What signifies compacted soil? Is there a generally applicable definition?
Soil is always defined as compacted and deformed when it can no longer guarantee the water and air balance, susceptibility to root penetration for the plant and even ground-water formation. In short: when the cavities that are present in soil are no longer sufficient to guarantee all of soil’s functions as a nutrient and water reservoir and as a plant production site. However: There is no definition that is generally applicable to soil. There are soils that are much more susceptible to compaction than others, for example clay in comparison to sand. And within a soil type there are individual soil horizons with different levels of susceptibility to compaction.
What effects does soil compaction have on plant production?
Soils always have a three-phase system: the gaseous phase, liquid phase and solid phase. This is to say pores that are filled with air, pores that are filled with water and the solid components. When the soil is compacted, the pore system is always the first thing to change. That means the gas-permeability becomes lower and the water conductivity falls to the degree that the pores are compacted. Furthermore the storage capability for water available to plants drops.
If the water conductivity falls below a certain value, this also decreases the aeration, since then the pores, as well as being finer, retain more water for longer. If the air capacity is less than 8-10 %, this is a case of significant soil compaction. If the saturated water conductivity falls below values of 10 cm per day, the soil can likewise no longer fulfil its functions for feeding the plant, since one is the dealing with stagnant moisture.
When the soil is too heavily compacted, the roots can only extend along the surface, making it difficult for the plant to reach water in deeper soil layers.
In terms of physics, when the soil is too heavily compacted, the roots can only extend along the surface, making it difficult for the plant to reach water in deeper soil layers. In terms of chemistry, the more densely compacted the soil is, the lower the chance is for the nutrients, which the farmer applies by using fertiliser, to get to where they can be accumulated, stored and even reabsorbed by the roots. In addition this increases the risk that the nutrients are washed out faster with the groundwater running off sideways in the compacted soil, even when the terrain is only very slightly inclined, and thus ultimately being flushed into the rivers.
How can a farmer tell if their soil is compacted?
There are a few simple methods for identifying soil compaction. First they should look at the surface. This is usually silty and densely packed together in compacted soils. There are no fissures, or no fissures that are close together, and that also run perpendicular to each. Then observe the cracking on the surface: Areas that look like large honeycomb are a sign of compaction. In compacted soils, there are often horizontal slabs at ploughing depth, i.e. approx. 30 cm, called plough soles, the effect of which reaches several decimetres deep into the subsoil.
Furthermore the farmer should look at the colouration of the topsoil: If there is not enough oxygen in the soil, iron present in the soil minerals is reduced and as a result also mobilised in the soil. This can be seen from the bluish or blackish colouration. Finally the plants’ root pattern may give some indication, since compacted soils show no or only limited root patterns going deep and with an even distribution.
What does a farmer have to be aware of to keep compaction of their soil to a minimum?
The general rule is that dry soils can take greater strains that wet soils. The consequence of this is that I have different cultivation regimes that I need to manage depending on the precipitation. The wetter the soil, the more sensitive it is. The drier it is, the more stable it is. This is the case both over the course of a year and for different natural environments.
With regards to strain from pressure exerted by machinery as it drives over the soil, it must always be ensured that the weight of the machinery and the pressure transmitted through the contact surface of the tyres are kept below the soil’s inherent strength. If the strain is instead greater and it also has a repeated and/or longer effect, there may be significant compression, which is further strengthened by shearing deformation (= slippage). The general rule is that at a constant pressure as the contact surface increases, the soil is more deeply compacted than by lighter units of machinery with the same contact surface pressure. These effects are particularly pronounced in annual ploughing work, which is carried out at a soil depth of e.g. 30 cm.
While the whole soil structure is being aerated in the first 30 cm, the subsoil below the drive sole is compromised, because the tractor always drives with two tyres in the subsoil and the pressures propagate downwards from there. In addition the slippage has a shearing effect that compromises the soil through and this causes the conducting pores (similar to the effectiveness of a straw) to be destroyed. So conventional cultivation leads, over time, to shallow soil, where the topsoil may have been penetrated by plenty of roots, but the plants likewise have just this area available for nourishment.
Conventional cultivation leads to shallow soil, where the topsoil may have been penetrated by plenty of roots, but the plants likewise have just this area available for nourishment.
This effect and the consequences for root penetration, water, air and heat transfer can as a result be used as indicators for the soil function at the arable sites. They can also very simply explain the significant reductions in yield in 2018! If we reduce the tillage or do away with it altogether, that means that to begin with the soil is less open to root penetration in the first year after the changeover and the plants have to create the cavities themselves over time. We have to take a period of at least 5 – 7 years into consideration here until a new pore system, which the plant can also root through easier and deeper, has emerged as a result of repeated desiccation and subsequent swelling with rainwater.
However this requires at the same time that the weight of the machinery used must not increase compared to the units used previously. Over a long term perspective, the result is soil that is more open to root penetration and at the same time is packed together to a more stable and less dense degree. So a changeover of this kind allows the farmer in the long term to save not just time, but also energy (diesel etc.) when doing less. Biology helps him with this.
What can you do to aerate soil that is already compacted?
Firstly the general rule is that a generous supply of nutrients ensures better root penetration and the desiccation and shrinkage = cracking associated with it. Supplying burnt lime dries out the soil at the same time in this regard and strengthens cracking in the compacted areas or stabilises the previously loosened soil layers. The soil horizons and as a result the storage spaces for nutrients, water or even the infiltration for water and/or the gas exchange with the atmosphere thus get back into working order. They also become more stable over long periods (up to several years).
Then the farmer should prolong crop rotations with deep-rooted catch crops, e.g. alfalfa. The soil dries out significantly and the stability and accessibility of the nutrient supply in the subsoil, along with the groundwater stored there increases through more intensive cracks down to deeper layers (> 1 m in depth). At the same time the soil strength increases. Which measures are most effective for which soils must be decided based on the type of soil, natural environment and degree of compaction.
How long can it take until these measures achieve success?
When it comes to soil compaction, negative processes unfortunately proceed over the long term. There are records that clearly prove that nothing at all happens for the first 10-15 years. We need a really long time until a soil structure can be mobilised again. Earthworms can help with this, but then we need a stock of 200 – 300 earthworms per m².
What technical options for counteracting soil compaction does the agricultural engineering industry have in your view?
Firstly there is the possibility of reduced tyre pressure on fields, which has been a topic of discussion for years. But in my view this will only help us to a little degree. Tyres with lower pressure don’t become uniformly flatter and wider. Instead they distribute the mass of the machinery, and as a result the pressure, on the soil unevenly. This means there is no constant contact surface. Instead peak values occur whenever the tyres expands to the side – pressure differences of up to 300% can occur.
The second option is larger tyres. Of course, these also produce pressure on the soil based on the mass of the machinery. The following must be taken into account here: The larger the tyre is while the mass of the machinery is increasing at the same time, the deeper the pressure is propagated into the soil. So if you want to better protect the subsoil, field work has to be done with smaller and self-driving machine units – robots!
The third option is caterpillar drives. If we look at the pressure propagation under a caterpillar track, then the greatest pressure is always at the front and back at the deflection pulley. In the middle, where the small wheels are, the pressure is much lower. So we cannot bring the pressure to the soil evenly. And finally we have the slippage, whose effect on the soil structure is often neglected although its destructive effect plays a huge role. Slippage, meaning the sliding of tyres over the soil surface, causes shearing forces that act diagonally into the soil layers. This shearing deformation in part reaches below the plough sole and leads to the soil’s natural pore structure being destroyed.
So my conclusion is this: The industry has to move away from large, and also expensive, machinery towards smaller, more efficient units that are adapted to the soils. Because of the irreversible damage that the soil has already sustained, it is much more difficult today to improve it in its function for plant production or even as a filter and buffer for clean groundwater and drinking water, but we can at least make it so that the situation does not deteriorate further.
How prepared do you judge both politics and agricultural practice to be to respond to the soil compaction situation?
No farmer is destroying their soil deliberately, since it is the foundation for food production for future generations. And we have meanwhile seen that the awareness of the various interest groups has at least increased. That is good news!
No farmer is destroying their soil deliberately, since it is the foundation for food production for future generations.
Now it’s about going from “I’m aware of the problem” to “what do I have to change to improve the situation?”. In Germany there are currently discussions being held with the Federal Environment Agency whether to introduce a system with which we can classify soils, for example based on physical properties such as water conductivity, air balance, oxygen availability, etc. Even dividing the soils into various hazard categories can help to make the problem more tangible for individual farmers. And then agricultural machinery manufacturers would have to offer them options for buying and deploying machinery adapted to their location.
Finally, what are your thoughts on controlled traffic farming?
It works if it is carried out every year from 1st January to 31st December, always on the same lane, with all machinery and applications. I then abandon these areas as support space that nothing grows on any more, but between them I have such loose soil that the plant can grow reasonably well. That without doubt only works if I standardise all the machinery accordingly, which is not yet possible over the full season as things stand today. So this would be another challenge for the agricultural machinery industry: Build machines with standardized working widths!