Coming off the back of drought, crop managers may have concerns that prolonged periods of fallow will cause a rise in the incidence of long fallow disorder.

Long fallow disorder is characterised as emerging crops growing poorly following periods of fallowing paddocks. In the 1970 and 80s, the disorder was associated with low densities of arbuscular mycorrhizal (AM) fungi. The theory was that propagules of the AM fungi decreased over time during long fallows and this caused the disorder. However a body of cotton industry research over 30 years tells a different story.

Research undertaken by Professor Peter McGee in the 1990s showed that AM persisted in soil columns that were not subjected to wetting and disturbance. Field trials undertaken over seven years in late 90s onward by many of the cotton industry’s pre-eminent cotton researchers — Stephen Allen, David Nehl, Chris Anderson and Peter Lonergan — showed that AM persisted under drought over this period of time. The conclusion of all this work was that a lack of AM colonisation is a symptom of long fallow disorder and not a cause.

Long fallow disorder is likely due to all organisms that make up the soil biology competing for organic matter and nutrients after extended dry periods coupled with the fact that these microbes are better at consuming organic matter than plants. Long fallow disorder appears to be associated with a reduced colonisation of the roots by mycorrhizal fungi, but the exact cause remains elusive.

Figure 1: A cotton root that has been cleared and stained. The dark oval structures in the root are the vesicles of AM fungi that have colonised the cotton. Colonisation of cotton, like this, remained; similar across seven years of sampling of a field that received no rain during the millennial drought.

“Some of the biology dies. This dead biology does not decay until the soil becomes wet again, at which point it decomposes rapidly, releasing nutrients that feed rapid growth in the surviving soil biology. The resultant feasting of the soil biology on this nutrient flush is accompanied by rapid growth and a reduction in available nutrients”, Oliver said.CottonInfo soil health technical lead, Dr Oliver Knox of the UNE School of Environmental Science says that when soils dry out, some of the biology can enter a survival stage. Some bacteria and fungi produce drought-resistant spores, while nematodes and other invertebrates undergo anhydrobiosis and ‘sleep’ until it becomes wet again.

“Germinating seed with a pathetic root system in this environment will have slow growth (Figure 2).

“The good news though is that the feasting can’t last forever.

“Over time the soil biology will return to a more balanced system in which crops have a better chance of getting the nutrients they need and, hopefully, that slow start due to long fallow disorder becomes a distant memory.

“This is also important as these crops provide new sources of food and organic matter that the system needs.”

In periods of wetting followed by drying, with no plant input to the system, the flush of biology does not get the new crop inputs it needs to survive. As a result, the soil biology reduces with each successive wet/dry cycle, as the organic matter pool is used up. In these instances, AM spores may well germinate, but die having not found a plant host, reducing the propagule load.

Figure 2. Schematic view of (A) the soil biology under a normal cropping cycle, where the biology might decrease and recover between crops. (B) Under drought, where the prolonged lack of water in the system leads to a further decline in the soil biology, whilst AM survive. Upon water returning to the system, the biology responds rapidly on the available flush of carbon and nutrients, to the detriment of the crop, but the system recovers.
During a drought with wetting and drying cycles (C), the biology is stimulated with each wetting event, living off the organic matter in the soil, but with each cycle this organic matter declines. With no new organic matter/plant inputs the overall biology declines, which includes the AM. When growing conditions return the system takes longer to return to normal and we see more pronounced long fallow disorder and AM colonisation issues as a symptom.

What can growers do?

AM propagules survive well in soil, as long as there are no wetting and drying cycles.

“During this drought we barely saw a storm, so even cultivation is unlikely to affect your AM numbers,” Oliver said.

“Weeds or cover crops could help your AM fungi, but they will also feed your other soil biology.

“Overcoming long fallow is likely to involve helping all members of the soil biology.

“Keeping fields clean in a drought is best management practice when it comes to weeds, but a cover crop planted on enough moisture to emerge and establish may assist with erosion, water infiltration and maintaining or improving your soil biology.

“On the back of this drought you may experience some signs of long fallow disorder, but the AM fungi, just like you, are a victim and not the cause, the real culprit is the incredible soil biology responding to favourable conditions.

“Only time or changes to field management will lessen long fallows effects.”

For more information, contact our CottonInfo soil health technical lead Oliver Knox