher rainfall.
“However, our ability to predict wetter seasons before they happen is still limited.”
Dr Davies said soil amelioration was also more beneficial on light textured soils such as deep sands and deep sandy earths.
“Crops on sandier soil types tend to have deeper roots to access water stored deeper in the soil profile, and constraint removal on sandy soils increases root penetration and access to water and nutrients,” he said.
Dr Davies said the GRDC funded research showed there was more value in addressing more severe subsoil constraints.
“Particularly on loamy sandy soils, you will get paying responses to ameliorating more severe levels of subsoil constraints in almost all seasons,” he said.
“A severe constraint in deep loamy sand at Mingenew limited the final rooting depth to about one metre, compared with unconstrained roots which grew to about 1.8m, and those affected by a more typical constraint which grew to 1.7m.
“This corresponded with grain yield which averaged three tonnes per hectare when there was no constraint, compared with 2.6t/ha for wheat affected by a typical constraint and 1.4t/ha where it was affected by a severe constraint.”
The research also confirmed that removing subsoil constraints can sometimes reduce wheat yields.
“While sandier soils produced some of the biggest yield improvements from amelioration, these soils also produced larger and more frequent negative yield responses when there were drier finishes to the season,” Dr Davies said.
