Soil compaction – the silent killer of conservation agriculture?

In this article, the effect of compaction on water infiltration and redistribution, root growth and on aeration will be discussed.

It is republished from the proceedings of the Soilborne Plant Disease Symposium 2022, with permission from the author and the organising committee.

Introduction

The population of the world is growing at a fast pace. According to a prediction released by Le Mouël and Forslund (2017),²⁰ the earth’s population will be more than 9.7 billion in 2050. This population growth will be the highest in Asia and Africa.²⁰ This means that more food is needed, while the arable land available for food production will probably stay constant or will shrink.²⁰ Only 14% of South Africa is regarded as arable.¹² Currently, there is a big drive to conserve this limited soil resource. This can be done by changing cultivation practices to conservation tillage practices.⁴ Conservation practices for crop production are currently supported by many farmers to conserve both soil and water.

South Africa’s water resources are limited and must be conserved. Without water, farmers will not be able to produce food and fibre for the increasing population of the world. In dryland farming, crop production is completely reliant on rainfall.¹² This research clearly showed how climate change aggravated the situation through a change in rainfall patterns. The long-term average annual rainfall declined from 510 in 1994 to 430 mm in 2020.¹² It is therefore essential to conserve our soil water for crop production as far as possible. This can be done by reducing the evaporation of water from the soil using stubble mulching on the surface.²¹ Mulching also creates a more suitable environment for macro- and microfauna in the soil.

Conservation tillage is defined as practices that cause minimum disturbance of the soil to maintain at least 30% of plant residue on the soil surface.⁴ This includes no-till, strip-till, ridge and mulch-till systems. Cultivation practices are thus adapted to till the soil less, while still achieving high production levels. No deep soil cultivation is practised.⁴

Farmers that changed from conventional (clean) tillage to conservation tillage practices experienced a yield decline for several years before the yields slowly increased to previous levels.⁷ However, after several years the yields started to decline again. This is attributed to the effect of soil compaction.⁷ Due to the ever-increasing cost of production, farmers must produce more, work bigger farms, and have less time to perform all the necessary cultivation actions.²⁵ To achieve this, they must mechanise by using larger (heavier) and fewer tractors, larger combine harvesters and larger bulk grain trailers. This larger equipment is heavier and causes soil compaction.¹³ Soil compaction influences soil physical properties which also have a secondary effect on soil water movement and aeration.¹⁴

Soil compaction’s effect on soil physical properties

Soil compaction is defined as the dense packing of soil particles (higher bulk density) due to external forces, such as equipment travelling over the soil.^7,18 In normal uncompacted soil, 50% of its total soil volume consists of soil pores.^5,6,14 At higher bulk density values (higher compaction) the soil contains fewer soil pores. The size of soil pores is classified as follows: Macro pores are larger than 100 ɥm, mesopores between 30 – 100 ɥm, while micropores are smaller than 30 ɥm.¹⁴

Figure 1 illustrates how macro pores are destroyed by compaction. Before compaction, the macro pore system allows water and air to flow freely (Figure 1 left). After compaction, the macro pores are only found in the lower part of the profile, below a depth of 250 mm. It is also evident that the interconnectivity of the pores was destroyed. Research done by Berisso and other authors (2013)^5,6 found that the macro pores which are responsible for rapid water and air movement in the soil declined drastically due to a lower porosity.²⁷ This lower porosity was mainly found below the wheel tracks of both tractors and combine harvesters.^5,6,9 Lower porosity also means that water is mainly conducted in micropores, and this causes the infiltration of rain or irrigation water to be slower.²³ This slow movement of water can cause a temporary water table which leads to poor aeration and even anaerobic conditions.¹⁷

FIGURE 1. The change in macro porosity due to wheel compaction (adapted from Practical Traction Knowledge, 2022).²⁴

Keller and other authors (2019)¹⁷ demonstrated how subsoil compaction due to heavy equipment causes local flooding, because the rainfall cannot infiltrate fast enough (Figure 2). According to their research, harvesting of sugar beet was done by some sugar beet harvester that weighs more than 60 tons. Harvesting is normally done when the soils are wet and compaction of the subsoil occurred.¹⁷

FIGURE 2. Report flooding events from 1980 to 2015 indicated by a grey dot. The trend is indicated by the red line (adapted from Keller and other authors, 2019).¹⁷

A compacted layer can negatively influence the redistribution of water into the subsoil.¹⁷ Water moves slowly through compacted layers²³ and can move sideways on a slope in the soil profile and not reach the subsoil.

Temporary waterlogging on top of a compacted layer can also lead to poor aeration (O₂ deficiency) and CO₂ toxicity.^5,6,23 Root growth depth and rate were also negatively impacted by compaction.¹⁷ Research done by Szatanik-Kloc and other authors (2018)²⁹ showed how roots grow slower in compacted soils and roots are often stunted. Compaction also limited the depth to which roots grow, because of too high soil penetration resistances which the roots cannot overcome.^17,29 Root development is also seriously restricted by compaction. Research done by Andersen and other authors (2013 – 2018)^2,3,10,29 found that both the rate of root development and the depth of roots are lower under compacted soil.

Soil compaction due to mechanisation in conservation tillage practices

Because of a lower profit margin and higher labour costs, farmers have invested heavily in mechanisation. Farmers now use larger tractors to produce crops when compared to 20 years ago.¹⁷ The power output of tractors increased from an average of 20 kW in 1950 to 350 kW in 2003.²⁵ This means a stronger, but also heavier, tractor that travels over the field.¹³ The weight of tractors has doubled in the last 20 years¹⁷ and thus the axle load causes compaction in the soil.^13,27 The effect of deeper compaction with soil due to higher axle loads is illustrated in Figure 3. From Figure 3 it is evident that a higher axle load can cause soil compaction up to a depth of 60 cm (24 inches). Keller and other authors (2019)¹⁷ showed how the soil stress increased from 1940 up to the present as illustrated in Figure 4(a).

FIGURE 3. The effect of axle load and soil moisture on compaction depth.¹³

This increase in stress resulted in higher bulk density and therefore also in lower porosity.¹⁷ Not only has the weight of tractors increased, but also the weight of combine harvesters as shown in Figure 4(b).^15,17 The weight on a fully laden combine can be up to 30 tons on the front axle. The biggest contributor to soil compaction is caused by the bulk grain trailer into which the combine unloads its grain.²⁸ The bulk grain trailer is used to transfer grain from the combine harvester to trucks and is a one-axle trailer which can weigh up to 60 tons. This causes considerable compaction to a depth of 60 cm.²⁸

The number of passes also influences the depth of compaction.⁹ More passes by heavy axle loads result in a build-up of compaction in the subsoil, at a depth of 40 – 60 cm.^9,18,23 A fully laden combine weighing in at 40 – 50 tons and a bulk grain trailer can compact moist soil up to a depth of 60 cm.²⁸ These trailers are very heavy and are pulled by 250 – 300 kW tractors which can also weigh up to 35 tons.²⁴ The tractor and bulk grain trailer sometimes travel over the same tracks as that of the combine, but normally create their tracks and over time cause compaction of the whole field.¹¹

Compaction problems due to multi-wheel passes also apply to the planting action in no-till and conservation tillage practices.⁷ The planting action in no-till practices requires a strong and heavy tractor (250 – 350 kW), as well as heavy planting units to penetrate the soil. The grain/fertiliser trailer which is pulled behind the planter can weigh up to 40 tons. This causes severe compaction in the subsoil.^5,6,7,17

FIGURE 4. Change in weight of (a) front wheel loads of combine harvesters as quoted by Keller and other authors (2019)¹⁷ from data of Schjønning and other authors (2015) and (b) rear wheel loads of tractors (data source: Deere & Company, Moine, IL, USA).

Research with maize under irrigation in Iran showed significantly higher bulk densities in the conservation tillage practices compared the conventional tillage practices.¹ If this compaction is not removed or eluviated by deep tillage, the compaction will remain for several years.²⁴ This will lead to yield losses which will cost farmers millions of rand in the long term. Research results cited by Practical Traction Knowledge (2022),²⁴ showed losses amounting to millions of dollars in North Dakota and Minnesota (Figure 5). Compaction over years by wheel traffic of heavy implements slowly progressed deeper and deeper, restricting water movement and aeration.¹⁷

FIGURE 5. Change in wheat yields between 1961 – 2016 in Scandinavia. Conservation tillage started in the late 1980s. Note the declining trend since 1990.¹⁷

The increase in yields in Figure 5 from 1961 – 1990 was due to better-performing new cultivars and improved use of fertilisers. During the latter part of the ’80s, conservation practices were included with an increase in yield. Since 1990 the yields slowly declined with time. This decline is mainly due to the build-up of compaction in the subsoil which influenced root growth and water movement.^2,3,7,10,29

Root growth rate and rooting depth are also restricted by subsoil compaction.^8,29 Research by Andersen and other authors (2013)² showed that the rooting depth of wheat was severally restricted by subsoil compaction to only 20 cm depth. This compaction also has a negative effect on yield. The roots cannot develop deep enough to take up water from the deeper soil profile and die off early in the season, therefore limiting the yield.¹⁷ Plants in compacted areas have lower emergence rates than plants in uncompacted soils. This also contributes to lower yields. Research by Berisso and other authors (2013b)⁶ also showed how compaction causes more nitrogen to be lost into the atmosphere and the uptake of potassium to be lowered.

Soil compaction’s influence on soil macro- and microflora

Conservation tillage practices enhance the number of soil macrofauna in the soil.²⁶ Compaction in the soil causes a reduction in the number and occurrence of macro pores,⁵ the habitable pore spaces for macrofauna are reduced, and therefore also the numbers of macrofauna like earthworms,¹⁶ springtails and arthropods. The numbers of Mesaphorura macrochaeta and Protaphorura armata were impacted by an increase in bulk densities. Their numbers decreased when the bulk density (compaction) increased from 1.02 to 1.56 g cm^-3.¹⁹ Soil compaction lowers the species richness of surface-active arthropods, while it does not have a big influence on the numbers present in the soil.²²

In a study done on the black soil of Northeast China, it was found that the abundance and richness of bacteria and arbuscular mycorrhizal fungi were greater in conservation tillage than in conventional tillage practices.³⁰ The microfauna and flora are more proliferated in the micro pores than in the macro pores. The effect of compaction on microfauna in the subsoil will therefore be small. Bouwman and Arts (2000)⁸ found in their research that the total numbers of nematodes were not affected by compaction. They have found that their distribution under soil compaction over the various feeding types has shifted increasingly towards a population with more herbivores and fewer bacterivores.⁸

Conclusion

The main benefit of conservation tillage of reduced soil disturbance and the build-up of organic material with all its benefits, cannot be denied. This does not apply to all soil types and under all climatic conditions. Due to an increase in conservation tillage equipment size and weight, like 350 kW tractors and combine harvesters that can weigh up to 60 tons, subsoil compaction tends to increase and build up over time. A reduction in soil pore spaces due to compaction will also influence water infiltration, water redistribution to the deeper profile, aeration, root growth rate and rooting depth. A temporary water table can also develop on a compacted soil layer which will cause anaerobic conditions and roots to die. This will reduce yields and the profitability of farming.

Soil compaction also negatively impacts macro soil fauna. Earthworms and springtail numbers decline due to soil compaction. It seems from the literature that micro-fauna and -flora are not that sensitive to soil compaction.

If this subsoil compaction is not alleviated annually, or at least every two years, the farmers will experience yield losses of up to 20% in the first year after compaction. Only in soils that freeze annually, can the compaction be alleviated naturally through the freezing and thawing action.

References

1. Afzalinia, S., & Zabihi, J. (2014). Soil compaction variation during corn growing season under conservation tillage. Soil and Tillage Research, 137, 1–6. https://doi.org/10.1016/j.still.2013.11.003.
2. Andersen, M. N., Munkholm, L. J., & Nielsen, A. L. (2013). Soil compaction limits root development, radiation-use efficiency and yield of three winter wheat (Triticum aestivum L.) cultivars. Acta Agriculturae Scandinavica Section B: Soil and Plant Science, 63(5), 409–419. https://doi.org/10.1080/09064710.2013.789125.
3. Banerjee, S., Walder, F., Büchi, Lucie, Meyer, M., Held, A. Y., Gattinger, A., Keller, T., Charles, R., & van der Heijden, M. G. A. (2019). Agricultural intensification reduces microbial network complexity and the abundance of keystone taxa in roots. The ISME Journal, 13, 1722–1736. https://doi.org/10.1038/s41396-019-0383-2.
4. Bergtold, J., & Sailus, M. (2020). Conservation tillage systems in the Southeast: Production, profitability and stewardship (J. Bergtold & M. Sailus, Eds.). Sustainable Agriculture Research and Education program. www.sare.org.
5. Berisso, F. E., Schjønning, P., Lamandé, M., Weisskopf, P., Stettler, M., & Keller, T. (2013a). Effects of the stress field induced by a running tyre on the soil pore system. Soil and Tillage Research, 131, 36–46. https://doi.org/10.1016/j.still.2013.03.005.
6. Berisso, F. E., Schjønning, P., Lamandé, M., Weisskopf, P., Stettler, M., & Keller, T. (2013b). Effects of the stress field induced by a running tyre on the soil pore system. Soil and Tillage Research, 131, 36–46. https://doi.org/10.1016/J.STILL.2013.03.005.
7. Botta, G. F., Tolon-Becerra, A., Lastra-Bravo, X., & Tourn, M. (2010). Tillage and traffic effects (planters and tractors) on soil compaction and soybean (Glycine max L.) yields in Argentinean pampas. Soil and Tillage Research, 110(1), 167–174. https://doi.org/10.1016/j.still.2010.07.001.
8. Bouwman, L. A., & Arts, W. B. M. (2000). Effects of soil compaction on the relationships between nematodes, grass production and soil physical properties. In Applied Soil Ecology (Vol. 14).
9. Bukovská, P., Burg, P., Masán, V., & Cížková, A. (2021). Simulation of Soil Compaction by a Tractor Passing. IOP Conference Series: Earth and Environmental Science, 906(1). https://doi.org/10.1088/1755-1315/906/1/012105.
10. Chen, G., & Weil, R. R. (2010). Penetration of cover crop roots through compacted soils. Plant and Soil, 331(1), 31–43. https://doi.org/10.1007/S11104-009-0223-7.
11. Damanauskas, V., & Jablonskytė-Raščė, D. (2021). New tillage system with additional renovation of soil properties in tramlines. Applied Sciences (Switzerland), 11(6). https://doi.org/10.3390/app11062795.
12. Dove, M., Lang, Y., Zermoglio, F., Johnston, J., & Sato, M. (2021). Climate risk profile: South Africa. World bank publications. www.worldbank.org.
13. Gürsoy, S. (2021). Soil Compaction Due to Increased Machinery Intensity in Agricultural Production: Its Main Causes, Effects and Management. In Technology in Agriculture. IntechOpen. https://doi.org/10.5772/intechopen.98564.
14. Hillel, D. (2004). Environmental soil physics (D. Hillel, Ed.). Elsevier.
15. Holthusen, D., Brandt, A. A., Reichert, J. M., Horn, R., Fleige, H., & Zink, A. (2018). Soil functions and in situ stress distribution in subtropical soils as affected by land use, vehicle type, tire inflation pressure and plant residue removal. Soil and Tillage Research, 184, 78–92. https://doi.org/10.1016/j.still.2018.07.009.
16. Jégou, D., Brunotte, J., & Rogasik, H. (2002). Impact of soil compaction on earthworm burrow systems using X-ray computed tomography: Preliminary study. European Journal of Soil Biology, 38(3), 329–336. https://doi.org/10.1016/S1164-5563(02)01148-2.
17. Keller, T., Sandin, M., Colombi, T., Horn, R., & Or, D. (2019). Historical increases in agricultural machinery weights enhanced soil stress levels and adversely affected soil functioning. Soil and Tillage Research, 194. https://doi.org/10.1016/j.still.2019.104293.
18. Kumari, R., Kumari, P., Sharma, B., Vigyan, K., Swami, K. (, & Rajasthan, K. (2020). Agricultural soil compaction under the tractor and its management. In Modern Technology of Agriculture, Forestry, Biotechnology and Food Science. Mahima Research Foundation and Social Welfare.
19. Larsen, T., Schjønning, P., & Axelsen, J. (2004). The impact of soil compaction on euedaphic Collembola. Applied Soil Ecology, 26(3), 273–281. https://doi.org/10.1016/J.APSOIL.2003.12.006.
20. Le Mouël, C., & Forslund, A. (2017). How can we feed the world in 2050? A review of the responses from global scenario studies. European Review of Agricultural Economics, 44(4), 541–591. https://doi.org/10.1093/erae/jbx006.
21. Liao, Y., Cao, H. X., Liu, X., Li, H. T., Hu, Q. Y., & Xue, W. K. (2021). By increasing infiltration and reducing evaporation, mulching can improve the soil water environment and apple yield of orchards in semiarid areas. Agricultural Water Management, 253. https://doi.org/10.1016/j.agwat.2021.106936.
22. Magoba, R. N., Samways, M. J., & Simaika, J. P. (2015). Soil compaction and surface-active arthropods in historic, agricultural, alien, and recovering vegetation. Journal of Insect Conservation, 19(3), 501–508. https://doi.org/10.1007/s10841-015-9771-8.
23. Nawaz, M. F., Bourrié, G., & Trolard, F. (2013). Soil compaction impact and modelling. A review. Agronomy for Sustainable Development, 33(2), 291–309. https://doi.org/10.1007/s13593-011-0071-8.
24. Practical Traction Knowledge. (2022, September 14). Recent study estimates that soil compaction is costing farmers millions. https://www.ntstiresupply.com/ptk-shared/recent-study-estimates-that-compaction-is-costing-farmers-millions[14.
25. Reicosky, D. C., & Allmaras, R. R. (2003). Advances in Tillage Research in North American Cropping Systems. In Journal of Crop Production (Vol. 8, Issues 1–2, pp. 75–125). https://doi.org/10.1300/J144v08n01_05.
26. Sithole, N. J., Magwaza, L. S., Mafongoya, P. L., & Thibaud, G. R. (2018). Long-term impact of no-till conservation agriculture on abundance and order diversity of soil macrofauna in continuous maize monocropping system. Acta Agriculturae Scandinavica Section B: Soil and Plant Science, 68(3), 220–229. https://doi.org/10.1080/09064710.2017.1381276.
27. Sommer, C., & Zach, M. (1992). Managing traffic-induced soil compaction by using conservation tillage (Vol. 24).
28. Svoboda, M., Brennensthul, M., & Pospíšil, J. (2016). Evaluation of changes in soil compaction due to the passage of combine harvester. Acta Universitatis Agriculturae et Silviculturae Mendelianae Brunensis, 64(3), 877–882. https://doi.org/10.11118/actaun201664030877.
29. Szatanik-Kloc, A., Horn, R., Lipiec, J., Siczek, A., & Szerement, J. (2018). Soil compaction-induced changes of physicochemical properties of cereal roots. Soil and Tillage Research, 175, 226–233. https://doi.org/10.1016/j.still.2017.08.016.
30. Zhang, S., Li, Q., Lü, Y., Sun, X., Jia, S., Zhang, X., & Liang, W. (2015). Conservation tillage positively influences the microflora and microfauna in the black soil of Northeast China. Soil and Tillage Research, 149, 46–52. https://doi.org/10.1016/j.still.2015.01.001.

For more information, contact Eduard Hoffman at ehoffman@sun.ac.za.

Click here to get your copy of WineLand Magazine and here to subscribe to our newsletter.