A look at herbicides and soil biology

Initial research has found that some of the seven active ingredients in herbicides may affect soil biology and suppress beneficial organisms.

By Tegan Kukulies, Tony Pattison and Paul Dennis

The Australian banana industry has a growing focus on improving soil biology, in order to suppress diseases and maintain productivity. In the pursuit of improved soil health, there is a reoccurring question being asked by banana growers – do herbicides affect soil biology? Until recently, there has been very little research conducted on herbicide use in bananas and the impact on soil ecology and plant productivity.
However, preliminary research being conducted by the Queensland Department of Agriculture and Fisheries in partnership with the University of Queensland to determine the potential impacts of herbicides has found that some of the seven active ingredients registered for bananas can affect the growth of soil organisms.
A screening experiment using a Trichoderma viride, considered to be ‘beneficial’ fungal species capable of suppressing soil borne diseases, found reduced growth of the fungus when placed on growth media containing paraquat (e.g. Gramoxone®), diquat (e.g. Sprayseed®) or glufosinate (Basta®); nearly completely inhibiting the growth of the fungi.
This implies that vital soil functions such as nutrient cycling and disease suppression may be suppressed when herbicides are applied to the soil, even at spot spray rates, due to the suppression of the soil organisms that perform these roles.
No growth impact
The second screening experiment used bananas grown in pots and was conducted to determine if the seven herbicides affected the growth of plants over a six-week period.
When the herbicides were applied around the base of the plants at recommended rates there was no reduction in plant growth, leaf-emergence rates or leaf chlorophyll content.
There was a minor trend for plants treated with glufosinate to produce fewer leaves, but overall this experiment showed that a single application of herbicides at the recommended field rates do not affect the growth of banana plants.
Nematode increase
Soil nematode communities can be used as indicator species to determine shifts in the composition of feeding groups and were used as indicators when the soil was treated with the seven different herbicides.
The application of glufosinate increased the percentage of bacterial feeding nematodes in the soil community (from 39 per cent to 63 per cent), which implies that glufosinate has caused a disturbance in the soil ecosystem leading to a bacterial-dominated biological system.
DNA sequencing
A further experiment which is currently being conducted to measure a larger range of soil biological indicators when treated with the different herbicides will use the latest molecular technology to sequence soil DNA.
Initial results indicate that glufosinate, paraquat, and paraquat/diquat are producing small changes to the soil microbial populations.
Overall, the initial results indicate that herbicides registered in bananas, if applied at recommended rates, do not have any harmful effects on banana plant growth. However, long term impacts of repeated applications and indirect impacts to the potential disruption to the soil ecosystem are yet to be completely understood.