Soil Compaction

Heavy machines and the subsoil

Over the last decades, farm machinery has grown enormously in weight. A study by soil physicists from Switzerland and Sweden traced the development of combine harvesters: a laden machine weighed around four tonnes in 1958 and around 36 tonnes in 2020, almost ten times as much. Over the same period the load on a single front wheel rose from about 1.5 to around 12.5 tonnes. Fully loaded sugar beet harvesters can weigh around 60 tonnes. The researchers compared the heaviest farm vehicles to the sauropods, the largest dinosaurs.

The physics behind the damage is well understood. The pressure at the surface depends on how the load is spread: wide tyres with low inflation pressure distribute it over a larger area, and modern machines are often equipped with exactly these tyres. But the stress that reaches deeper layers depends above all on the total load on each wheel. The heavier the wheel, the deeper the pressure travels. With today's loads, it reaches well below the depth that a plough loosens, into the subsoil.

That is where the problem begins. Topsoil compaction is unpleasant but repairable: roots, soil life, wetting and drying, and in cold regions frost loosen it again within a few years, and tillage can break it up. In the subsoil, these forces are weak. Long-term experiments in several countries, evaluated by an international working group, found that compaction at 40 centimetres and deeper could persist for decades, even where the ground freezes deeply every winter. In a Swedish field trial, damaged pore systems in the subsoil were still measurable more than a decade after compaction. Soil physicists therefore regard deep subsoil compaction as practically permanent.

The authors of the study mentioned above estimate that around a fifth of the world's arable land is at risk of harmful compaction, mostly in North and South America, Europe and Australia, where heavy machines are common. The FAO's world soil report of 2015 counts compaction among the main threats to soils worldwide.

A second, older form of damage lies a little higher: the plough pan. Where a field is ploughed to the same depth year after year, a dense layer forms just below it, pressed by the base of the plough and by tractor wheels running in the open furrow. Above it the soil is loose, below it is a barrier. Water stands on it after heavy rain, and roots spread out along it instead of passing through.

None of this is a reproach to farmers. Heavy machines spread because they save labour, because harvest windows are short, and because farms have to make ends meet with ever fewer people. Agricultural research and many farmers are working on solutions themselves. But the consequences of a heavy wheel on wet ground do not show up in the year's harvest. They show up decades later, often for someone else.

What helps

Prevention is far more effective than repair, because subsoil compaction is so hard to undo. The most important rule is to keep heavy loads off wet soil, since wet soil compacts much more easily than dry soil. Lower tyre pressure, lighter machines and fewer passes reduce the stress. In controlled traffic farming, all machines run on the same permanent lanes, so that the rest of the field is never driven on.

Once damage has been done, mechanical deep loosening is often tried. Its effect tends to be short-lived: the loosened soil is unstable and readily compacts again under the next load. More durable help comes from living roots. Deep-rooted plants with strong taproots, such as lucerne, chicory or forage radish, can push through dense layers. In an American study, forage radish penetrated compacted soil considerably better than cereal rye. When such roots die, they leave channels that later roots, water and earthworms use. Researchers call this biological drilling. Permanent ground cover protects the surface in the meantime, and soil life gradually rebuilds the crumb structure that pressure has destroyed.

How Terra Humana understands soil compaction

For Terra Humana, compaction shows how easily soil damage stays out of sight. A compacted subsoil looks no different from above. Only the water that runs off, the roots that stay shallow and the plants that suffer early in the dry season reveal it.

It also confirms why we work the way we do. On a slope, compaction and erosion reinforce each other: water that cannot soak in runs off and takes soil with it. That is why we rely on roots rather than machines to open the ground. Pioneer plants and deep-rooted plants do the loosening that no tool can do in the subsoil, and they do it in a way that lasts.