Soil compaction is a complicated problem and, despite many decades of research, there are still a lot of unknowns. De-compaction by deep ripping is a proven and profitable solution on many soil types so it is being widely used, but re-compaction in the years after ripping and knowing if, when and how to deep rip again are emerging issues.

How long ripping lasts

Controlled-traffic farming (CTF) maximises the longevity of de-compaction by deep ripping. Research under CTF at Balla (in the northern wheatbelt of Western Australia) shows deep ripping persisting for 10 years in pale yellow sand with less than 15 per cent clay.

Without CTF, ripping is unlikely to last more than three years, with up to 80 per cent of the re-compaction happening in the first machinery pass. In un-matched CTF systems, the rip does not last much longer as 50 to 60 per cent of the paddock is wheeled each season.

Ripping benefits on deeper duplex soils (45 to 50 centimetres of topsoil) can be highly variable. In higher-rainfall zones or zones susceptible to waterlogging, ripping might only be beneficial for about 12 months as water pools on top of the heavier subsoil layer and collapses the pores made by the rip.

Ripping is not generally recommended on shallow (less than 25cm sand over clay) duplex soils or duplexes with hostile subsoils.

Re-compaction is a risk on all soil types but it is a bigger risk:

• If the soil is waterlogged;
• If driving on the soil when it is wet;
• On soils with a more even distribution of sand, silt and clay particle sizes (for example, loam) making it easier for the particles to pack tightly together. Soils with gravel can better withstand re-compaction;
• Where organic matter is low;
• On soils with poor structure (weak aggregates). Soils with better structure are more able to resist compaction; and
• With regular ripping. Ripping busts soil structure so if organic matter and clay are not present to help rebuild stable aggregates, the soil is more likely to settle into a hardened mass.

Even if traffic is perfectly managed, some soils are naturally prone to hard setting. These soils have weak aggregates (peds) that swell and collapse (slake) when wet. When the soil dries it becomes a hardened mass with little pore space. Hard setting is more likely in loamy sands to sandy loams with low organic matter (Unkovich et al. 2022).

Anecdotally, Western Australian farmers are re-ripping about:

• every second year on poorer (white) sands (less than 5 per cent clay);
• every third year on medium (yellow) sands (5 to 12 per cent clay);
• every fourth year on coloured loams (12 to 15 per cent clay); or
• every four to six years to counteract natural soil hardening from wetting and drying cycles.

Tapering yield responses from the last rip often drive the decision to re-rip. For example, research on a deep white sand at Kojaneerup found the yield benefit from ripping was substantial in the first year, limited in the second year, and gone by the third year.

But soil re-compaction might not be the culprit in all cases.

Is it re-compaction?

There could be other causes behind the decline in ripping response. If the yield benefit is declining but soil penetration is not increasing, something else is driving the decline.

Declining soil nutrient stocks is one suspect. Ripping increases the yield potential and therefore the crop’s demand for nutrients. This demand might be initially fed by nutrient reserves below the previously compacted layer.

As roots use up these nutrient stores, the higher yield potential can only be met by applying more fertiliser. Researchers around Geraldton have noticed yield response tapering off after a few years even though the soil has not recompacted.

The compacted layer still present between ripping tines. Source: Bindi Isbister, DPIRD

Deep soil sampling before and after ripping is useful to gauge whether or not the decline in ripping response is a result of low potassium or other nutritional issues.

Leaching rains events (more than 25 millimetres) on sandplain soils will also contribute to nutrition issues, as water infiltration increases after ripping.

Other suspects include root diseases or a bad season.

Check before you re-rip

If considering re-ripping, it pays to check first that soil compaction is the culprit.

The best way to check for compaction is to dig a hole and look at the soil. You can often see the compacted layer, which has a distinct upper boundary and a blocky appearance.

A penetrometer is the best tool to measure soil compaction. Use the penetrometer when the soil is wet (not saturated) because dry soil can appear compacted, and hard-setting soil will be very hard when dry. Readings above 1.6MPa indicate compacted soil.

A simple version is a metal rod pushed into the ground. If you can feel a hardpan, check in a few places how deep it is to work out a ripping plan.

How a soil aggregate breaks when squashed is another clue. If it fractures where pressure is applied, it is likely compacted. If it crumbles into smaller aggregates along random paths it is less likely to be compacted.

In gravel soil, pieces of gravel leave an imprint in the soil after they are removed.

Crop root growth can provide another clue. Soil compaction could be the issue if:

• growth is good and suddenly stops;
• roots have swollen, stubby tips; or
• crops have deformed and bent roots, especially in canola.

However, root issues could also be from chemical problems.

The “Soil Quality: 6 Soil Compaction” ebook has more detail on identifying soil compaction.

Consider economics

At the very least, yield increases need to cover the cost of re-ripping to make it worthwhile. The WA Department of Primary Industries and Regional Development and GRDC have developed an excel-based tool to help growers explore pricing scenarios with different input costs and yield responses to determine if there is a benefit to deep ripping.

Conclusion

Soil will eventually re-compact after ripping. In some situations this is only a couple of years, in others in can take more than 10 years. Checking the soil needs re-ripping first saves time and money as a declining yield benefit could be from other causes such as poor crop nutrition.

References

Bryce A and Pluske W, 2021. Assessing the profitability of soil amelioration. GroundCover Issue 55, Nov-Dec 2021.

Parker W, Isbister B, Hoyle F & Leopold M. “Soil Quality: 6 Soil Compaction.” SoilsWest, Perth, 2021. Apple Books.

Unkovich M, McKenzie D and Parker W. 2022. New insights into high soil strength and crop plants; implications for grain crop production in the Australian environment. Plant Soil.