Assessing the productivity function of soils. A review

The development and survival or disappearance of civilizations has been based on the performance of soils to provide food, fibre, and further essential goods for humans. Amongst soil functions, the capacity to produce plant biomass (productivity function) remains essential. This function is closely associated with the main global issues of the 21st century like food security, demands of energy and water, carbon balance and climate change. A standardised methodology for assessing the productivity function of the global soil resource consistently over different spatial scales will be demanded by a growing international community of land users and stakeholders for achieving high soil productivity in the context of sustainable multifunctional use of soils. We analysed available methods for assessing the soil productivity function. The aim was to find potentials, deficiencies and gaps in knowledge of current approaches towards a global reference framework. Our main findings were (i) that the soil moisture and thermal regime, which are climate-influenced, are the main constraints to the soil productivity potential on a global scale, and (ii) that most taxonomic soil classification systems including the World Reference Basis for Soil Resources provide little information on soil functionality in particular the productivity function. We found (iii) a multitude of approaches developed at the national and local scale in the last century for assessing mainly specific aspects of potential soil and land productivity. Their soil data inputs differ, evaluation ratings are not transferable and thus not applicable in international and global studies. At an international level or global scale, methods like agro-ecological zoning or ecosystem and crop modelling provide assessments of land productivity but contain little soil information. Those methods are not intended for field scale application to detect main soil constraints and thereby to derive soil management and conservation recommendations in situ. We found also, that (iv) soil structure is a crucial criterion of agricultural soil quality and methods of visual soil assessment like the Peerlkamp scheme, the French method "Le profil cultural" and the New Zealand Visual Soil Assessment are powerful tools for recognising dynamic agricultural soil quality and controlling soil management processes at field scale. We concluded that these approaches have potential to be integrated into an internationally applicable assessment framework of the soil's productivity function, working from field scale to the global level. This framework needs to serve as a reference base for ranking soil productivity potentials on a global scale and as an operational tool for controlling further soil degradation and desertification. Methods like the multi-indicator-based Muencheberg Soil Quality Rating meet most criteria of such a framework. This method has potential to act as a global overall assessment method of the soil productivity function for cropping land and pastoral grassland but needs further evolution by testing and amending its indicator thresholds.