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How Much Effect Does Soil pH Have on Weed Management?

Content credit: agfaxweedsolutions.com. The original article can be accessed here.

Aside from the way proper pH can boost yields, hitting the right levels can affect how well the crop competes against weeds. Soil pH can even affect herbicide performance and/or how long its residual activity remains in place.

The role of soil pH is a pretty big deal in Australia. Soil acidity limits productivity on about 50% of Australia’s cropland, “and several important weed species have a competitive advantage over crops on soils where the surface soil pH level is less than 5.5 or the subsurface pH is less than 4.8,” according to the article.

Bringing soil pH up to optimal levels increases the crop’s biomass, according to a trial by Chris Gazey and Joel Andrew in Western Australia. Compared to plots with an un-limed, acidic profile, the total biomass increased by 160 where researchers properly limed the soil. The weed biomass also was cut to only 3% of the total amount of vegetation produced in the plot.

Hashem points out that it takes time – “usually several years” – for liming to fully adjust soil pH to the point that farmers will see strong gains in weed control. He cites one University of Western Australia trial that ran from 2010 to 2014. In that project researchers “saw reductions in populations of wild radish, annual ryegrass and barley grass of between 40% and 70% in the fourth year of the trial as a result of lime applications.”

Surface pH can also influence how residual herbicides break down.

In acidic soils, the chemistry imidazolinone breaks down more slowly. That increases its persistence. “The opposite occurs with triazines and sulfonylureas, which persist much longer in high pH (alkaline) soils,” according to the article.

Two related points in the article:

  • Soil pH strongly influence how water (through hydrolysis) breaks down certain herbicides. “Triazines (Group C sub-group) and sulfonylureas (Group B sub-group) are broken down through chemical hydrolysis in neutral or acid soils,” the article noted. “This process is much slower in alkaline soils, potentially restricting crop choice and exposing late germinations (of weed seed) to sub-lethal doses of the herbicide.”
  • Microbial break down of some herbicides also can be altered by soil pH. “Soil pH generally does not affect this process, except in the case of imidazolinones (Group B sub-group), which persist for longer in acidic (low pH) soils.” That, in turn, could prolong herbicide carryover into the next crop.

Getting back to biomass, the more vegetation the crop produces the more competitive it can be. In one study, DAFWA’s Abul Hashem reduced annual ryegrass counts from 139 to 56 per square meter in a 4-year examination of liming’s benefits. That amounted to a 59% reduction.

Australian scientists have found that even a small shift in soil pH can greatly reduce available soil aluminum. “Soluble aluminum restricts the growth of plant roots and this limits the plant’s ability to access water and nutrients, particularly in seasons where there is a dry finish.” That, in turn, reduces yield and grain size.

“Weeds do not usually prefer acidic soils, they are just able to take advantage of the reduced competition for resources,” the article emphasized. “By adding lime to acidic soils it is possible to lift the pH, allowing crops to perform better and out-compete the weeds.”