World Soil Day: the disappearing ground beneath our feet

"We cannot be complacent if we want to conserve the soil and the carbon, nutrients and biodiversity it holds for future generations"

Our soils are under threat from ever more intensive agriculture and climate changes. Extreme rainfall events such as Storm Frank in 2016 are predicted to become more common: a recent report from the Met Office suggested that there is a 34% chance of somewhere in the UK breaking a rainfall record each winter. Apart from flooding, these rainfall events can cause widespread erosion on unprotected or damaged soils, loss of soil nutrients and carbon, reduced crop growth and pollution of our rivers and streams.

Erosion is a natural process where soil particles are loosened from the ground by rain and wind and transported by our rivers and streams to the sea over long periods of time. The 18^th century Scottish scientist James Hutton observed this on his own farm in the Scottish Borders and realising there was a process of renewal and loss, went on to develop his world view changing theories on Deep Time and became the Father of Geology.

However, accelerated erosion, caused by inappropriate land use or land management is a serious global issue that affects our environment and ability to feed a growing population. The Food and Agriculture Organisation (FAO) of the United Nations has estimated that a third of the world’s soils are already degraded. That means they may produce less crop yield and need more artificial fertilisers and inputs, or both.

While much of this degradation is found in low income countries, Scotland is not immune to the adverse effects of soil erosion. Every year, we lose some of our precious agricultural land to erosion. When we lose soil, we lose carbon, we lose nutrients and we lose biodiversity.

Much of what we know about erosion rates on agricultural land in Scotland comes from a few, individual studies of erosion events, but the evidence from many different situations is now gathering weight. Scientists at the James Hutton Institute have developed a simple way to assess soil erosion risk which is freely available on the Scotland’s Soils website. It allows land managers to mitigate risks when planning what to grow.

While increased mechanisation has brought many benefits to farmers and helps produce the food we eat, some common practices, if not properly managed, can increase erosion risk. For example, ‘tramlines’ - paths regularly used by tractor wheels - running with a slope, are routes for water to gather and flow, leading to erosion.

Work in England, supported by Defra, has shown that tramlines are associated with soil erosion particularly within winter sown cereals and other combinable crops. Although cereal crops need to be sprayed in autumn for crop health and yields, this practice can further compact the soil along the tramlines and exacerbate erosion associated with winter rainfall. A UK-based study has shown that tramlines increase runoff by 46% and generate five times more sediment loss. Methods such as the use of very flexible tyres with low ground pressure are being tested that will help reduce this loss but the issue remains, for now.

Typically, high-value crops require fine seedbeds which means the topsoil has many more small soil aggregates that are then more readily washed down slope or blown away when dry and exposed to strong wind. At the James Hutton Institute, we are trialling a diverse range of cover crops and novel tillage methods to bind the soil and maintain a soil structure that is more resilient to erosion.  

It is often overlooked that normal ploughing – which turns the ground surface right over to expose bare earth - can cause soil to be gradually moved down-slope exposing subsoil and potentially damaging buried archaeological remains such as burial sites. Work at the Institute’s Balruddery Farm is exploring alternative ploughing practices, such as changing ploughing direction, to mitigate these effects.

While soil erosion in Scotland due to traditional cultivation practices is not on the same scale as in some parts of the world, it does impact on the water quality of our streams and rivers, and we cannot be complacent if we want to conserve the soil and the carbon, nutrients and biodiversity it holds for future generations.

Dr Allan Lilly is a soil scientist within our Environmental and Biochemical Sciences group, and wrote this article with colleagues Blair McKenzie and Pete Iannetta to highlight World Soil Day, held annually on 5 December to focus attention on the importance of healthy soil and sustainable management of soil resources.