Site adjusted organic matter balance method for use in arable farming systems

Summary

Common humus balance methods give distinct inexact results and do not meet nowadays requirements in Germany. Outgoing from the method of KOERSCHENS et al. (2004) an improved, site adjusted, semi-quantitative method was developed for manual use in agricultural practice and consultation. In the validation and optimization process over 300 variants from 39 long-term field trials were used, which represent the most important site conditions of Central Europe (Germany). The optimization work was done in four steps.
Identification of site specific groups with homogenous humification levels. Distinct different humification characteristics were recognized for the organic matter of different German site conditions (soil, climate). Six site specific groups could be identified when comparing the results of the humus balancing with the field trial organic matter content changes of the soils.
Humification coefficients of the crop species. The effects of the crop species cultivation, and the climate and soil conditions on the humification process were combined and expressed in the crop species humification coefficients. Optimal values were obtained when the results of the humus balancing were in accordance with the organic matter content chance of the trials (objective function: 0 kg Corg ha-1 ≈ 0 % Corg content change). Equal assessments of the site specific groups were reached by systematic adjustments of the humification coefficients of the crop species until the objective function was observed.
Humification coefficients of the organic materials. Additional analyses of multiple long-term field trial results have shown, that the organic material coefficients of the common balance method were fixed at somewhat too high values. Over this, the humification values were negatively related to increasing supply of organic materials. Therefore, these coefficients were corrected according to the field trial results.
Classification system for the humus balance results. The nitrogen surface balance of the field trials was suitable for to evaluate the soil fertility and environmental tolerance of the humus balance results. Under a specific N surplus constraint (e.g. 50 kg N ha-1), arable cultivation systems without mineral nitrogen fertilization (e.g. organic farming) can tolerate much higher humus surplus values than systems with increasing nitrogen fertilization. For use in different arable farming systems, therefore, the common classification scheme (A to E system of VDLUFA) was corrected.
Through installation of humification coefficients in site-specific groups adjusted to the Corg content change in the soil, and adaptation of the humification coefficients of the organic materials, the optimization process resulted in a large improvement of the method accuracy (s2=0,034 to s2=0,011). For practical use, only a little information about site specific characteristics, the crop species in the crop rotation and the amounts of added organic materials are necessary in the calculations.