What happens to the soil under heavily concreted environments such as central London? Is it still full of worms and is the soil still rich in nutrients and life? Alison Givney Send new questions to nq@theguardian.com. Readers reply What most people think of as soil exists only near the ground surface and has terrible engineering properties: whatever gets built on top settles unevenly and rain ingress can lead to swelling and erosion. So all the organic stuff containing the worms and dead leaves tends to be excavated, leaving firmer soil below to build structures on. Sadly, removing all that soil organic matter is taking quite a toll ecologically and environmentally (globally, soil is our No 2 carbon sink, after the oceans) so we need to be doing more to ensure we always return the nutrients to the earth. Ted O’Hare If you look at the challenges faced by urban street trees, that’s probably an indication of the soil quality under heavily concreted areas. City trees grow in soil that is very depleted in nutrients due to a lack of top soil and organic matter from fallen leaves and other plant and animal debris; the soil will be much more compacted and less aerated, and surrounding surfaces are largely impermeable to oxygen and rainwater, which is also likely to be more polluted. Urban areas also act as heat islands increasing moisture evaporation. Consequently urban trees have a much shorter lifespan than suburban or rural trees, reflected by the soil quality. MrsLessing Before the concrete is laid, any topsoil will be stripped off and removed, so this will automatically remove most of the soil microbe population, the seed bank, important nutrients such as nitrogen and sulphur, plus a good proportion of other mineral nutrients and trace elements. What’s left is the subsoil – or historically made ground if it was a brownfield site – which is typically inert, inorganic and impoverished. So, initially, the answer to these specific questions is: no, it is lifeless. However, over time, concrete cracks, which reconnects the soil to the surface and atmosphere, and the weathering forces that are partly responsible for soil creation in the first place. Trees also extend their roots under the concrete/hard surfacing, causing further cracking but also sub-surface connections to the surface. Underground services, such as drainage pipework also crack and break, further enhancing the influence of air and water. These connections allow micro, meso and even macro fauna (including earthworms) to re-colonise the parts of the soil profile where growing conditions start to become favourable. This is usually still only a small proportion of the total area, but it is a start. So, I would say that over time, some life will return to the soil beneath concrete and other hardstanding. Tim O’Hare, Sustainable Soils Alliance We had a mid-40s house in California (ancient by CA standards) with a heavily concreted back yard. It took us about four weekends to pull up and dispose of the concrete and we thought that it would take several years to regenerate the soil. But the newly uncovered ground proved incredibly fertile and within a year we had tomato plants taller than 6ft. So I guess that there was a great deal of biological activity that must have been taking place underneath the concrete. Presumably water and air could get through the cracks in the concrete? Robert Cripps There is no viable soil symbiotic interactions under either concrete or asphalt. Basically cut off from both the atmosphere and water, there is no activation of the beneficial microbes, or nutrients. Therefore no bugs, except ants and possibly a few others will benefit from existing in a non-viable environment cut off from oxygen and water. Robin Lloyd I recently picked up two tonnes of top soil that was underneath a demolished garage – there’s no worms in it and nothing has randomly started to grow in it over the last week. Powers I can’t speak for heavily concreted, but early last summer in our garden we removed a thick layer of gravel with multi-layered weed suppressant underneath it, which had been there for many years. There was not a sign of life in the soil underneath, certainly no worms, and I fretted about bringing in some top soil as it felt so dead. Needn’t have worried, as all of that quickly came back once open to the elements. We’ve added localised compost when planting certain plants, but for a wildlife garden, the initially nutrient-poor soil has been something of a bonus, as it tends to suit many native wildflowers. drizzleinthebrizzle Soil needs the activity of insects, fungi, bacteria and birds/mammals in order to maintain the richness and life within it. Soil is built from the actions of different organisms, each contributing towards it in some way. When you cover it, or pave over it, or even dig it up (to some degree), you inadvertently change its structure and the way it functions. This is why no-till gardening/farming is making a comeback. We need to start recognising widely the importance of soil and begin to address how to improve it (for the benefit of all organisms) instead of stripping it bare. Apfreely If you’re going to build something, it doesn’t matter what, and there is going to be concrete laid, all topsoil and organic matter have to be removed. That means underneath your road, house foundation, terrace, shed base etc, there’s just compacted subsoil. Sorry, no worms, creepy-crawlies or other interesting creatures, and no vegetation. Inert, and stable – or it should be, otherwise you’ll have the district surveyor on your neck. At least it used to be when I was laying bricks in southern England. Hard to say, though, these days anything is possible – they might have sold the quality control to a consortium of California hairdressers, or a private equity gorilla, who knows? bricklayersoption I’m a geologist. Analysis of buried soils under the permafrost layer in Siberia show quite bioactive soil horizons, and the research found that the buried soil was higher in microbial life than the soil horizons above it that were exposed to air and falling vegetative matter. In soil horizons you have lateral and vertical permeation of water and gas; the water table in London for example, is about 45 metres below the surface. When I studied geo-tech, it’s vertical structures and pumping that has more local influence on the water table’s hydrological movement than capping from rainwater. You would still get water and gas diffusion, as anyone digging a building foundation in London could attest. So I would say though you would get a strong reduction in insect and arthropod and possibly worms and any other creature that needs occasional surface access, the microbial and fungal life is much more dependent on what was there originally, the soil profile, than any capping. Was it a black humus rich in organic matter, or one of the famous London clay formations outcropping at the surface? Both would be richer in microbial life even capped by concrete, than would an exposed laterite soil of Western Australia. A lecturer told me: “It takes a million years to make a good soil.” He was referring to the loss of top soil from poor land use, the fact that it takes so long (like a lot of geological processes; oil can take 10m years to mature) and can be gone so quickly. But it also means that a concrete cap of 20 years or even 300 makes little difference to the soil. It’s just a hiatus in the addition of organic material in the very uppermost layer, and some of the organisms that are involved in exploiting this. This isn’t an argument that urban sprawl isn’t environmentally damaging – it clearly is – just that when you go a few feet down there’s less influence from what’s happening on the surface. WombatsRamble Sorry – as a soil scientist I think you are off the mark here. Under a single paving slab there is probably very little impact on the soil beneath – indeed it may create additional habitat. Lift up a single slap and there are lots of worms and other creatures making use of the space. However, concrete over an area the size of Heathrow runway 3 and this will have a dramatic impact on the gaseous and hydrological status of the soil, eliminating all macrofauna and dramatically altering the microfauna into anaerobic-tolerant species. Yes, it’s life Jim, but not as we know it. FishSocks When I took on an allotment 10 years ago, it was covered by a double layer of carpet, so there was nothing growing there at all, and just like a paved street, the soil was pretty lifeless, no earthworms etc. As the soil health movement’s famous agronomist Ray Archuleta is fond of saying: “Plant and soil are one.” Plants make soil, so without plants and the myriad organisms they support, what do you have? Geology. So concrete is death unless you can break it up with an earthquake or something and start over again. frostfinger4 We bought a vineyard in the south of France that had been subjected to about 30 years of “modern agriculture” involving heavy equipment and some pretty nasty chemicals. The soil was dead, concrete hard with no insects nor birds living there. We quit spraying the nasties and encouraged the natural weeds to grow. It took about seven years for the soils to regenerate, mostly through common sense and benign neglect. Interestingly, several years into this project, the ag-advisory people became very concerned about the degraded soils locally and persuaded a bunch of vignerons to dig two-metre pits in various places. They then gathered us up and we went around inspecting each other’s soil. Alarmingly, every single pit uncovered soils that were remineralising – turning back into rock! Except the two on our domaine, which made the ag advisers very happy. It also had the effect of finally stopping all the other vignerons from laughing at us for all the weeds in our vineyard. I guess the moral here is that even dead, seemingly inert soils (as you would expect to find under concrete) can regenerate if you just leave them alone. winebore I lived in a 15-year-old house with a near solid concrete pad (14ft wide) as a patio in the back yard. Our neighbours made the mistake of planting a willow tree 6ft from the fence; its roots were able to send out runners that went more than 25ft underground before coming up in our yard. The extension service told me willow roots could travel over 50ft under concrete from the host tree. water_moon It sits and contemplates the question which most of nature asks itself on a daily basis: why are humans so stupid? _Meat_ Humans being an important part of nature, your question should really be: “Why is nature so stupid?” Unless, of course, you’re actually saying that humans aren’t a part of the natural order? Perhaps you might try to be more specific in your naming and shaming terms of reference … or even just less stupid! NewMe359