Agriculture is the main culprit. A raft of practices – from deforestation for land clearance to the types of intensive farming that exhausts nutrients or make the soil too salty for crops – contribute to depleted soils. Farmers are then forced to seek new soil, often by invading forests. If we want to preserve the few remaining forests and other diverse ecosystems around the world, we are going to have to better manage our existing agricultural land.
Deforestation is a major driver of soil loss. Once shrubs and grasses have been cleared from the land, there are no roots to hold soil, nutrients and water in place. A dry spell can turn the ground to dust that is easily blown away, as happened with disastrous consequences in America in the "Dust Bowl" 1930s. This also frequently happens in China, Australia and parts of Africa, where large areas of agricultural land are becoming deserts.
Great green wall
Farmers are fighting back by planting trees and shrubs that help keep the soils moist, buffer the winds and slow rainwater. West African countries, for example, have already achieved remarkable improvements in soil fertility as a result of tree-planting programmes, and there is now an ambitious plan for a 'Great Green Wall of Africa' to cross the continent from Djibouti to Senegal in a tree barrier against the encroaching deserts.
In Indonesia, I found farmers planting vetiver – an Indian grass – to protect their soils from erosion, because its roots grow down deep and hold it in place. Solutions like this are liked because they offer multiple benefits – vetiver reduces weed and pest incursion, produces marketable oil and is also useful for animal feed.
Essential minerals, such as nitrates, that crops suck up from the soil take a particularly long time to be replaced naturally. Although nitrogen is very common in the air, only a few organisms are able to break the tough chemical bond in the gas to create the nitrate form that plants and animals use to develop proteins.
Animal faeces contain many of these chemicals, and recycling essential nitrates from biomatter is the easiest and cheapest way of replenishing the soil. In many places in the world, livestock dung is the only fertilizer available, but it is becoming scarcer because, in trying to preserve the few trees and shrubs around cropland, farmers are burning dung for cooking.
One solution to this problem, which I saw on a farm in India and which is now being trialled in Cameroon, is to use the dung to feed a biodigester – a tank in which bacteria break down the waste. The methane gas produced by the bacteria is then used to fuel cooking stoves, and the decomposed manure can be spread on the fields as fertiliser.
Another way of getting nitrates into the soil is to introduce the few organisms that can create it for you – so-called 'nitrogen-fixers', which includes plants like legumes that contain in their root systems bacteria that can make nitrates from the air. At the Tropical Soil Biology and Fertility Institute in Nairobi, I met soil scientist Peter Okoth, who has developed different types of promiscuous soybean – a nutritious nitrogen fixer – that can be intercropped with cereals and can concentrate an impressive 265 lb (120kg) of nitrate per hectare of soil. Okoth says his soybean variants are, "more effective than industrial fertilisers", which are not only expensive but are also made from oil, meaning they produce greenhouse gas emissions during production.
Give peas a chance
A growing body of scientists is arguing that the common pastoral scene of a man in a hat driving a plough is causing some of the biggest damage to our soils. By churning up the soil, farmers break down its important structure and integrity, reduce its fertility by exposing the minerals to oxygen, release quantities of carbon dioxide and remove the nutrients in the leftover crop stalks.
