Deep tillage research commenced on the Southern Tablelands in 1960 and 22 experiments have now been established on properties from Bookham in the west to north-east of Braidwood. The sites have covered a range of soil types, from free-draining friable red loams to poorly-draining duplex hard pan soils. Superphosphate histories have varied from very high to low; and perennial and annual pastures, and wheat, rape and field peas have been tested for responses to tillage, the depth of which has ranged from b0cm to 2cm. The implements tested have been the Wallace Soil Reconditioning Unit, tile Agrowplow, the Domino Sub Tiller and a single tyne ripper. During the period, seasonal conditions ranged from extremely wet to extremely dry.

Measurements have included:

• herbage, grain, and root production;
• soil pH, bulk density, strength, and chemical composition;
• herbage chemical composition.

Results from the programme are summarised in Table 1.

Table 1. Summary of Results. Effect of deep tillage on herbage and grain yield on the Southern Tablelands, NSW, 1980-1963

*Observational (3 observers) only; no yield measurement available.

It is obvious that responses have varied widely - from large increases to large decreases. The most common result, however, was a nil response. There were more positive responses in the wet year of 1963 than in the other, drier years.

In general, deep tillage:

• increased soil moisture
• reduced soil bulk density
• and strength had no effect on soil pH

On an intensively-sampled site in 1983, positive responses to deep tillage were associated with:

• increased soil available phosphorus, exchangeable cations, and percentage of calcium, phosphorus and magnesium content of herbage; and
• slightly increased pH and decreased aluminium.

These effects did not occur in an unresponsive situation on the same site.

Where positive responses to deep tillage do occur, they could result from any combination of the following reasons, plus others not listed.

• Runoff interception)
• Increased moisture penetration) Improved soil moisture status
• Easier root penetration
• Biologically-induced nutrient release from enhanced microbiological activity - nitrogen, phosphorus, etc.
• Accelerated soil mineral oxidation - manganese.
• Improved soil micro flora activity, including that of mycorrhiza.
• Improved soil micro and macro fauna population - earthworms, etc.

Factors affecting whether responses to deep tillage will occur include:

• Soil type: Soils with a restricting hard pan should be more responsive to deep tillage than friable loams without hard pans.
• Seasonal conditions: Where responses to deep tillage are strongly associated with improved soil moisture status, the benefit of this will be greater when it occurs in autumn when temperatures are still sufficiently high to promote good plant growth. Good late summer- autumn rainfall is therefore desirable to ensure, on responsive soils, good growth responses in the following winter-spring.
• Tillage timing: From the previous point, it is obvious that soils should be tilled so that they are receptive to late summer-early autumn rain. Tillage following summer storms is appropriate for annual pastures, but perennial pastures could easily be killed by such tillage. If summer tillage is considered undesirable, spring tillage, when soil moisture is still adequate, is appropriate, again bearing in mind the possibility of substantial damage, particularly to perennial pasture, should dry conditions ensue. If the effects of tillage are transitory, repeat tillage at appropriate intervals will be necessary to maintain response.
• Fertiliser history: Tillage cannot make available - to plants -nutrients unless they are already present in some form in the soil. In soils with low native reserves of minerals, therefore, greater responses to tillage can be expected in situations with significant fertiliser history.
• Crop or pasture type: As discussed earlier, perennial pastures are more vulnerable to root pruning damage following tillage than an inactive annual pasture. Taprooted plants such as rapeseed should be more responsive to deep tillage than fibrous rooted plants in soils where a subsurface compacted layer occurs.