Soil degradation is one of the most pressing problems facing our planet. More than 80% of the world's farming land is 'moderately or severely eroded', with 75 billion tonnes of soil lost every year.
Over the past half-century, the intensification of agriculture has led to failure of soil structure and organisms, reducing it to little more than ‘dirt’ – a sterile medium in which little can grow without artificial fertilisers. It is a self-perpetuating cycle of destruction and chemical dependence. Without soil organisms and soil structure to retain them, water and nutrients leach away, and the soil compacts and becomes prone to erosion.
Land degradation costs up to $10.6 trillion a year. In the UK, that cost is between £900 million and £1.4 billion a year – half of which is caused by loss of organic matter, over a third by compaction, and about 13% through erosion. The depletion of topsoil is so severe in the UK that in 2014 Farmer’s Weekly magazine announced we have only a hundred harvests left.
However, a ‘regenerative farming’ movement is gathering pace, from naturalistic grazing systems to agro-forestry and ‘no-till’ (zero ploughing) agricultural systems, all of which focus on restoring soil structure and soil biology.
Rewilding is part of this soil restoration movement – an approach particularly applicable to marginal land, like Knepp, where intensive farming is inappropriate and/or unprofitable.
At Knepp we have seen life returning to the earth in a surprisingly short space of time post-industrial farming, suggesting that rewilding is a very effective, low-cost way of restoring depleted soils. A study of our soils by Cranfield University in 2018, compared to an arable control, showed a doubling of soil organic carbon, a doubling of soil microbial biomass, a recovered fungal/bacterial ratio and an increase in fungal biomarkers. These are major indications of both soil health recovery and carbon sequestration. In 2022 we are undertaking an extremely in-depth carbon analysis of our soils to work out exactly how much carbon they have stored over the past 20 years of ecosystem recovery.
Earthworms are also a key indicator of soil health. Surveys of anecic, epigeic and endogeic earthworms at Knepp in 2013 recorded a total of 18 species. Using neighbouring farmland as a baseline, the comparison with Knepp showed an exponential rise in the populations of all three types of earthworm in all three areas of the rewilding project.
Dung beetles perform a range of vital functions, including pulling down organic matter into the soil. In a study by the University of the West of England in 2018, Knepp was found to have 11,677 beetles compared to an organic control with only 518. Organic farms are thought to have 38% more dung beetles than non-organic farms (Hutton and Giller, 2003) due to reduced use of wormers and antibiotics. One hypothesis for Knepp so dramatically outperforming even organic farms is the diversity of herbivores in the system and the fact that the animals winter outside so there is no break in the availability of dung for the beetles which are active all year round. Twenty-one different species of dung beetle have been recorded at Knepp.
Ants, too, increase the fertility of the soil, creating micro-habitats for numerous other species with their ant-hills, some of which, at Knepp, are now over a half a metre high.
The eruption of fruiting bodies of fungi across the land is another exciting indicator, demonstrating the spread of mycorrhizae through the soil. Mycorrhizae are the fungal filaments that convey water and essential nutrients to the roots of plants. They can extend for miles in the soil, an invisible web transmitting chemical communications between plants. Mycorrhizae can even mine rock, extracting minerals and bringing them into the plant food cycle.
Encouraging, too, is the emergence in our post-arable fields of southern marsh, early purple and common spotted-orchids – plants that depend on an exclusive, symbiotic relationship with mycorrhizae.