Technical Yearbook 2023

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: Macropores are larger than 100 ɥ m, mesopores between 30 - 100 ɥ m, while micropores are smaller than 30 ɥ m. 14 Figure 1 illustrates how macropores are destroyed by compaction. Before compaction, the macropore system allows water and air to flow freely (figure 1 left). After compaction, the macropores 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 macropores 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 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 A compacted layer can negatively influence the redistribution of water into the subsoil. 17 Water moves slowly through compacted layers 23 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 (O 2 deficiency) and CO 2 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 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 years 17 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). 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 is also seriously restricted by compaction. Research done by

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

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TECHNICAL YEARBOOK 2023