Technical Yearbook 2023

FIGURE 2. Reported 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). 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. 17 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. 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. 17 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. 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

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

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TECHNICAL YEARBOOK 2023