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.