The
              Report

No. 76: Conservation tillage practices will aid farm profitability

RESEARCHER DETAILS
Name: Dr C.E.Pankhurst Organisation, CSIRO Land and Water Davies Laboratory, Private Mail Bag P.O. Aitkenvale,Queensland 4814, Phone:(07)4753 8500, Fax:(07)4753 8600
Email:Clive.Pankhurst@tvl.clw.csiro.au

Adopting conservation tillage practices will improve soil health throughout the south- eastern Australia wheat belt, according to research undertaken for the Rural Industries Research and Development Corporation (RIRDC).

The research at sites near Harden and Cowra in NSW compared effects of conventional cultivation and direct-drilling practices on various biological properties in soil. Investigations during autumn/winter and spring from 1995-97 were led by Dr Clive Pankhurst of CSIRO Land and Water, Adelaide. Other team members included Clive Kirkby, Bruce Hawke and Heather McDonald.

The team's focus was on the soil microbial biomass and how its size,activity and composition are linked to the soil organic matter status and the root disease status of the soil.

It showed that for Australian agriculture to retain profitability and sustainability, soil management requires a predictive understanding of soil processes and a way of monitoring them.

The study

The project 's aim was to increase understanding of the role of soil biota in soil processes and the maintenance of soil health.

Its objectives were to:

1. determine the effect of conventional cultivation and direct-drilling practices on selected soil biological properties, the composition of the soil microbial biomass and the development of soil suppression to root disease; and

2. define links between tillage-induced shifts in soil biological properties and the composition and activity of the microbial biomass, and

(a) soil chemical properties,

(b) organic matter levels in the soil,and

(c) the development of suppression to root disease.

The Harden site was at the CSIRO Tillage and Stubble Management Trial which had red earth with a sandy clay loam surface texture.The Cowra site was at the NSW Department of Land and Water Conservation Research Centre where there was soil of the red duplex type with a hard setting sandy loam surface texture.

The effect of a change of tillage practice on soil biological and chemical properties was investigated with three tillage practices studied:

• conventional cultivation (with residues burnt and the crop conventionally sown);

• direct drilling (no cultivation, residues retained); and

• single cultivation (incorporating crop residues, and with the crop conventionally sown).

Each tillage practice was superimposed on the other for the three-year period. Figures 1 and 2 show the layout of the two sites.

Fig 1.Harden

Figure 2.Cowra

The results

At both sites, direct-drilling practices caused a significant increase in the total carbon and nitrogen content of the soil.

Significant increases in soil pH,C.E.C, soil moisture, microbial biomass,and carbon and nitrogen mineralisation also occurred.There also was an increase in the ratio of fungi to bacteria in the microbial biomass.

In contrast,rates of cellulose decomposition,a surrogate measure for the rate of organic matter decomposition,and populations of cellulolytic bacteria and fungi,were higher in soil under conventional cultivation.

This was consistent with the lower carbon and nitrogen levels in the conventionally cultivated soils.

Data for grain production for the selected treat- ments at each site are shown in Figures 3 and 4.

At Harden,the plots were sown to lupin in 1995 and wheat in 1996.In 1997, peas were sown,but the crop was sprayed out during pod fill to control herbicide-resistant weeds.Lupin production was similar for the direct drilling and conventional cultivation treatments in 1995,but wheat production was higher in the latter plots in 1996 (Figure 1).

Figure 3. Grain production in the selected treatments at Harden in 1995 and 1996.

dFigure 4. Grain production in the selected treatments at Cowra in 1995,1996 and 1997.


 

At Cowra, the plots were sown to wheat in 1995, canola in 1996, and wheat in 1997.

The single cultivation plot proved more productive than the direct drilling/single cultivation and conventional cultivation/single cultivation plots.

But the conventional cultivation/direct drilling treatment yielded better than either the direct drilling or the single cultivation/direct drilling treatments,especially in 1995 and 1996 (Figure 2). An explanation for the poor production of the direct drilling and single cultivation/direct drilling plots at Cowra is greater density of weeds and possibly higher disease levels in these plots compared to the cultivated plot.

Application of conventional cultivation or single cultivation tillage practices onto the original direct drilling plots resulted in a significant decline in total carbon and nitrogen, microbial biomass and microbial activity.

There also was a measurable decline in the amount of moisture retained in the soil.

The microbial quotient (the ratio of microbial biomass carbon to total carbon)declined as did the ratio of fungal to bacteria in the microbial biomass).

This suggests that increased soil disturbance results in a shift in the composition of the microbial biomass from a fungal dominated biomass under direct drilling to a bacterial dominated biomass under conventional cultivation.

In contrast,direct drilling onto either the conventional cultivation or single cultivation- treated soil resulted in a significant increase in microbial biomass in both soils.

There also was a measurable increase in total carbon and nitrogen in the original conventional cultivation soil but not the original single cultivation soil.

These changes were accompanied by an increase in the microbial quotient and in the ratio of fungi to bacteria in the microbial biomass. They can be expected to have important effects on soil structure and the turnover of energy and nutrients in the soil.

Soils from each of the tillage treatments at Harden and Cowra were assayed in two experiments for evidence of suppression towards the cereal root pathogens, Take-all and Rhizoctonia .

Suppression towards Take-all was higher than that towards Rhizoctonia ,and the suppression was higher in the direct drilling soils than the conventional cultivation soils at both sites. This suggests reduced tillage in higher rainfall regions of Australia may suppress some cereal root pathogens.

The work also showed that suppression was linked to higher organic carbon levels in the soil,and the development of a microbial biomass dominated by fungi.

Summary

Significant differences in a range of chemical and biological properties were found between soils subjected to conventional tillage and direct-drilling practices at both locations.

Depending on the tillage practices at Cowra, there was either increased or decreased organic matter and other soil biological properties.

Evidence for root disease suppression was found at both Harden and Cowra and developed more strongly under direct-drill practices. The suppression was more effective against disease caused by Take-all than Rhizoctonia .

This study makes a significant contribution to sustainable agriculture by highlighting the potential benefits of adopting either minimum tillage or direct-drilling practices.

Major benefits are increased organic matter in the soil,more topsoil moisture, improved capacity of soil to retain nutrients, increased microbial biomass and microbial activity, and positive steps towards the development of a soil microflora that suppresses soilborne root disease.

Other RIRDC organic produce reports: