History gives up its secrets... slowly by Alex Nicol

History tells the tale. Twenty years of stubble burning and cultivation rob the soil of N and organic matter

Damien Heenan is the guardian of a rarity in modern agriculture, a truly long-term field trial.

The 5-hectare site he inherited at NSW Agriculture, Wagga Wagga, was established in 1979 to test the impact of a range of farming systems on soil organic matter and soil nitrogen.

The trial is doing that — see graph this page— but it's now yielding information on the impact of practices unthought-of 20 years ago, on everything from soil acidity, disease control and weed control, to the impact of various farming practices on the emission of greenhouse gases.

Organic matter central to soil health

Organic matter was chosen as the field of study because, in Dr Heenan's words, "It's the supply sink for the soil. It constitutes no more than 2.5 per cent of the soil volume in the top 10 cm, yet it determines soil structure, nutrient availability, biological activity, and until recent times was the principal source of nitrogen for plants.

"In this trial we're truly learning the lessons of history," he says. "It was so skilfully set up that we've been able to introduce new cropping techniques without losing the vital ability to test theories over time.

"It's very difficult to measure the impact of different rotations and cultivation practices on the organic matter in the soil. Differences between years are tiny and it was 10 years before any definite picture began to form. The fascinating thing is that the picture is still changing."

Effects of continuous cropping and cultivation

The trial compares 13 crop-rotation, stubble-management and cultivation treatments. The old practice of continuous cropping, stubble burning and conventional cultivation has had the most serious impact on the level of soil organic matter.

At the other end of the spectrum, no-till wheat crops in a subclover-wheat rotation have had least impact.

Early this decade liming treatments were added, and canola and lucerne introduced into the rotations. Pressed for recommendations following the work, Dr Heenan says that the trial was never designed to provide a quick-fix answer to anything.

"There's no 'Eureka!' on this job," he says. "We began 20 years ago to test a number of theories. These days there are new theories but the bank of basic information built up by this trial keeps providing answers.

"We even have the original soil samples stored away like some sort of wine library to provide a benchmark for tests that may be conducted 50 years from now, using techniques we haven't yet dreamed of."

Program 3.5.2 Contact: Dr Damian Heenan 02 6938 1999

Region North, South, West