Lately, I’ve spent a long time thinking, reading, watching & learning about soils, and I’ve gone from marginally interested to fascinated. Here’s my introduction to why soils are so important.
Why do soils matter?
Soil is a combination of minerals (effectively, broken down rocks), organic matter (dead &/or decaying plant or animal tissue), living organisms (bacteria, fungi and larger organisms) and all the water, gases, and chemicals that they produce / consume / recycle.
As any gardener will tell you, if you have good soils you can grow bountiful beautiful plants. If you don’t your plants will struggle to get enough nutrients and water, maybe killing them. However the importance of soils for crops is even more complicated than that, bringing together physical, chemical and biological disciplines.
Firstly, soils have their own complex and extremely important microbiome. We’re probably more familiar with this in the context of the human gut, but the same concepts apply in soils. Soils involve a vast array of microbial life competing, cooperating and complimenting each other and themselves interacting with the larger lifeforms around them.
It’s long been known that these microorganisms are essential in decomposition and recycling of nutrients, and ever since bacteria were identified as the cause of diseases like anthrax by researchers such as Robert Koch, a connection to pathology has been known. What recent discoveries are showing is how much this community – invisible to the naked eye – is contributing to plant nutrition and even plant defence against pests and diseases.
Secondly, soils are major storehouses of at least 2 environmentally critical chemicals; water and carbon dioxide. Soils are hugely variable in the amount of water they can hold, which has implications for flooding and drought. Healthy soils can absorb & slow the rush of water to rivers when it rains heavily, and they retain more water and release this to plants for longer in drought conditions. This matters because both drought and floods are becoming more common as climate change advances, and unhealthy soils exacerbate both.
In addition, good soils are also massively important carbon sinks. There is more carbon in soil than in the atmosphere and all plant life combined, (as quoted here) Using soils as a means of carbon sequestration may not be as eye catching as futuristic geoengineering ideas, but it’s probably cheaper and more effective.
Lastly, soil (specifically top soil) matters because it is essential for agriculture and its being lost faster than it can be replaced. 99.7% of human food comes from cropland, but the statistics on soils loss are sobering. A report for the Committee on Climate Change calculated that the UK has lost 84% of its fertile topsoil since 1850, while a study by Cornell University found that the United States is losing soil 10 times faster than the natural replenishment rate, China and India a staggering 30-40 times faster. (Btw, it’s often cited that soil degradation is causing food to be less nutritious, but on balance, the evidence seems to suggest this isn’t actually the case)
Where is the soil going? It’s being washed into rivers, blocking them up and making them more prone to flooding. The run-off also takes agrochemicals with it, causing algal blooms and harming aquatic life. Soil is also blown away, creating air polluting microparticles and dispersing human infectious disease organisms.
The world in general is waking up to the importance of soils, but to do something about it we need to know
- What’s causing soil degradation / loss?
- What can be done about it?
What’s causing soil degradation and soil loss?
One of the biggest culprits is having bare soil in fields. This is observed in almost any agricultural area, but most famously in the corn and soy growing regions of the USA. Here, a rotation between soy and corn is widely practiced, but since both crops only sprout in spring (say, May) and are harvested in autumn (say, October) this leaves large swathes of land without plant material growing on them for months of the year. The effect of wind, rain, snow and sunshine causes the soil and its nutrients to wash and blow away much more readily than if a “cover crop”, crop residue or over-wintering species are present.
Another factor is tillage and ploughing – the process of turning over & breaking down the soil in preparation for planting. This practice, like bare soil, makes it more likely that soil will blow or wash away by increase disaggregation. When it’s dry, tillage causes the soil to dry out more quickly turning it to smaller particles, and when it’s wet, rain drops fragments the soil into splashes (it hits the soil at up to 20mph!). These splash-born particles also clog surface pores, accelerating run-off.
In addition, tillage changes the microbial composition which helps to “glue” the particles together. For instance, the hyphae of mychorrhizal fungi create a mesh which has a significant impact on soil aggregation, and tillage disturbs these networks.
Application of agrichemicals can contribute to degradation in the quality of soils. The many complex interactions in the rhizosphere (“the soil zone surrounding and influenced by the roots of plants”) include remarkable symbiotic loops. Crudely speaking, plants use photosynthesis to create carbohydrates, some of which they send into the soil as root exudates. This complex broth influences and is consumed by the microorganisms in the rhizosphere, who in return make nutrients (eg nitrogen) accessible the plant that would otherwise exist in forms that the plant cannot, or cannot as easily take up. Adding agrichemicals to the soil can destabilise this relationship, and since this is an ecosystem, the presence and balance of many other species along the food chain, including larger lifeforms are impacted. The result can be degraded soil fertility.
Finally, loss of organic matter is a key factor in soil degradation. This dead &/or decaying plant or animal tissue makes up typically single digit % of the volume of soil but has an out-sized impact on the aggregation, water holding capacity, fertility, carbon holding capacity and ecosystem vibrancy of the soil. Essentially, this component of soil alone underpins more or less all the major reasons that soil is so important, and is involved in most of the soil-degrading factors discussed above.
Organic matter concentrations vary massively depending upon climate and ecosystem type (think Irish peat bog vs Mediterranean sandbanks), but a key fact is it typically takes decades for organic matter to accumulate in the soil, whereas it can be lost very rapidly, especially when native vegetation is converted to agriculture. It’s probably fair to say that the most common arable farming practices are not actively building or sustaining soil organic matter.
What can be done about it?
There are many changes to practice that can be undertaken to improve soil health. As interest in and recognition of the importance of good soil increases, and as climate change makes existing practices less and less feasible – for example in Australia – these practices are being more widely adopted. Bear in mind, however, that any antidote is always adopted in the context of a farming business which is location (climate, soil type) and business model (crop type, route to market) specific. One size will not fit all.
To avoid leaving bare fields, cover crops (and/or leaving crop residues) are increasingly common ways of protecting soil against damage. Cover crops can be planted immediately after harvest, and grow through until the following year’s planting season. Cover crops not only provide protection from erosion, they can deliver many other benefits. For instance, legumes fix nitrogen into the soil, deep-rooted prairie grasses and radishes help with breaking up compaction, and if animals are to be grazed on the cover crop, plants which are nutritionally beneficial to them (eg chicory) can be grown.
An important interjection here, is that in many farming systems terminating cover crops for the subsequent cash crop to grow involves the use of herbicides, frequently glyphosate. There is, in some quarters, a concern about the ecological and health implications of this chemical. Reducing or eliminate the use of this herbicide will create major complications for farmers trying to do the right thing for their soil by growing cover crops between cash crops. A reminder, if it were needed, that black-and-white issues are anything but simple.
To avoid ploughing, no-till or minimum-till agriculture (in the US and Australia this is also called zero tillage or direct drilling), is one important change. Under this practice, planting is done with a drill which can cut a small slot in the soil and insert the seed at the appropriate depth. Not only does this mean less damage to the soil & greater carbon sequestration and it also means the farmer has to cross the fields fewer times in her/his tractor which also has benefits of reduced soil compaction and lower fuel consumption.
Another step that can be taken to increase biological activity, increase organic matter content and manage cover crops, is to integrate animals into the farming process. This has two benefits. One, perhaps obvious, is that they produce dung which is an excellent fertiliser. Another is that in just the right amount, grazing stimulates shoot and root growth & death, increasing the levels of plant matter in the soil that can be digested by mesofauna and microorganisms alike (see this USDA article).
It’s fair to say that the separation of livestock and arable farming practices is a major trend in western farming over the last 50 years; businesses models are specialised, the economics of different land and climate types have also influenced this. Leaving aside business logic, it is probably to the detriment of our soils.
Included in the most passionate soils advocates are the practitioners of regenerative agriculture practices (eg the Savory Institute). Farmers who have adopted these practices have demonstrated remarkable recovery in soil health and topsoil quantity. Indeed, many regenerative agriculture practitioners would advocate zero-chemical-inputs and have success with those methods.
The problem is that many aspects of the business model that these systems advocate sit awkwardly with the consumer end of the food system. When we rely on supermarkets that are conveniently located, homogenised and fully stocked year round; when suppliers to major food manufactures are heavily specialised and consolidated; and when food inputs are commoditised (“no.1 hard red winter wheat”, “no. 2 field corn”) many of the solutions become less tenable.
Ever the pragmatist, I would advocate for learning from what works and minimising what causes damage, then integrating these changes it into ”conventional” farming practices rather than waiting for wholesale overhauling of the entire food production value chain.
Which leads us neatly onto the barriers, economics, and emerging businesses involved in soil management. Part 2.