Heavy Soil Loading and Its Consequence for Soil Structure, Strength and Deformation of Arable Soils – Can Risk Values for Soil Degradation Be Derived and Verified?.

The aim of this lecture is to clarify the effect of soil management and the consequences of physical degradation on the processes of soil aggregation and the changes in the accessibility and functionality of structured soils. In the last 5 decades the mechanical stress applied to arable or forest soils increased by a factor of approx. 6-10 with respect to the mass transported over the fields during field preparation or harvesting and reached values of greater than 60 Mg, by far beyond any accepted load on highways worldwide.  Site management strategies under various climatic conditions have to consider the maximal soil internal strength as the main prerequisite because mechanical soil strength quantifies all former processes which took place during the former soil development, soil structure formation due to swell shrink processes, biological activity and addition of organic carbon, but also due to farming or forest management concepts.

If due to soil management strategies the internal soil strength will be or was exceeded by external stresses applied not only the available pore space is reduced while the solid mass is increased but especially the pore continuity and the accessibility of exchange places for nutrients or heavy metals are altered. In addition, the root distribution in soils becomes not only reduced but also unevenly distributed which again hinders the possibility to evenly uptake plant available water and nutrient out of the pores. The applied stresses also create an anisotropic arrangement of the pores and their functions and also result in a more pronounced lateral mass transport by surface runoff including soil erosion by water and wind. Thus, also the water balance on all scales (from the pedon to the catchment) shows an increased water loss by evaporation and surface water while the groundwater recharge and the transpiration rates are reduced.

 If we analyze the scale dependency of nutrient adsorption and desorption phenomena it has to be stated, that the accessibility of exchange places within soil aggregates are the better the more structured and the more rigid these aggregates are, while with increasing mechanical stress applied aggregates are deformed and as a consequences they become less porous with a severe differentiation between the outer and the inner part of the aggregate volume. On the bulk soil scale, we can analyze the platy structure as a visible hint for anthropogenic soil degradation which affects amongst others also the ion exchange processes. The differences e.g. between the cation exchange capacity and the corresponding actual intensity values increase and can result in soils with platy structure in a reduction to less than 20 % of the theoretically available exchange places which are actually accessible. Thus, not only the efficiency of fertilizer application but also the corresponding filter and buffer functions are reduced and finally result in an increased ground water or surface water pollution risk. Furthermore an uneven aeration within such degraded soils results in a very pronounced anoxia on all scales (from the aggregate to the hectare scale) which can be quantified not only by corresponding measurements of the spatial distribution of redox potential values. In recent studies under “normal” wheeling conditions in arable fields and also in harvested forest sites it was proofed that Eh values close to 0 mV orf even a methane emission have to be expected (with consequences for the global warming). Also denitrification is considered as a main threat for the environment. Finally anoxic conditions also affect the redox induced mobilization of Fe and Mn which even results in a more rapid soil development with consequences also for yield and further soil functions. In the lecture,  these effects will be discussed as well as a soil protection guide line (based on the German soil protection law) will be presented in order to prevent further irreversible soil degradation phenomena.   

Keywords: mechanical strength, mechanical stress and strain effects soil degradation, pore functions, ad-/desorption processes,  redox potential, anoxia;, global change