Assessment of cotton soils essential

A leading soil scientist has recommended cotton growers carefully assess their fields for soil structural damage after the 2010-11 crop in preparation for the large planting in 2011.

Speaking on CSD’s Web on Wednesday broadcast, Dr Pat Hulme from Sustainable Soils Management said many fields across the cotton growing regions had sustained a lot of damage due to tillage and wheel traffic in the wetter than normal spring period.

It was done when the soil was wet so it’s caused some compaction, however, because of the controlled traffic farming system we’ve got, the compaction is mainly restricted to the area underneath the wheel track while the structure beneath the plant lines is pretty good,? he said.

Dr Hulme said soil pits are a good way of assessing a whole soil profile however digging a smaller trench with a spade was also useful and the Cotton CRC publication SOILpak was an excellent reference on how to do this.

You need to review in your mind the functions of each part of the field, or soil profile. The top of the hill in the plant line is your seed bed and you want it to be relatively fine the peds to be smaller than the seed so you get good seed-soil contact.

You want the root zone to be a good medium for root growth to allow water, air and roots to move in and out to store the nutrients.

You want your wheel track areas to act as a roadway to support the machinery so you want that to be compacted, but ideally though you’d like the soil though to have enough holes in it that the roots could still explore the zone beneath the road way. You want your soil surface to be able to take in water particularly the non-trafficked furrows.

If the shoulders of the hills are compacted then the decision as to the type of tillage needs to bear in mind a number of factors.

The ideal moisture content for tilling the soil is when its slightly dryer than the plastic limit where you can roll the soil into a 3 mm diameter rod in your hand. If the soil is really dry, the products of tillage will be large and you’ll be left with the challenge of making them into a seed bed.

It’s obviously also going to depend a lot on how long until you’re going to be planting your next crop and the third thing is to decide whether you can just live with the compacted nature of the soil by nursing the next crop and fixing the problem up next year.

Dr Hulme said nursing the crop involves more frequent irrigation than in soils with better structure, and higher fertiliser rates.

You’re just walking a tightrope there’s less room for error in a compacted soil, he said.

According to Dr Hulme, soil tests or a nutrient budget are important, particularly for growers planting cotton on fields that had crops in last summer.

I believe it is important to have a history of soil testing and to use a nutrient budget to work out, firstly, whether the soil nutrient levels are adequate and secondly, whether you’re supplying sufficient nutrients to replace the major nutrients that have been exported by the crops.

With repeated measurements you can see the increase or the decline you can see if you’re winning or losing, he said.

For farmers who had fields flooded during the 2010-11 summer, Dr Hulme said they should not expect major changes in their soil chemistry apart from some denitrification the loss of nitrogen to the atmosphere under waterlogged conditions.

The silt that’s deposited during the flooding process is likely to have nutrients and it’s likely you could be improving the soil a little bit – thats the way a majority of the soil that we farm in the cotton industry was built was by deposition by flooding.

The material that’s being deposited to form the soil that we farm has a high clay content because it’s been deposited in the back plains away from the streams. Because of the high clay content, that soil shrinks and swells which allows it to rebuild its structure that is damaged by the trafficking, however what we’re seeing is that the structure of the soil benieth the plant line is generally in good condition.

The goal for soil management at this stage would be to have a look, work out how much or how little tillage needs to be done, get in and do it early while there’s still some months to get your seed bed happening at the top of the bed.

The SOILpak manual is available from the NSW Department of Primary Industries website http://www.dpi.nsw.gov.au/agriculture/resources/soils/guides/soilpak/cotton

16 June 2011

Watch the full interview.
http://www.csd.net.au/wow/show/1529

Further Information:
Dr Pat Hulme, Sustainable Soils Management, Warren. Ph 0268 473367 pat@soilman.com.au