- Soil compaction is caused by compression from machinery traffic or stock trampling.
- Poor root growth and swollen root tips can indicate a compacted layer (hardpan), usually between 10 and 40 cm.
- Removal of compacted layers is usually best achieved through deep ripping.
- Prevention of compaction through adoption of controlled traffic farming can also minimise yield penalties.
The majority of agricultural soils in Australia have developed subsoil physical constraints, in particular compaction (Greacen and Williams, 1983). An estimated 13 million hectares (70 %) of Western Australia’s agricultural soils have moderate to high susceptibility to subsurface compaction (DAFWA, 2006). Subsurface compaction is caused by compression from agricultural machinery traffic with the compacted layer forming between 10 and 40 centimetres. In contrast, compaction from stock trampling is confined to the surface 15 cm of soil. In addition to compaction, hard layers can form as a result of natural soil packing and chemical cementation processes and these may occur throughout the soil profile. These hard layers slow or in extreme cases prevent root growth and restrict root access to water and nutrients.
Detecting soil compaction
By examining the soil profile it is possible to identify a compacted soil layer because it is physically stronger (harder) and more dense than the soil above or below it. Compacted layers often have distinct massive or blocky appearance (figure 1) and are clearly defined horizontal layer that occurs between 10 and 40 cm.
Figure 1: (a) A dense compacted layer in yellow loamy sand at 15 – 30 cm (Photo by Noel Schocknecht, DAFWA); (b) A distinct compacted layer in a sandy loam, note fractures in hard pan through which roots preferentially grow.
In loam, sandy clay loam and clay soil, a hardpan will often fracture into large clods when dug up which can only be broken with some force. Plant indicators include; poor root growth, swollen root tips and horizontal root growth as roots try to force their way through compacted soils. Roots growing through compacted soils can be confined to macropores, soil fractures or be at a reduced density.
Hand probes are basically steel rods that are pushed into the soil by hand. Compacted layers are more difficult to push through, and easier once past the compacted zone. This needs to be done when the soil is wet to depth (preferably the upper drained limit) as many soils become hard when dry regardless of compaction status. Hand probes can be made from steel rod (about 8 – 10 mm diameter) or heavy gauge (3 mm) fencing wire about 40 cm long with one end looped to make a handle. Depth increments can be added.
A cone penetrometer (figure 2) works by the same principle as the hand probe except that it measures and records the force required to insert a standard sized cone into the soil profile. The penetrometer is inserted at a steady speed by hand and the instrument uses a gauge to measure the force required to penetrate the soil at a given depth, measured in mega- (MPa) or kilopascals (KPa). The data are stored in a data logger and can then be downloaded and the strength of the soil profile assessed.
Penetration resistance has been related to crop root growth in wet soils close to the drained upper limit. In general crop root growth starts to be restricted when the penetration resistance exceeds 1.5 MPa and is severely restricted at 2.5 MPa or more.
Figure 2: Cone penetrometer
In spring when the soil surface layers have been re-wet, the soil may still contain dry subsurface layers that resist a probe or penetrometer. Soils can be wet up before using a probe or penetrometer but it is essential to make sure the soil is wet deep enough to detect the compaction layer.
Management of subsoil compaction
Few Australian agricultural soils exhibit strong shrink-swell characteristics and soil insect activity tends to be low in our semi-arid agricultural environments. Consequently there is limited opportunity for physical improvement of compacted layers except through direct physical disruption using tillage.
Deep ripping involves breaking up the hard pan using strong tynes usually to a depth of 30 – 40 cm (figure 3). Removal of these compacted layers by ripping has been shown to increase root growth rate from 0.5 – 1.5 cm/day through the compaction pan of yellow sandy earths (Delroy and Bowden, 1986). This allows the roots to better utilise nitrogen and water as it moves down the soil profile (Delroy and Bowden, 1986). In high and medium rainfall areas this has proven very successful with average yield increases of 22 – 37% for wheat (Davies et al., 2006; Jarvis, 2000). However it should be noted that negative responses to deep ripping can occur when there is a dry finish to the season and the bigger deep ripped crops have used the available soil water faster, leaving little water for grain filling (Blackwell et al., 2005).
Other methods of removing the compaction, such as the use of deep tillage points and deep working at seeding, can also be successful and cost less as they don’t involve a separate operation. Subsurface clay in shallow duplex soils and gravel layers can resist a penetrometer or probe but these soils are less likely to respond to ripping.
Yield benefits resulting from ripping have been measured for various soil types ranging from sands to clay loams but in heavier textured soils benefits can sometimes be short lived (Hamza and Anderson, 2003). This can be due to recompaction by machinery or a loss of soil structure due to slaking and dispersion when soil is wet and benefits can be maintained through prevention of compaction and use of gypsum to stabilise the improved soil structure (Hamza and Anderson, 2003). Prevention of compaction by restricting traffic to tramlines has been shown to increase yields in WA by 10 – 15 % (Webb et al., 2004). Most of this is attributed to reduced compaction with some additional benefits from reduced crop damage.
Figure 3: Plot showing the difference in penetration resistance between a deep ripped and not ripped (compacted) deep yellow sand.
Further Reading and References
Blackwell PS, Ford R, Webb B (2005) Inter-row deep ripping. In Northern Agricultural Region Trial and Demo Reports 2005. Department of Agriculture, Western Australia.
DAFWA (2006) Map unit database accessed 25 May 2006. Department of Agriculture and Food, Western Australia.
Davies SL, Gazey C, Gilkes R, Evans D, Liaghati T (2006) What lies beneath – Understanding constraints to productivity below the soil surface – Proceedings Geraldton Crop Updates, Geraldton 2006. Department of Agriculture, Western
Davies S, Lacey A (2011) Subsurface compaction: A guide for WA farmers and consultants. Department of Agriculture and Food, Western Australia.
Delroy ND, Bowden JW (1986) Effect of deep ripping, the previous crop, and applied nitrogen on the growth and yield of a wheat crop. Australian Journal of Experimental Agriculture. 26: 469-479.
Greacen EL and Williams J (1983) Physical properties and water relations. In ‘SOILS an Australian view point.’ CSIRO Academic Press pp 499-530.
Hamza MA, Anderson WK (2003) Responses of soil properties and grain yields to deep ripping and gypsum application in a compacted loamy sand soil contrasted with a sandy clay loam soil in Western Australia. Australian Journal of Agricultural Research. 54: 273-282.
Jarvis R (2000) Deep tillage. In The Wheat Book: Principles and Practice. (Eds. WK Anderson and JR Garlinge) pp 185-187. Department of Agriculture, Western Australia Bulletin 4443.
Webb B, Blackwell P, Riethmuller G, Lemon J (2004) Tramline Farming Systems Technical Manual. Department of Agriculture, Western Australia Bulletin 4607.
Authors: Stephen Davies (Department of Agriculture and Food, Western Australia)
This soilquality.org.au fact-sheet has been funded by the Healthy Soils for Sustainable Farms programme, an initiative of the Australian Government’s Natural Heritage Trust in partnership with the GRDC, and the WA NRM regions of Avon Catchment Council and South Coast NRM, through National Action Plan for Salinity and Water Quality and National Landcare Programme investments of the WA and Australian Governments.
The Chief Executive Officer of the Department of Agriculture and Food, The State of Western Australia and The University of Western Australia accept no liability whatsoever by reason of negligence or otherwise arising from the use or release of this information or any part of it.