Soil Compaction I: An Introduction

Soil compaction occurs when the amount of open spaces between soil particles (referred to as porosity) is reduced, and soils become denser, physically hard, and more difficult to permeate.(1)

Soil compaction can be induced by a natural cause (as with the case of heavy impact of rain drops) or by anthropogenic causes including; excessive machinery use, intensive cropping, short crop rotations, intensive grazing and poor soil management.(2,3) A compacted soil is a disturbed soil; removed from its natural state. It will fall short of providing support or food to its surrounding ecosystem. Soil compaction is prevalent on agricultural lands and the chief cause of this is the excessive use of heavy farm machinery. The force from heavy machineries exerts pressure on soil aggregates (group of soil particles) pressing them together until they are disintegrated. The consequence of this is a dense soil with reduced porosity and permeability.(3,4)

Soil compaction exist on the soils surface layer (surface compaction) or deeper below the soil surface (subsurface compaction).

• Surface compaction: This level of compaction is easily dealt with. It can be removed by ploughing the surface layer experiencing compaction. It is caused by the use of equipments weighing less than 10 tons per axle. Soils with surface compaction become vulnerable to water erosion if not dealt with. Run-off water can carry soil nutrients and deposit them in neighbouring water bodies causing water pollution.

• Subsurface compaction: This type of compaction is more problematic than the surface compaction; it cannot be resolved by merely ploughing the compacted layer. It is caused by the use of machines or equipments weighing more than 10 tons per axle (such as manure tankers and combine harvesters).
The soil condition can also contribute to increasing the occurrence of soil compaction. Compaction occurs easily in wet clay soils, soils with less organic matter (as they are easily disintegrated), and sandy soils.(4)

Detecting soil compaction

Soil compaction can be detected by visual assessment, by using a cone penetromenter/hand probe and by measuring soil physical properties related to soil compactness (the extent of soil compaction).(1)

• Visual assessment: Compaction can be diagnosed by visually assessing a soil profile. With this method a soil pit is dug and the soil layers are observed for indications of compaction. A compacted layer will be dense or hard with little root growth. Another method of visual assessment is by looking out for evidence from crops growing in the suspected area. For example (a) roots on compacted soils tend to be horizontally spread because of their inability to penetrate the soil (b) nutrient deficiency symptoms will appear on crops in suspected area because of their inability to reach deep for nutrients (c) there will be low germination rate (e) and finally the most devastating effect is the decline in crop yield.

• Cone penetrometers and hand probes: A cone penetrometer is an instrument used by soil scientist and engineers, to measure the force required to penetrate a standard sized cone through a soil. Compaction of a soil layer is judged by how much resistance a penetrometer encounters through the soil profile. The hand probe works just like the cone penetrometer, except in this case a wooden or a steel rod is used to test the soils penetration resistance. The rod is pushed down the soil by hand and soil compaction is judged by how hard it is to push the probe down the soil profile.

• Soil physical properties related to soil compaction: Scientist measure the compactness of a soil by studying the changes in some soil physical properties which are affected by compaction.(1) The soil bulk density (defined as the dry weight of soil per unit volume) is the most common property measured with regards to soil compaction. Changes in the soil weight per unit volume give an indication of the level of compactness of soil particles. A soils bulk density is important because it relates to plant root growth. The less porous a soil, the more weight per unit volume and the more difficult it is for plant root penetration. Other physical properties related to soil compactness include: pore void ratio (volume of voids/volume of solids), total porosity (volume of voids/total volume), degree of saturation (volume of water/volume of voids).(1,5)

Table 1. Estimated bulk density at which root growth is impeded for different soil textures

References and further reading

1. Soane BD and Ouwerkerk CV (1995) Soil Compaction in Crop Production. Elsevier. Amsterdam. Pp 684.
2. Hamza MA and Anderson WK (2005) Soil compaction in cropping systems: A review of the nature, causes and possible solutions. Soil and Tillage Research 82: 121-145.
3. Horn et al (1995) Soil compaction processes and their effects on the structure of arable soils and the environment. Soil and Tillage Research 35:23-36.
4. Wolkowski R and Lowery B (2008) Soil compaction: causes, concerns and cures. Retrieved from: https://www.soils.wisc.edu/extension/pubs/A3367.pdf.
5. Rowell DL (1994) Soil science methods & applications. Longman Group, London. Pp 70.