Precompression Stress

The contents of this page have been introduced by Rainer Horn, Heiner Fleige. Corresponding author. Tel.: +49 4318802575; fax: +49 4318802940. E-mail address: h.fleige@soils.uni-kiel.de 
Christian-Albrechts-University zu Kiel, Institute of Plant Nutrition and Soil Science, Olshausenstrasse 40, D-24118 Kiel, Germany

The precompression stress as a factor to assess the mechanical stability of soils is predictable by using pedotransfer functions. Over the past decades arable soils have been deteriorated because of the heavy machinery on the farm. Increase loads enhance subsoil compaction which is mostly irreversible. As a consequence of this deformation, the soil productivity decreases while the erodibility increases thus affecting additional compartments of the surrounding ecosystems. If soil compaction phenomenons exist on field (traffic lane, plowpan layer, subsoil compaction) because of too intense soil loading, soil erosion plays a major role in soil degradation. In order to prevent those negative developments, agricultural soils should only be wheeled at suitable times. If heavy machines are used in farming, then soil compressibility at the given moment should be compared with internal soil strength at all corresponding soil layers.

The physically defined concept ‘‘precompression stress (Pc)’’ is presented at farm scale, including two operation methods in order to define precaution and critical values for the legislation and executive level according to the German Soil Protection Law. The first step is the prevention of subsoil compaction in general by the definition of the mechanical strength of soils, which is defined by the Pc. This Pc value is used as the precaution value, to ensure site-adjusted land use. The second step is to predict the change of soil functions after exceeding the Pc and furthermore to assess if critical values (test and action values) caused by subsoil compaction are reached or already exceeded. Criteria for the definition of critical values by subsoil compaction concerning crop production are discussed in order to also establish such values in the European Soil Framework Directive.

The ‘‘Pc’’ concept, which includes predicted and regionalized ‘‘Pc’’-maps, was verified on a research farm in the weichselian moraine landscape in Northern Germany for areas resistant or susceptible to soil deformation at the given water content throughout the year. Furthermore, the stress-dependent changes of the air capacity after exceeding the Pc was predicted by pedotransfer functions and linked with the farm soil map. As an additional proof for the validity of the Pc concept, a field experiment on a Stagnic Luvisol was also conducted in order to measure the stress distribution up to 60 cm depth using the Stress State Transducer (SST) system at two different wheel loads (3.3 and 6.5 Mg) using a tractor-pulled mono-wheeler. According to the effective soil strength, the wheel load should not exceed 3.3 Mg at field capacity to avoid subsoil compaction.

As Precompression Stress (Pc) is defined as the maximum major principal stress that a soil horizon (and in the sum the whole profile, respectively) can withstand against any applied external vertical stress, the quantification of this Pc value as the precaution value is the first step within this concept for the given site, in order to avoid subsoil compaction and, respectively, impacts on soil functions in general. Soil functions include besides yield functions for cultivated plants also e.g. habitat functions for natural plants, buffering, filtration or regulatory functions. regulatory functions. Not exceeding Pc is the best protection to avoid subsoil compaction and resulting (negative) consequences.

We use two different ways to prepare Pc-maps in order to predict the susceptibility of arable subsoils to compaction. Because the vulnerability of soils could change throughout the year depending on climatic conditions, two water suctions (pF 1.8 and 2.5) were considered

The Pc values for the main and accompanying soils were predicted by pedotransfer functions at two different water suctions [pF 1.8 and 2.5, reduction of the Pc at pF < 1.8 for hydromorphic soils (see Horn and Fleige, 2003)], which are extrapolated to the soil-type map. A soil strength map (Pc-map, Fig. Below) was constructed for a site-adjusted land use to avoid subsoil compaction in general. Every field is characterized by varying soils with different Pc/precaution values. At field capacity medium Pc values (Luvisols and Cambisols) dominate, associated with low (Anthrosols, Stagnosols and Gleysols) and very low ones (Histosols). At dryer field conditions medium (Stagnosols and Anthrosols) to high (Luvisols and Cambisols) are predicted, associated with low/very low Pc (Gleysols and Histosols).

Farm scale, Subsoil: 40 cm 
pF 1.8 and pF 2.5

Precompression stress

Modeling of Pc at pF 1.8 (left) and pF 2.5 (right) for subsoils (40 cm). 
Classification of Pc (kPa): very low < 30, low 30-60, medium 60-90, high 90-120

 

Precompression stress versus load

To characterize the mechanical stability of a soil, both in situ stress measurements and mechanical stability parameters determined in laboratory are needed. Soil stresses exceeding the stability limit result in further soil compaction. Field experiments have shown that at tire loads with 3.3 Mg a mean normal stress of about 50 kPa in 40 cm depth is achieved, whereby at 6.5 Mgstresses of more than 100 kPa (even in 60 cm) are possible. According to measurements, the recommendation of wheel load at conventional tillage is not to exceed 3.3 Mg (respectively 2.5 Mg at hydromorphic mineral soils like Stagnosols, Gleysols or weak Anthrosols) at field capacity to avoid subsoil compaction.

Precompression stress versus load 
Farm scale, Subsoil: 40 cm, pF 1.8

Precompression stress versus load

Classification of the effective soil strength is defined by the relationship of precompression stress to stress impact in 40cm depth: 
ratio of Pc to s 
1.5-1.2 stable, 1.2-0.8 labile, Less than 0.8 unstable, plastic deformation

Air Capacity

Air capacity is considered to estimate the applicability and reliability of the concept for the prediction of load-dependent changes in soil physical properties. The Figure below shows the potential effects at a defined stress of 90 kPa at field capacity on air capacity (macroporosity, pores > 50 µm) in comparison to the initial state in the subsoil. Because oxygen supply of hydromorphic soils is limited, a classification into a lower air capacity class (e.g. medium to low) is justified.

Critical value: Air Capacity 
Farm scale, Subsoil: 40 cm, pF 1.8 
Stress impact: 90 kPa 

Critical value: Air Capacity

Modeled changes of air capacity (considering the oxygen supply of hydromorphic soils) for subsoils (40 cm) at a stress impact of 90 kPa (right) in comparison to the initial state (left). 
Classification (Vol.-%): very low: Less than 2, low: 2-5, medium: 5- Less than 13, high: >13

Conclusions

  • There is a general consensus among soil scientists and many agronomists that the Pc value can be applied for the description of the soil mechanical strength. The Pc value is used as the precaution value to conserve the existing soil structure.
  • The machinery of a farm or a company has to be adjusted to the strength of the (sub)soil bearing capacity
  • It is suggested to choose machinery according to the lowest Pc in the subsoil, as far as the loaded area exceeds 10%
  • The wheel load at typical tire inflation pressures should not exceed 3.3 Mg at field capacity to avoid subsoil compaction

Precompression Maps

Relationship precompression stress to actual soil pressure at pF 1.8 for topsoils (0 - 30 cm) of Europe. Low topsoil load (tyre inflation pressure: 60 kPa, rear axle load: 17kN, high topsoil load: 200 kPa, rear axle load: 200 kN). Data are taken from the Soil Map of Europe (1:1.000.000) and the corresponding explanations.

Precompression stress versus load    Full Image

Classification of the effective soil strength by the relationship of precompression stress to soil pressure: 
>1.5 very stable, elastic deformation, 
1.5-1.2 stable, 
1.2-0.8 labil, 
< 0.8 unstable, additional plastic deformation. 


Calculated values of the precompression stress (kPa) at pF 1.8 and pF 2.5 for subsoils (30-60 cm) of Europe. Data are taken from the Soil Map of Europe (1:1.000.000) and the corresponding explanations.

Precompression stress versus load    Full Image

Classification of the precompression stress (kPa): 
< 30 very low , 
30-60 low ,
60-90 medium , 
90-120 high , 
120-150 very high . 


Change of air conductivity classes at pF 1.8 and 2.5 for topsoils (0 – 30 cm) of Europe, stressed with a high topsoil load of 200 kPa

Precompression stress versus load    Full Image

Classification of air conductivity: (10-4cm s-1): 
< 5.5 very low, 
5.5-12 low , 
12-25 medium , 
25-55 high , 
>55 very high

 

References

  • Horn, R. & Fleige, H. (2009): Risk assessment of subsoil compaction for arable soils in Northwest Germany at farm scale. Soil & Tillage Research, Volume 102, Issue 2, 201-208.
  • Horn, R., Fleige, H. (2003) A method for assessing the impact of load on mechanical stability and on physical properties of soils. Soil and Tillage Research, 73, 89-99.
  • Horn, R., Fleige, H. (2005) Editorial: Introduction to the special issue on experiences with the development and application of a new simulation model predicting the dynamics of agro-physical soil state for selection of management practices to prevent soil erosion (SIDASS project). Soil and Tillage Res. 82, 1-3.
  • Soil deformation as a threat to soil functioning - how intense can soil functions be influenced and what are the possibilities to define critical values, Eurosoil 2008, 25-29 August 2008, Vienna, Austria

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