Keywords: earthworms, soil health, tillage, soil, aggregation.
A healthy soil associated with sustainable crop production is likely to be considered a sustainable agricultural system. Soil health indicators only have value if they influence management decisions that support soil and food security. The surface layer (<5 cm) of a field is disproportionally affected by arable land management practices. For silty-clay soils, structural degradation of this layer leads to slaking under the impact of rain, with implications for nutrient leaching, capping, crop emergence, infiltration, runoff and erosion. Thus, aggregate stability (rapid wetting) measurements have relevance for both soil and food security. Also, earthworm activity is a major factor regulating aggregate stability, and important for both soil functions and supporting plant productivity.
Three responsive soil health indicators, aggregate mean weight diameter, earthworm populations and Lumbricus terrestris (indicator species) midden abundance, were measured in arable field trials. Results showed that all soils tested were unstable, contained small earthworm populations and very few L. terrestris earthworms, although the actions of the limited numbers of L. terrestris anecic earthworms, specifically their middens, were associated with high biological activity and soil aggregation, highlighting their role as an ecosystem engineer.
The findings on soil health suggest that these agricultural systems may not be resilient to changes in rotation, climate and weather variability. Contrary to expectations, organic amendments did not improve the indicators. A more fundamental change in management practices addressing tillage and/or cropping is likely to be needed to improve soil health and the sustainability of the agricultural system. Despite the indications of poor soil health, crop yields have been sustained and, in many cases, increased by appropriate nutrient management.