Discipline: environment; Keywords: soil erosion, soil formation, soil lifespan, sustainability, soil conservation, sustainable management.
Soils underpin the requirements for health and longevity of mankind. They are critical, providing the basis of food production, a store and filter for water resources, the largest organic carbon store and a platform for development. Pressures on the soil resource grow as food demands rise and land degradation increases. According to the FAO, to date, 36% of the world’s cultivable land has been farmed and in many areas of the world conventional plough-based agriculture is accelerating soil degradation. It is estimated that human-induced soil erosion outpaces soil formation by more than one order of magnitude. It is therefore important that land managers, policy makers and society at large understand the timeline over which soil ecosystem services may be severely affected by the complete loss of topsoil. Here, Dr D.L. Evans and co-workers provide the first scientific global estimate of soil life spans and the degree to which changes in land use or management can extend them. They defined an upper limit on the productive lifespan of soils as the time it would take for the top 30 cm soil to erode, assuming that current rates of erosion and soil formation do not change. It is accepted that because the 30 topsoil is enriched with nutrients and organic matter, it is essential for farming and other ecosystem services. Their work was published in Environmental Research Letters, Volume 15 of 2020, 0940b2. https://doi.org/10.1088/1748-9326/aba2fd; the title being: Soil life spans and how they can be extended by land use and management change.
The data was compiled from 4285 plot-based gross erosion rates representing 10030 plot years, from 240 studies, comprising 255 unique locations across 38 countries. The data were assigned into three categories with respect to land management: bare soil, non-bare conventional agriculture and conservation-based agriculture. The bare soil dataset was from soils that were kept free of vegetation on experimental plots. Whilst instances of constantly bare soil are only likely to occur periodically (e.g. prior to crop emergence), the use of bare soil data provides a worst-case baseline against which conventional agriculture and soil conservation practices can be assessed. The non-bare conventional agriculture dataset includes observations from plots undergoing non-conservation agricultural practices, including down slope cultivation, non-terraced cropland and conventional tillage. The conservation-based agriculture dataset comprises plots that have been subject to soil conservation techniques, such as land-use change and modifications to agricultural practices. These were combined with existing soil formation data to estimate soil sustainability expressed as the life span for the topsoil of 30 cm to be eroded.
Interpretation of the results obtained: First, an assessment of soil life spans using soil loss rates measured from non-bare soils under conventional management systems suggest that, under a worst-case scenario, 93% were thinning and 16% had life spans of less than a 100 years. At these sites, soil erosion was a significant threat to the soil’s capacity to grow food, support ecosystems, store and regulate water, cycle carbon and nutrients and thus to the overall functioning of the soil system. Second, it was shown that land use and land management practices can extend the soil lifespan and, in many cases, even promote the onset of soil thickening. Only 7% of the conservation plot dataset had life spans of less than a 100 years, with 48% of the estimated life spans exceeding 5000 years and 39% exceeding 10000 years. The estimates for forested and grassland sites suggest that conversion to these land uses would be most effective in achieving both of these outcomes, closely followed by the introduction of cover cropping as in regenerative agriculture. Given the need to meet the growing demand for food, cover cropping should to be considered. Additional strategies to extend soil life spans and promote annual soil gain include conservation and zero till practices, contour cultivation and terracing. In general, conservation practices extended soil life spans and may promote soil thickening, increasing the potential for water, carbon and nutrient storage, and thereby soil conditions which could enhance crop yields.
Conclusion: The message is clear, poor management practices which result in soil erosion and conventional practices with tilling in crop production are wasting our topsoil, and SA farmers should seriously consider changing to regenerative farming where cover crops are an integral part of production systems.