Dateline WA: No-till' s impact on soil microbes by Margaret Roper* Centre for Mediterranean Agricultural Research, CSIRO

Graingrower Clint Pitman gets close to the world of microorganisms living under stubble at 'Valema Farms' near Corrigin. Trials on the Pitmans' farm over four seasons indicate that no-tillage concentrates micro-organisms near the surface and stimulates cellulose decomposition.

SOILS ARE ' living' due to the presence of micro-organisms including bacteria, fungi, algae and protozoa. There are large (to billion bacteria/kg soil) and diverse (more than 40,000 known species) populations of microorganisms in soils.

top 10 cm of fertile soil may have more than 2 t/ha of micro-organisms

Soil micro-organisms playa significant role in the function of biological systems, including farming systems. Micro-organisms contribute to decomposition of organic matter, agrochemicals and pollutants; biological control (including pathogens and insects); nutrient cycling; and soil structure.

Organic matter in soil is the most important fraction that supports microbial populations. The microbial biomass (the weight of microbes present) constitutes 2-5 per cent of the organic carbon in soils. It is the interaction between micro-organisms and organic matter in the soil that largely determines the fertility of the soil. Therefore, as a principle, it is extremely important to use farm management practices that maintain organic matter levels in our soils.

There is evidence (some anecdotal) that notillage practices can increase organic matter in soils. How?

  • Stubble retention is a food source for micro-organisms and can increase the size and activity of microbial populations including nutrient transformations such as nitrogen fixation. Once digested by soil micro-organisms, stubbles can in tum contribute to the soil organic matter pool.
  • The lack of disturbance under no-tillage results in a concentration of microbial populations nearer to the soil surface. This has two advantages: it provides a plentiful supply of oxygen to micro-organisms to support their activity; and it allows for a rapid microbial response, e.g. to the breakdown of crop residues located at or near the surface.
  • Microbial populations increase in size partly due to improved soil structure supporting more efficient microbial interactions.

How about other impacts of no-till?

No-tillage has been associated with increased root disease, and reduced early vigour. But there is another twist to this. While root diseases can be managed using crop rotations, there is also evidence from long-term trials in SA that soil microbial populations adjust to the no-tillage regime by developing microbial groups that suppress root diseases, thus eventually reducing or even eliminating the incidence of disease after a number of years. (See story 'The war below ... ' p12.)

As well, beneficial colonising bacteria are being researched as inoculants for increasing plant growth, and early results are promising. (See story 'Bacteria in wheat roots combat lake-all' p13.)

WA trials (over four seasons) have indicated that no-tillage concentrates micro-organisms near the surface and stimulates cellulose decomposition, but these results have not translated into increased yields compared with cultivated systems.

And measurements of N mineralisation (by Art Diggle, WA Department of Agriculture, a collaborator in this project) supported the hypothesis that tillage stimulates mineralisation of nitrogen. However, the results indicate that only in the first year of no-till was there any significant reduction in N mineralisation compared with the cultivated treatment.

Experience elsewhere has shown that adaptation of microbial populations to changed tillage conditions can take up to five years. Longer-term monitoring may be necessary to assess the benefits or otherwise of no-till systems, particularly on fragile WA soils.

Program 4 Contact: Dr Margaret Roper 08 9333 6668 email

* Initially presented to the WANTFA (Western Australian No-Tillage Farmers Association) conference 2002.

Region North, South, West