Cotton Seed Distributors - Web on Wednesday

David Nehl - Research Scientist, NSW Agriculture and Cotton CRC, discusses the impact of drought, long fallows and mycorrhizas (VAM) in cotton soils.

David, many growers have had some enforced long fallow because of the drought, does that have any implications to this year’s crop now that some water is available?

Long fallows are a concern, cotton is very reliant on mycorrhizas fungi, you probably know them as VAM fungi. They are an important part of how cotton grows in normal situations. Usually we have plenty of VAM fungi in the soil, we have had a number of experiments over several years during the 1990s that showed an 18 or 17 month bare fallow is generally not a problem for VAM fungi. Plenty of VAM fungi survive through that period until they get another crop or cotton crop to grow on.

The concern is that with a very long bare fallow, that we are seeing in some areas with the drought some fields are out for two or even three years, possible these VAM fungi might die back and when the field comes back into cotton, the cotton is going to have some trouble growing. We have been looking at some fields at Bourke and we have been quite surprised, it is looking quite positive.

Some of the fields we looked at there have had 28 and 35 months bare fallow up to March this year – we planted some little test strips of Linseed and cotton, then we came back in four or five weeks time and assessed colonisation in those plants by VAM fungi, and they were actually quite well colonized.

In one field they had around the 30% of their roots colonised, in the other field it was even higher at around 40 to 50%. That’s not too bad – its fairly well in line with what we would normally see in a normal cotton crop. Those fields still have a little bit of time to go, one of them will actually have 41 months of bare fallow by the time the cotton is sown and we are quite hopeful that there will be sufficient levels of VAM fungi in the soil. You might be wondering – why is this so, how can they survive so long? We think it is because of the drought, because the soil is dry, everything shuts down.

All the microbes go into suspended animation and they are just sitting there waiting for rain. If there are areas where we have had wetting and drying cycles during the drought. If you have had rainfall events that wet the soil and the soil shrinks and cracks or swells with rainfall events, that may be an issue.

Certainly in those areas where we have two or three years of bare fields, no weeds to support the VAM fungi, no cotton, if its been dry we are quite confident that reasonable numbers of VAM fungi will have survived for adequate cotton growth.

If you are in one of the areas where there has been sporadic rain and you have had some of the wetting and drying, what would be the approach to take there if you feel that there has been some moisture in the soil?

One aspect we have been looking at is to just grow a sacrificial crop as a nurse crop, something like oats, barley or wheat just to build the numbers up. We are running some trials at Bourke in some of those fields that have had very long fallows. You need to be sure that you really are going to have a VAM problem before you go to that trouble, its something that is not easy to test for. The best way to do it is to grow some plants in the field, at this stage it’s a bit too cold to be growing cotton but certainly barley or winter crops would give you an idea of how that VAM colonization of roots is going to progress.

If you felt that you were in the situation where you would have the problem, what would be the remedy?

Growing that nurse crop is about all we could suggest. If you came back with cotton in an area where the VAM fungi were very depleted, that could lead to a delay in colonisation. I doubt you would have a complete elimination of the fungi in the soil, they are after all native Australian fungi, no-one introduced them to these soils.

They have adapted to thousands, millions of years of having a flood and drought cycle in our climate and if the soil is dry they are surviving. But, if they were at very low numbers, this could lead to a transient depression in cotton growth and if that was sufficiently severe it would lead to a delay in growth and possibly yield loss.

Sometimes we do see that sort of delay, especially where thee has been a deep cut in a field, something more than 40 cm, you will get a yield loss if the VAM fungi have been severely depleted. Usually though, by the following season, that difference is not visible.

They recover very quickly during that season and the following year is fine. A nurse crop is possibly the only way that you could build those fungi up, it is something that you can cheaply put in, hopefully get a bit of winter rain just to boost those numbers. As I said earlier, you really need to be sure that you might have a deficiency  or lack of VAM fungi in the soil before you would go along those lines.

Further Information:  Robert Eveleigh, John Marshall,  Craig McDonald or David Kelly