UC Weed Science (weed control, management, ecology, and minutia)
Article

ASDoing the weeds

  • October 15, 2015
  • By Oleg Daugovish

You may have heard about ASD (anaerobic soil disinfestation) as ‘biological alternative to soil fumigation'. This is the practice of adding easily degradable carbon sources to soil and developing anaerobic conditions under plastic mulch in moist soils for 3-5 weeks. Then the aeration is provided by cutting holes in plastic and you are ready to plant. The mechanism of action is a mix various processes: anaerobiosis itself, reduction in pH, development of volatile compounds, changes in microbiology and physical properties of soil and, of course, substantial increase in nitrogen that usually comes along with the incorporated carbon sources. The end result is pathogen suppression, big yield improvements compared to untreated soil and rapid adoption by organic growers. But what about the weeds?

In 2007 we took a look at ASD with 9 t of rice bran/acre under clear plastic mulch in a very weedy site, something that even bravest organic growers would discourage. Anaerobic conditions developed well as expected and we saw a lot fewer weeds compared to untreated plots. In fact during three weeding operations (pulling weeds from under plastic via small planting holes, which requires flexible wrists) we had 55%, 68% and 90% fewer broadleaf weeds in ASD plots and we spent a lot less time weeding them compared to untreated checks. Last year we used opaque mulch and after ASD saw >90% reduction in weed number in planting holes where competition with crop is most critical. The weeds that remained in ASD plots were field bindweed (not a surprise) and those with wind dispersed seed (annual sowthistle and common groundsel) suggesting that this technology was effective against annual broadleaf weeds germinating from soil seed bank.

Yellow nutsedge shoots from tubers excavated from untreated soil (left) and from soil after 3 weeks of anaerobic disinfestation with 9 t/acre of coffee grounds (right).
Figure 1. Yellow nutsedge shoots from tubers excavated from untreated soil (left) and from soil after 3 weeks of anaerobic disinfestation with 9 t/acre of coffee grounds (right).
This year we were ready to make a next step and test ASD on one of SoCal favorites – yellow nutsedge. We collected and buried nutsedge tubers in soil in permeable bags and for carbon sources we used grass clippings, coffee grounds and spent grain (brewery waste). The number of nutsedge shoots in all ASD plots was similar and 87% less than in untreated check (Figure 1).
Yellow nutsedge in untreated bed (left) and beds treated for 5 weeks with ASD with 9 t/acre of rice bran (right)
Figure 2. Yellow nutsedge in untreated bed (left) and beds treated for 5 weeks with ASD with 9 t/acre of rice bran (right)
In a large weedy strawberry field ASD with 9t/acre of rice bran for 5 weeks provided dramatic reduction in nutsedge germination (Figure 2). However, here in SoCal the tubers send new shoots every week of the year and we'll see how viable those post-ASD tubers are during the remaining months of the 9-month season. But so far we're cautiously optimistic and for this organic grower this is as good of a place as he can be at.