As a soil scientist you would expect me to be enthusiastic about the benefits that soil gives to humanity and very happy that the United Nations designated 2015 as International Year of Soils. During this year there have been numerous activities throughout the world to draw attention of a wider public to the value of soil. In the UK, the British Society of Soil Science has been active in organising events in schools – recognising the need to enthuse future generations.
The properties of natural soils under forest or grassland are especially impressive; by "natural" I mean soils largely unaltered by humans by processes such as ploughing. In these soils the interactions between physical, chemical and biological processes are particularly amazing. Part of me want to be a "soil-hugger" and stop any interference with these habitats so that the wonderful structures and biodiversity, established over many years, are preserved. However, as we all know, a great deal of land, and therefore soil, needs to be used for agriculture. About 40% of the total global land surface is used for some form of agriculture but even this figure understates agriculture's global impact. Large areas of land are unsuitable for agriculture, being made up of deserts or mountains or are too cold. In the UK, some 70% of the land area is used for agriculture in some way and values in the range of 50% to 70% are common elsewhere. Hence, in many regions, the majority of land suitable for agriculture is already utilised, and this means the majority of soils are modified by human activity. So if we are concerned about the quality of soil on our Planet, it is these agricultural soils where we need to focus our attention.
Producing food, as well as fibre and fuel, is just one function we derive from soils – albeit a vitally important one. But soils also provide a range of so-called "ecosystem services" including storage and filtering of water, influencing the composition of the atmosphere through regulating flows of greenhouse and other gases, and providing a habitat for a vast array of animals and microbes (biodiversity).
When thinking about the way we treat our soils at least two issues need to be considered. First, it is usually inevitable that soil used for agriculture, at least for growing annual crops such as wheat as opposed to grass (termed "arable soils"), will be somewhat less valuable for "ecosystem services" than a soil under natural vegetation. This is because the soil will normally have a lower organic matter content than the corresponding natural soil and is likely to have increased inputs of nutrients whether from fertilizers, manures or biological nitrogen fixing plants such as clover. So an acceptable trade-off has to be determined in different regions for the proportions of land used primarily for agriculture and those maintained primarily for other "ecosystem services" – but bearing in mind that people need to eat!
Second, for soils under agriculture a key issue is sustainability – ensuring that the practices used permit continued production of crops at an acceptable level for many years as opposed to good yields for a short period followed by decline. In human history, serious damage to soils was almost certainly a factor in the collapse of civilizations. A key factor for achieving sustainability is maintaining soil organic matter content as this component influences virtually all soil properties – biological, chemical and physical. Adding manure is an obvious way of increasing organic matter content but, in most places, there is simply too little manure available to rely solely on this. Using other organic by-products such as sewage sludge, domestic waste and "wastes" from food processing can be valuable provided these do not introduce pollutants or pathogens. There is evidence that even small changes in soil organic matter content, either increases or decreases, can have disproportionately large impacts on soil physical properties. So there is scope for research to find ways of making limited supplies of organic resources achieve maximum impact – making a little go a long way. In fields used for growing annual crops, a move to reduced tillage, as an alternative to ploughing, can be helpful as this concentrates organic matter near the soil surface, but reduced tillage is not successful in all soil types or environments.
There can be conflicts between the different uses of soils and between factors conducive to long-term sustainability compared to short-term production. Developing management practices to meet the range of different requirements, production and environmental, and developing sensible policies to address any necessary trade-offs, is not easy. Soils researchers regularly grapple with these issues, as do farmers. It is my hope that these groups will work more closely together to find solutions; innovative forms of funding are essential for encouraging such cooperation between researchers and farmers, in addition to funding for gaining more fundamental knowledge of soil processes using exciting new techniques that are becoming available. Because soils differ so widely in their properties (for example light sand compared to heavy clay, soils prone to flooding compared to those facing drought), simple prescriptions of a "one size fits all" nature are generally unhelpful.
It is also essential that the quality of soils is taken seriously by policy makers on behalf of the citizens they represent. Current policies and regulations vary between countries but are generally weaker than those for protecting the quality of water or air. If the International Year of Soils leads to some strengthening of relevant policies, and greater embedding of soil protection within other policy areas, something valuable will have been achieved.
Professor David Powlson is a Lawes Trust Senior Fellow for Rothamsted Research, Harpenden, UK.