Topsoil is created by the physical and chemical weathering of rock into sand, silt and clay. It is estimated that this process builds topsoil at the rate of one inch every 500 years. Such a timescale effectively makes soil a non-renewable resource in our lifetime. Perhaps it was for this reason that President Franklin Delano Roosevelt stated “The nation that destroys its soil destroys itself.”
We continue to destroy our topsoil at an alarming rate. About a third of the world’s topsoil has already been degraded and if current rates of degradation continue unabated the world could be without topsoil in around 60 years.
While farming more sustainably may put the brakes on some of the destruction, what is really needed is a way of speeding up the recovery. One possible option may have been staring us in the face for more than a century – earthworms.
Charles Darwin spent time researching earthworms and their role in soil formation. Through some backyard experiments he concluded that topsoil levels could be increased in only a matter of years rather than centuries, primarily as a result of the digestive work of earthworms. Although there is much anecdotal evidence to support Darwin’s conclusions, there is scant scientific research to be found on it. There are, however, studies showing that earthworms are important actors in the regeneration of compacted soil, speeding up its repair.
In New Zealand attempts were even made some fifty years ago to translate the value of earthworms into monetary terms (fitting neatly with the Sustainable Food Trust’s work on true cost accounting). Researcher D. R. Crump, in his book Earthworms – a profitable investment (1969), estimated that for every dollar investment in earthworms on New Zealand sheep farms, the farmer could expect a return of $3.34 and an increase in productivity of 25 to 30%.
Earthworms offer multiple benefits to the soil ranging from the positive effects of aeration, drainage and soil structure to the enhancement of nutrient availability and soil biology. These benefits vary according to the type of earthworm: endogeics live on the surface and make horizontal burrows whilst anecis live in the subsoil, burrowing more than a metre deep.
The tunnelling of anecis earthworms, particularly, opens up channels for water and air down to the subsoil level. These worms live in permanent vertical burrows that can be as much as two metres deep, although they collect their food from the soil surface. Their tunnels break up the soil, facilitating root growth by allowing plant roots to penetrate deeper into the ground, thus making additional nutrients and moisture available to them. The tunnels also allow for improved drainage which has been shown to be as much as ten times faster than in soil without earthworms.
Additionally, earthworm casts directly enhance the aggregation of the structure of the soil, cementing soil particles together and thereby significantly increasing the water-holding capacity of soils.
A further benefit of earthworm casts is that they return nutrients to the soil in a form readily available to plants. Worms feed on plant debris such as leaves and dead roots, as well as the soil itself. The organic and mineral constituents of their food become concentrated during the digestive process, thus making their casts a rich source of nutrients, in particular, nitrogen. The concentration of nitrate has been shown to be up to eight times higher in worm casts than in the surrounding soil. Other research has shown that worm casts release four times more phosphorus than the soil in which they are found. In fact, it is likely that the availability of practically all nutrients is improved when organic material passes through the earthworm gut. It is particularly through this action of leaving their casts on the soil surface that earthworms can help to rebuild topsoil.
Further, soil biology as a whole is also improved in the presence of earthworms. They enhance the activity of microorganisms and actively spread fungi and bacteria through the soil. As organic matter passes through their digestive system it is mixed with microorganisms in a manner that actually renders the level of microorganisms higher in their casts than in the originally consumed organic matter.
SFT director, Patrick Holden has a theory that soil is the digestive system of the plant. He postulates that, just as our bodies require good bacteria for optimal function, so does the soil. Continuing this analogy, in the same way that we manage our dietary intake for optimal health we must look to do likewise for the soil. Supporting the growth of the earthworm population is one key aspect of managing soil health that might pay big dividends. There are a number of ways that such a strategy could be approached.
Earthworms have two key requirements for good health. First, they need dead plant material to feed on. If crop residue is removed, earthworms lose their food source. We can increase organic matter not only with crop stubble but also with green manure crops, crop rotations to build up organic matter, or even permanent pasture which allows organic matter to die and decay.
Second, like so much of the natural world, earthworms need to be left alone to do their work. They do not react well to soil disturbance. Soil tillage greatly affects earthworms. It destroys their tunnel systems and is a particular issue in the autumn when breeding is taking place. Heavily compacted soil is difficult for earthworms to move through. Tillage also stimulates drying the surface soil and greater temperature fluctuations between day and night, both unfriendly conditions for earthworms. Earthworms need moist soil and are inactive when it is dry. A further negative impact of tillage is that it brings earthworms to the surface where they are subject to predators such as birds.
Several studies have shown significantly higher earthworm populations in no-till soil versus soil that has been ploughed. Although the research remains patchy, it is clear that earthworms do not co-exist happily with modern agriculture. They typically live only a few months in the face of today’s multitude of environmental threats (intensive farming, monoculture and pesticides to name but a few), while they have been observed to have a lifespan of up to ten years in a protected environment.
More concrete scientific research is certainly required to be able to confidently quantify the economic benefits of earthworms, although indications already point to their potential high value as an agricultural resource. What is clear beyond doubt is the urgent need to identify methods to speed up the recovery of topsoil. If Roosevelt was correct in his statement on soil, then earthworms might yet prove to be our saviour.
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