Back to Earth for Cotton Stubble

09 June, 2006

*The nutritional benefits of returning cotton stubble to the soil through mulching and tillage have been confirmed by CSIRO and Cotton CRC research. *

Dr Ian Rochester commented on the weekly CSD Web on Wednesday video that his team has studied the nutritional impact of stubble removal and stubble incorporation for several years.

A three-year study showed that yield was reduced by about 10 per cent when stubble was removed, which was equivalent to removing 60kg of nitrogen from the field.

“Basically we were depriving much of the microbial biomass in the soil of their energy source so they had nothing to live on; they just died off. So there were fewer organisms in the soil to recycle nutrients,” he said, noting that previous studies had also shown that burning stubble resulted in even more severe nutrient losses.

*“*You loose all of that carbon so it is not going back into the soil, but you also loose most of your nitrogen, you loose some of your phosphorus, there is a little bit of your potassium that goes up, and most of your sulphur is gone. So there are significant amounts of nutrients that go up in the smoke. This reduces soil fertility.

“We need to put every bit of stubble we can back into the soil, particularly in the western regions where we are starting off with very low levels of soil organic carbon, (higher levels on the more eastern side of the cotton belt). Generally, levels of organic matter are declining in our cultivated soils.

“That has enormous implications in terms of nutrient cycling, soil structure, soil health in general, and maintaining an active and dynamic soil microbial biomass is important.”

Dr Rochester said incorporating stubble back into the soil provides the best nutritional outcome. “That is the way to go. Even growing legumes or sacrificial cereal crops is a good way to go to get a little bit more carbon back onto the soil.”

Chris Anderson (NSW DPI/Cotton CRC) said on the CSD video that burning and mulching cotton stubble also had an important impact on diseases such as Fusarium.

“We set up a trial at the end of the 2003-04 season with some areas where stubble was root cut and some of it mulched, and then we had other strips that were raked into piles and burnt.

“The field was then put through a wheat crop through the winter, then it went summer fallow, winter fallow, then back into cotton.

“What we found was that where we had mulched, we got 19 per cent total survival of Fusarium (in a high incidence field), and where it had been raked and burnt that was increased to 27 per cent, which is statistically significant.

“We were also interested to look at whether if you mulch the field and then you leave it on the surface for as long as you possibly can before turning it in and doing your pupae busting, does that make a difference to disease in the following season.

“So in a back-to-back field in a different part of NSW, (again a severe Fusarium field), we found that where we had mulched it and then done the pupae busting straight away and incorporated that stubble, we had a survival of about 27 per cent, and where we had left it on the surface for a month before turning it in that was increased to 33 per cent.

“So again there are only small differences, but they were statistically significant, so anything that you can do to reduce that severity is going to help,” Chris Anderson said.

He also outlined the results of trials investigating the possible movement of Fusarium during irrigation, and whether or not this could be attributed to water, mud or trash movement.

“What we found was, (especially in a part of a field with a fairly high incidence of Fusarium) the trash coming off that part of the field definitely carried most of the pathogen on it.

“In a kilogram of trash coming off the highest severity parts of the field you would have up to 160 million colony forming units of Fusarium wilt fungus. When we looked at the amount that was just in water floating around it was only around 20,000 colony-units.

“When that was passaged back through a storage dam and then back out into the head ditch that was right next to the storage dam, we found that that was almost reduced to undetectable levels, probably due to the settling action of it, sitting there and all the spores and the trash rots and falls to the bottom of the dam.

*“Steve Allen from CSD and Joe Kochman and his team with QLD DPI & F *did a similar series of trials where they left the stubble on the surface for up to a month before they turned it in and they had some success with those getting differences of about 30% in total survival.

“So where you would have left the stubble on the surface your survival in your plants the next season was 30 per cent higher than where it had been turned in straight away. For some reason that seemed to correspond with a wet winter, so if you had a lot of rain over the winter, that survival difference was more exaggerated.

“But, if it was a dry winter and there wasn’t a chance for that stubble to break down, you didn’t often see much of a difference in leaving it on the surface for a month or turning it in straight away.

“The main thing to look out for is that the less stubble that you have in your field at planting time the less disease [Fusarium]. So if you can deal with that stubble at the end of the preceding season and get rid of it then you are going to be better off in the following season.

“If you have got no Fusarium wilt, there is no reason to get rid of all that trash at the end of the season. You may as well mulch it and turn it in because it is good for the soil, increases friability, organic carbon levels and things like that.

“David Nehl (NSW DPI/ Cotton CRC) and Stephen Allen (CSD / Cotton CRC) have both looked at the impact of stubble management on seedling disease as part of their ongoing annual disease surveys, and there doesn’t seem to be any correlation between the amount of trash in the field at planting time and seedling disease caused by rhyzoctonia and pythium, so you would have to say there is no relationship there at all.”

Further information: Dr Ian Rochester 02 6799 1520