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Procedures for the determination of nutriment potentials of soil under special consideration of the soil body and their integration into the soil condition survey in the forest (BZE II)


Traditionally, the soil skeletal fraction has been considered as an inert diluent of the reactive fine earth. However, several recent studies show, that the soil skeleton can have a nutrient potentials equalling or even surpassing those of the fine earth. The aim of this study is to integrate the soil skeleton`s cation exchange capacity (CECsk) to the BZE II procedure. Two approaches are followed: (i): The development of an simple batch method for direct measurement of the CECsk in order to replace an existing but expensive percolation method. (ii): Testing of selected parameters as proxies for the skeleton`s cation exchange properties. Furthermore, the relevance of the CECsk for the soils of the BZE sampling grid as well as the effect of sieving soil samples and rejecting the fine earth coatings of the skeleton particles on total soil cation exchange capacity are to be investigated.

Skeleton samples were washed to remove adhering fine earth. NH4Cl was used as extracting agent. Cation concentrations were measured by ICP-AES. The CECsk batch method follows the CEC method for fine earth according to Trüby & Aldinger (1989). Core parameters of cation exchange (e.g. duration, molarity) were tested for gneiss and granite samples from the Black Forest. The method was evaluated with a percolation method, embedding the skeleton samples in chemically inert quartz sand for the simulation of natural suction power and fluxes. The validation was conducted using 22 representative samples from different parent materials and soil depths all over Germany. Because of the CECsk decrease with increasing particle size, CECsk of distinct particle size classes was measured. As approximation for the skeleton cation exchange properties, exchange properties of the fine earth and the vapour adsorption of dry skeleton particles at a defined water suction were tested by linear regression. Calculations were conducted using 170 samples from 70 sites within the BZE grid. For soil skeleton, only 2-6,3 mm size fraction was used.

A 24h-batch extraction with a solid:liquid ratio of 1:5 corresponds best with the percolation method. The non-linear regression equations from CEC values of skeleton diameter classes relative to CEC of the 2-6,3 mm fraction describe the decrease in weight-based CECsk with increasing particle size. The function follows the form CECsk relative = a * mean diameter ^ b + c. Knowing the CECsk level and the particle size distribution, total skeleton CEC can be calculated. All parameters tested as proxies for CECsk show a significant linear correlation with the respective cation exchange parameter of the skeleton. Because of the inclined value distribution, valid regression equations with cation exchange properties of the fine earth can only be given for CEC (r² = 0,31-0,38, depending on depth), K (y = 0,343 x + 0,61; r² = 0,42) and Al (r² = 0,46). Vapour adsorption is able to predict total CECsk only. Regression equations are y = 2 x (r² = 0,86) for 0-10 cm soil depth and y = 1,62 x (r² = 0,85) for 10-90 cm soil depth (notice special meaning of r² in regressions without intercept). The interaction of the two different types of parameters tested as given by the term (exchangeable cation potential * vapour adsorption)–0,5, shows the best accuracy of prediction. (CEC: r² = 0,71, Al: r² = 0,72, K: r² = 0,57). Sieving and homogenizing soil samples results in a significant overestimation of nutrient cations (especially Mg) by a factor up to 4. The amount of the fine earth coating of the skeleton proved to be negligible. The estimation of volume based nutrient potentials show that the skeleton of all examined parent materials can significantly contribute to the total soil nutrient potential. This is especially true for subsoils with a skeleton content > 20 %. The soil skeleton reveals possible contributions to the total soil nutrient potential and must not be neglected. The batch method developped for direct and simple determination of CECsk enables the integration of the CECsk into soil monitoring.

Regressions established allow for a basic estimation of CECsk from database values of the fine earth. Using the vapour adsorption method for the assessment of CEC is not advisable because the efforts nearly equal those of the direct (batch) method.

The study was financially supported by the Federal Ministry of Food, Agriculture and Consumer Protection of Germany.

Executive Institute:

Institute of Soil Science and Forest Nutrition Details of Institute of Soil Science and Forest Nutrition

Parent institution:

University of Freiburg Details of University of Freiburg (Baden-Württemberg)

Contract period:

01. 11. 2004 - 31. 05. 2006

Project budget:

117,443 €

Funding Programme:


  • Forestry
  • Soil science

Purpose of research:

Applied research

Funding Institutions:

Project Management Agency: