Dr Matthias Lebert (Kiel - DE)

Complete List of Publications

Horn, R., C. Johnson, H. Semmel, R. Schafer and M. Lebert (1992). STRESS MEASUREMENTS IN UNDISTURBED UNSATURATED SOILS WITH A STRESS STATE TRANSDUCER (SST) - THEORY AND 1ST RESULTS. Zeitschrift Fur Pflanzenernahrung Und Bodenkunde 155(4): 269-274.

A method to quantify the spatial stress distribution will be introduced and first results will be discussed. This method allows the detailed analysis of principal and shear stresses as well as the determination of the direction angle of principal stresses and the octahedral shear stress angle. The described Stress State Transducer (SST) is composed of six single strain gage sensors that enable the accurate and recording of stresses in six directions in a wide load range. Their data form the base for calculation of spatial stress distribution. Some first results show that in a luvisol derived from loess wheeling at a wheel load of 4.0 Mg induces high shear stresses in a depth of 30 cm. This probably causes plastic soil deformation.

Horn, R., T. Baumgartl, S. Kuhner, M. Lebert and R. Kayser (1991). THE EFFECT OF SOIL AGGREGATION ON STRESS-DISTRIBUTION IN STRUCTURED SOILS. Zeitschrift Fur Pflanzenernahrung Und Bodenkunde 154(1): 21-26.

The mechanical compressibility of arable soils can be described by preconsolidation load value and by the shear resistance parameters of the bulk soil and single aggregates. In order to quantify the effective stress equation must be also known the hydraulic properties of the soil in dependence of the intensity, kind, and number of loading events. The soil reacts as a rigid body at very fast wheeling speed inclusive a very pronounced stress attenuation in the top soil while stresses will be distributed in the soil threedimensionally to deeper depths at slower speed. These variations can be explained by the mechanical as well as by the hydraulic parameters of the bulk soil and single aggregates. Thus, the pore water pressure value of the bulk soil as a parameter of the effective stress equation further depends on the hydraulic properties of the inter- and intraaggregate pore system and continuity. As can be derived from the results the pore water pressure values are identical irrespective of the predessication for clayey polyhedres at high load while in coarse textured prisms the pore water pressure value depends on load and predryness. The consequences for soil strength under dynamic loading are shortly discussed.

Lebert, M. and R. Horn (1991). A METHOD TO PREDICT THE MECHANICAL STRENGTH OF AGRICULTURAL SOILS. Soil & Tillage Research 19(2-3): 275-286.

During a 3-year period the physical and mechanical properties of 37 typical, differently textured and structured agricultural soils in Bavaria were determined in order to predict their mechanical compressibility and trafficability. The soil physical properties (bulk density, pore size distribution, saturated hydraulic conductivity, air permeability and penetration resistance) and the soil mechanical properties (pre-consolidation load and the shear strength parameters angle of internal friction and cohesion), were determined on undisturbed, differently pre-dried soil samples (60 and 300 hPa water tension). In order to quantify the changes in soil physical properties affected by loading, all soil physical parameters were measured before and after loading by the confined compression test (load range 10-800 kPa). It was found that in homogeneous, non-structured soils, such as sands and silts with low clay content ( 15% w/w), stability increased with increasing degree of aggregation (coherent

Lebert, M. (2010). A method to assess the vulnerability of agricultural subsoils to compaction. Proceedings of the 19th World Congress of Soil Science: Soil solutions for a changing world, Brisbane, Australia, 1-6 August 2010. Symposium 3.1.1 Further development of soil evaluation methods: 5-8.

The method proposed consists of three steps to derive the vulnerability of subsoils to compaction. In the first step, the mechanical susceptibility to compaction is determined by the pre-compression stress with pedotransfer functions. In the second step, soil structural quality of subsoils is determined by the evaluation of soil physical properties as air capacity, saturated water conductivity and packing density. In the third step, vulnerability to compaction is derived by combining the mechanical susceptibility to compaction with the soil structural quality. The method is based on the evaluation of actually analysed soil physical data from 1300 representative agricultural subsoils in Germany, offered by the soil surveys of the federal states. The results are shown in maps of 1:1.000.000 scale. At the water content of 100% field capacity, about 60% of the German subsoils are high vulnerable to compaction, including marshlands, clayey river sediments, glacial loams, loessian soils, periglacial clays and loamy and clayey soils derived from weather beaten rocks and sediments. Pure and loamy sands have low vulnerability. At the water content of 80% field capacity, low, medium and high vulnerability are distributed about one third each over the arable area in Germany.

Rucknagel, J. Brandhuber, R. Hofmann, B. Lebert, M. Marschall, K. Paul, R. Stock, O. Christen, O. (2010). Variance of mechanical precompression stress in graphic estimations using the Casagrande method and derived mathematical models. Soil & Tillage Research. 106, 165-170.

Mechanical precompression stress is a yardstick for the strength and compressibility of soils. The default method for the estimation of precompression stress is the graphic method according to Casagrande. It involves a Subjective perception by the engineer who not only determines the point of the highest curvature visually, but decides also which points are to be used for generating the virgin compression line. In order to avoid such subjective approaches, mathematical models for the determination of precompression stress have been developed emanating from the Casagrande method. These models estimate the smallest radius of the curvature based on the minimum of the second numerical derivative. The paper has the aim to quantify the variance of subjectivity implied by the person executing the graphic method, the variance of different model approaches and the accuracy of the latter in handling the graphic values. Additionally we wanted to investigate the effect of different parameters on the ordinate of the diagram and the effect of the first load step on the precompression stress. To understand these relationships, stress/bulk density functions and stress/void ratio functions measured on 13 sites were analysed by five experienced but independent engineers and by use of three mathematical models. The mean errors of precompression stress estimations by the different testers were 0.01-0.12 and by the models 0.10-0.87 on a logarithmic scale. Expressed in kPa, increasing mean errors were observed with rising precompression stress, due to delogarithmization. For the graphical determination, they reached approx. 10-20 kPa at precompression stress levels of 60-150 kPa in typical subsoils: this means 15% on average. The handling of graphically obtained values by help of mathematical models disclosed considerable deviations between them. In the logarithmic variant, the mean absolute errors varied from 0.09 (9 kPa) to 0.40 (30 kPa) and the determination coefficients from 0.71 to 0.96. Another influence on the level of precompression stress has been observed when different variables were plotted on the ordinate of the graph. The graphically obtained values of precompression stress and those shown in the dry bulk density graph exceed the values calculated on the basis of the void ratio by the factor 1.2-1.5. Furthermore, it can be stated that in soil-compression tests with an initial load of 25 kPa higher precompression stress values were obtained than with lower initial loads (5 kPa), if the precompression values were low.

Lebert, M., J. Brunotte, C. Sommer and H. Boken (2006). Protecting soil structure against compaction - proposed solutions to safeguard agricultural soils. Journal of Plant Nutrition and Soil Science-Zeitschrift Fur Pflanzenernahrung Und Bodenkunde 169(5): 633-641.

To safeguard the ecological soil functions and the functions linked to human activities, measures against harmful changes to the soil are required, in line with the precautionary principle. The German Federal Soil Protection Act sets obligations for precaution in agricultural land use and, if harmful changes to the soil are foreseeable, measures for averting a danger. The results of a research project of the Federal Environmental Agency show that it is possible to describe an impairment of the soil structure, using methods of soil analysis. But this as a sole information would not quality for the identification of harmful changes to the soil in the context of the Soil Protection Act, which requires an assessment of the severity of disruption of soil functions and the respective subject of protection. This would make additional soil investigations on site mandatory. Approaches in agricultural engineering and soil physics have introduced procedures to preserve the soil structure, in accordance with the precautionary principle. But these procedures have different goals and different ranges of application and hence offer partial solutions to safeguard against soil compaction. The assessment model of "trafficability by measuring the rut depth" provides information about the compaction status of the soil under applied conditions for farming gear, without providing detailed information about affected soil layers. The soil-physical model of classifying soils into "risk classes for harmful soil compaction" focuses on the relationship between topsoil compaction and crop yields. The soil-physical models "precompression stress" and "loading ratio" provide information for the assessment of subsoil compaction and a prognosis of a possible impairment of the soil structure at the water content of field capacity. It is necessary to validate the individual models with additional regional data about soil structure before a final assessment of the prognoses is made.

Springob, G. and M. Lebert (1994). CRITICAL LIMITS OF SOIL TESTS FOR K AS DERIVED FROM RELATIONSHIPS BETWEEN TOPSOIL K AND TISSUE WATER K OF PLANTS. Agribiological Research-Zeitschrift Fur Agrarbiologie Agrikulturchemie Okologie 47(3-4): 303-311.

A statistical procedure is proposed to simultaneously determine minimum critical soil test values for K and critical plant K concentrations from relationship between topsoil K and the K concentration in the tissue water of winter wheat plants. Soil K was extracted from both, fieldmoist and airdried samples, using the conventional NH(4)acetate (exchangeable K), CAL and CaCl2 procedures. The closest soil-plant correlation (r(2)=0.602) was found for the exchangeable K of moist soils, the weakest for the K-CaCl2 of dried samples (r(2)=0.547) and KCAL was inbetween (r(2)=0.583). In all cases saturation curves were obtained approaching constant levels of 207 mmol K l(-1) tissue water (plant critical value) when asymptotic models were fitted to the data. The amount of soil K related to the onset of the plateau is defined as the soil K critical level. It is derived by fitting several asymptotic functions to the data among which the RICHARDS function (RICHARDS, 1959) performed best. Tentative critical limits for the routine NH4 extraction are 1.25 mg K g(-1) clay for moist and 1.12 mg K-1 clay for dried soils, respectively, and for the CAL procedure 0.85 mg K g(-1) clay for moist and 0.93 mg K g(-1) clay for dried samples, respectively. For K-CaCl2, moist and dried soil samples yielded similar critical values of about 80 mg K g(-1) soil. Critical values like these appear to be useful limits when reductions of soil K levels in previousely overfertilized soils are in progress.

back to profile