Soil compaction and water repellency on high rainfall beef farms

Figure 1. Soil compaction in sandy soils (20 sites on 11 farms).

Surveys of soil constraints on high rainfall beef farms in 2019 suggest that soil compaction may be limiting pasture growth on some South West farms.

Almost half of the 39 sites sampled across 21 farms had a compaction level that is likely to restrict root growth at a depth of 100 millimetres.

Results suggest that sandy soils are more prone to compaction compared to clayey soils, especially at depth. Fifty percent of sandy sites showed some form of compaction at 100mm and 55 percent at 200mm, with the majority of these rated as severely compacted (35 and 40 percent respectively).

Compaction in clayey soils tended to occur at more shallow depths, with compaction typically declining below a depth of 100mm. The highest incidence of severe compaction in clayey soils occurred from 50 to 100 millimetres (21% of sites). Compaction at 50mm was more prevalent in clayey soils (37%) compared to sandy soils (15%).

Figure 2. Soil compaction in clayey soils (19 sites on 10 farms).

Surveys were conducted using a CP200 cone penetrometer (pictured below) which records resistance to penetration at 25mm intervals down the soil profile. Increasing resistance can reduce the percentage of plant roots that penetrate the soil, with a “critical value” of 2,000 kPa (300 psi) thought to slow or stop root growth. If 50% of paddock insertions exceeded 2,000 kPa, compaction was considered moderate, while 75% exceedance was considered severe (source: PennState Extension).

However, penetrometers do have some limitations such as not capturing pores formed by earthworms, dung beetles and root channelling. Anecdotal observations suggest that earthworms were more common in clayey soils and often absent in sandy soils.

Also, penetrometers can under-estimate compaction if soils are waterlogged, or over-estimate if soil is dry or gravelly. To control the influence of these factors, surveys were undertaken during August when soil moisture and conditions for root penetration were close to ideal. Any waterlogged sites or soils with more than five percent gravel were excluded from results. An additional check was made to see if there was any effect from low levels of gravel or dry spells during August by surveying along adjacent fencelines where possible as a comparison. Only one of the 21 fencelines sampled had severe compaction above 200mm and was also excluded from results. Minimal compaction along the other 20 fencelines suggests the impact of dry soil and gravel on results was minimal.

The main impact of compaction is limiting root growth, but germination can also be affected. However, another factor influencing germination and plant establishment is water repellency, which can reduce water infiltration early in the growing season.

Water repellency was assessed on 22 farms using a molarity of ethanol droplet test and categorised using a published rating system. As expected, sandy soils tended to be severely water repellent, while clayey soils tended to have low water repellency.

Given the potential constraints presented by compaction and water repellence, combined with the issue of soil acidity in high rainfall pastures and the recommendation to incorporate lime into soil, there is growing interest in soil amelioration techniques such as soil aeration, ripping and ploughing. However, while the effect of various techniques has been intensively researched in drier cropping regions, benefits and recommended techniques are less well known in the high rainfall zone where farmers are either understandably hesitant to cause soil disturbance, lack machinery or need research support.

Table 1. Water repellency ratings from 40 sites on 22 farms, with most common rating for each soil type highlighted.

For more on the science of soil compaction from the Department of Primary Industries and Regional Development, click on this link.

The project is supported by South West Catchments Council and Western Beef Association Inc., through funding from the Australian Government’s National Landcare Program.

Copyright © 2023 South West Catchments Council

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