Soil quality and fertility in sustainable agriculture, with a contribution to the biological classification of agricultural soils

Soils and crops are nowadays particularly vulnerable to climate change and environmental stresses. In many agrosystems, soil biodiversity and the ecosystem services, soil provides are under threat from a range of natural and manmade drivers. Agricultural soils are perturbated by the agronomic practices so far adopted that disrupt soil trophic networks to a large extent and make soils less and less productive in the long term. In this scenario, sustainable soil uses aimed at improving plant/root status, growth and development play a crucial role, as they have been found to enhance the biological capacity of agricultural soils. This review paper is divided into four main parts: (a) The contentious nature of soil organic matter. A few points in Lehmann and Kleber's article may mislead thinking. We just want to stop and think before moving on. (b) Soil biological quality/fertility. Soils are alive, organic and dynamic, because they are living systems. (c) Soil classification. The concepts contained in the first two points were well known to the fathers of pedology but, unfortunately, their principles have been lost over time. (d) Which agricultural practices can be defined as sustainable? As for point three, the desired spirit is that of a stimulating provocation. The published literature was analysed within a holistic view, with agrosystems considered as living systems where soil, vegetation, fauna and microorganisms co-evolve and are reciprocally influenced. Ultimately, this review will suggest a better stewardship of agricultural soils as a natural capital.