The science of soil compaction

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Compaction solutions by natural forces

Shrink and swell (especially by cracking clays), biological activity of roots (especially woody species), burrowing of soil animals (especially earthworms, ants and termites) and chemical stabilisation of soil by components organic matter can all contribute to improving soil condition and helping to alleviate compaction. Unfortunately such responses are not as rapid as mechanical loosening, but may be very cost effective in the long term, especially on some clay soils. Severe compaction of a cracking black clay in Queensland by 10t axle loads was ameliorated naturally in five years.

Evidence of controlled traffic farming allowing improvement of soil structure by natural processes has been shown for red-brown earth soils in South Australia. The avoidance of heavy wheelings enabled soil macropores (cracks and tunnels from soil animals and roots) to increase within six years, allowing better infiltration of water into the soil (Ellis et al. 1992).

Some grower experience with difficult subsoils seems to show that progressively deeper digging with points at seeding, encouragement of increased soil organic matter and use of lime or gypsum or both can provide a more cost effective improvement than deep cultivation or organic matter improvement and use of ameliorants alone.

Compaction solutions by deep cultivation

Mechanical reversal of subsoil compaction is provided by a range of deep tillage techniques including deep ripping (or subsoiling), deep ploughing, inversion (mouldboard) ploughing, spading, delving and slotting (Table 1). Davies and Lacey (2011) give more technical details of deep ripping, inversion ploughing and spading. Hamza and Penny (2002) provide more detail on the value of additional chemical stabilisation of gypsum responsive subsoils when deep ripping. The table below is a summary of some of the important features which many current methods offer and their capacity to decompact subsoils to depth.

Table 1 Summary of mechanical methods to remove compaction
Method Maximum depth Technical improvements Ease of mixing ameliorants Key value
Deep working seeding points About 0.2m (~9 inches) using narrow knife points N/A Some mixing possible with degree dependent on soil type and moisture content, working speed and tine spacings Partial decompaction without a separate operation; deep working points can be put on selected tines each year and rotated to reduce total draft impact in a given year
Deep ripping 1m or more with sufficient traction  tine breakout and winged points# (about 0.5m with largest current farm equipment) Wings on points or tine legs and shallow leading tines* or discs to to allow deeper effective working Comparatively poor with single tines at the same depth More commonly available, relatively lower operating costs for the same depth of working, deeper loosening
Inversion ploughing About 0.35m Relatively few for deeper loosening but share angle needs attention to maintain depth Relatively effective, can result in deeply buried layers with minimal mixing through disturbed profile Burial of topsoil, organic matter nutrients and weed seeds if skimmers are used
Rotary spading About 0.4m when combined with deep ripping Stronger and deeper working spades Very good Incorporation of clay, especially old ineffective claying; lime into acidic subsoils
Delving Perhaps 0.5-1m depending on design and draft Closer spaced narrow legs may reduce clod size Can be very good Cost effectiveness for depth loosened (not cultivation all the soil); capacity to lift subsoil clay into sandy topsoils and create sand seams into subsoil clay layers

#Forestry and mining rippers

*Hamza et al (2011)

Crop response to deep cultivation

General predictions to responses to deep ripping and the reliability of the response have been investigated by recent modelling. Poor response to deep ripping in the modelling is due to terminal drought from the larger biomass using too much water in drier seasons. Poor responses to deep ripping can also be caused by digging too deep below the critical depth where the soil does not break out and poor penetration of a hard layer, as well as the presence of another subsoil constraint.

Acknowledgement

Soil compaction research is supported by Department of Primary Industries and Regional Development and Grains Research and Development Corporation through DAW00243 Minimising the impact of soil compaction on crop yield. The input of Paul Blackwell is gratefully acknowledged.

Contact information

Jeremy Lemon
+61 (0)8 9892 8413