Australian scientists are only just coming to terms with what's going on under the ground or, more to the point, what's happening in that all important layer of soil in contact with plant roots - otherwise known as the rhizosphere.

It's been only a few years since scientists identified an interaction between bacteria and the roots and the soil structure as a reason for lack of vigour in direct drilled crops. Roots naturally tend to grow more slowly through compacted soil and the slower the growth, the greater the opportunity for microbes to gather around the root tips and slow the rate of root and shoot growth.

Minimum tillage is rapidly becoming the preferred farming system in cropping areas right across the country so it's important to understand more about the reactions going on in the rhizosphere and, if possible, get them working in our favour.

Reporting on GRDC-supported research over the past four years, CSIRO researcher Dr Michelle Watt says that it's now established that the seminal root - the first out of the seed - is the fastest growing but all roots play a role in shaping the environment around them. They exude sugars that feed the microbes in the rhizosphere and they send out chemical messages that influence the rate of development of particular microbes. There is variation between breeding lines in this level of interaction between roots and their environment. This is being exploited to develop more productive direct drilled crops.

It seems that the rhizosphere is anything but a passive, 'take it or leave it' zone. Dr Watt talks about signals initiated by the roots that can switch on particular types of bacteria and even produce a pathogenic response in some colonies. We're all familiar with the signals between the roots of legumes and the bacteria in the soil that lead to nodulation and the ability of those legumes to fix nitrogen. These are being used as a model to study the interaction between the roots of cereal crops and soil bacteria.

Dr Watt says that in the past four years well over 100 conventional wheat varieties and breeding lines have been screened for fast early root growth and selections are still being made. Those roots even have differing reactions to gravity - it's obvious when you think about it that gravity would have an impact on root growth, but she says the team is now working with some Japanese material that's sensitive to gravitational effects.

It's not simply a matter of giving a plant a good start. Growers who've survived four drought years and watched potentially good crops hay-off and fail to finish would like to know the varieties they're sowing have the ability to chase moisture down into the soil.

However Dr Watt says that while it's relatively easy to study roots in hydroponic systems or in sand in the glasshouse, it's a very different matter in the paddock. Only recently the CSIRO group found that the first out, fast growing seminal roots end up the deepest.

Dr Watt says researchers are still not sure how shoot characteristics affect the root system. Conventional wisdom has it that the time of flowering influences root growth and that early flowering varieties don't put their roots down as deeply as later flowering varieties but that, she suggests, is being revisited.

Acknowledging that minimum or no till systems are the way of the future, Dr Watt says that the good news is that there is significant variation in the way different cultivars and breeding lines handle the environment and that selections of more vigorous lines with faster, deeper roots is underway.

The Crop Doctor is GRDC Managing Director, Peter Reading