Land Use, Soil Quality and Management of Soil Organic Matter

Abstract

The Black Earth was made by grassland to suit itself. Its outstanding attributes are fertility, rapid permeability allied with generous available water capacity, and resilience in the face wind and weather. Breaking the sod in the eighteenth and nineteenth centuries, and ploughing ever since, has degraded the soil and its capacity to yield both crops and ecosystem services. A narrow focus on crop yield has created a gamut of environmental and societal problems: degradation of the uplands has rendered the land more susceptible to drought, loss of woods and meadows in bottomlands has increased vulnerability to floods. These problems are political rather than scientific. Given political will, they can be overcome: outstanding examples are the adoption of a nationwide program of soil conservation in the USA following the Dust Bowl in the late 1930s, and implementation of the Plan for the Transformation of Nature in the USSR after the 1946–1947 drought. Farmers’ experience and long-term field experiments show soil organic matter to be an integral index of soil fertility. Over the past century, Chernozem have lost half of their organic matter; relative to virgin soils, losses have been greater and they continue through increased mineralization caused by intensive tillage and insufficient inputs of crop residues and manure. Agriculture with a perpetual deficit of energy is not sustainable. Likewise, agriculture that neglects soil structure may well turn the steppes into desert. The regular release of plant nutrients depends on the labile fraction of soil organic matter, and stocks are maintained under crop rotations with perennial legumes and goodly dressings of farmyard manure. The greater the input of fresh organic matter, the greater the mineralization of SOM and the capacity of soil to meet crops’ nutrient demands. However, crop yields are not correlated with total SOM or, even, with the labile fraction: biochemical composition, rate of turnover and the capacity to release nutrients are the most important factors in yield formation. A diversity of crops in rotation, as opposed to continuous monoculture, increases the ability of the root system to absorb nutrients and water from soil. This reduces farmers’ dependence on industrial inputs for crop nutrition and control of weeds, pests and diseases: poor soil quality has to be compensated by costly external inputs.