Don’t take soil compaction lightly

Prof Rainer Horn qual­i­fied as a professor of soil science in 1981 and from 1998 to 2017 he held the chair for soil science at Kiel Univer­sity, Germany. His scien­tific inter­ests are soil physics and soil ecology with a partic­ular focus on phys­ical land degra­da­tion.

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.

Prof Dr Rainer Horn in the labo­ra­tory at the Chris­tian-Albrechts-Univer­sity, Kiel.

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!

Professor Horn, thank you for talking to us.