‘Once the land, like the air and the sea, belonged to the people. Then somebody stole it'. So reads some graffiti on Hackney flyover in North East London. It always used to make me laugh when I went past, although I could also see the logic of the statement. I'm not going to go into a political diatribe about whether land ownership is really appropriate, but I do want to talk a bit about soil.


If you ask most people what soil is they will usually say ‘minerals', ‘sand or rock particles' or something similar. But soil is more correctly defined as an eco-system. It's not even a micro-habitat, because it's not microcosmic at all; it is a big enough part of our world that the planet Earth is named after it (albeit two-thirds of the planet is covered by water). It is a conglomeration of living things that interact together in the same way a rain forest or a coral reef is, although we tend to take it for granted because it is constantly underfoot. Soil consists primarily of minerals but there is also the organic component usually made from decaying plants and animals. When this rots sufficiently it turns into humic acid which is a black jelly like substance that binds soil particles together and provides nutrients. There are also organisms living in the soil, from the enormous tap roots of some plants to the microscopic hyphae of some fungi; from large burrowing animals like badgers down to nematodes and other tiny creatures. The range of invertebrates that inhabit the soil is enormous. Everyone thinks of worms, but there are also beetles, slugs, snails, centipedes, millipedes, woodlice, tardigrades, lice, mites, ants, wasps, bees, spiders to name but a few. These creatures all interact with each other, some preying on other creatures, some eating decaying matter, others eating plant roots. Fungi, many of which are microscopic, are also a vital component, and so are bacteria. Without eukaryotic bacteria there would be no life of this planet so of course the soil is swarming with them. If you removed any one component, the ecology of the habitat would change irrevocably, just as it does in any other ecosystem.


Yet 90% of the time we ignore all these other components when we look at soil because we are only thinking about texture, structure, pH and nutrients. We don't want creepy crawlies in our potting compost because it is an artificial medium, not a true soil. But if we are to have a healthy soil all these ingredients are present in varying degrees. So why then are the four things I have mentioned so important and what are they?
Soil texture describes the individual grains, and the proportion of them. Soil grains are divided into three classification sizes: sand (largest), silt (medium) and clay (smallest). Most people don't realise that clay is the smallest component because they think of it as being hard and lumpy, but actually the individual components are so tiny they form a conglomerated mass which only break up easily when they are dry. With sand the individual grains are visible to the naked eye and flow freely (an egg timer just wouldn't work with clay!). Silt is somewhere in-between. Very few soils are made up of just one particle size, even deserts, so particle sizes are used to define soils such as sandy clays. The word loam is used to describe an approximate equal quantity by volume of all three components. This makes the best sort of soil because it has the best air and water retaining characteristics. These two things, by the way, are also vital ingredients of the soil ecosystem because without air and water nothing would survive.
Soil structure describes the arrangement of the inorganic grains in the soil and the pore spaces between them, vital for holding the water and air in place. Imagine a swimming pool full of footballs. If you fill it full of water (and the balls don't float), there would be big spaces for the water to flow between and not much in the spaces. If you have the same pool filled with marbles and added water all the water would be held by surface tension around the outside of each marble, but there would be very little air space. If you have a range of sizes, say footballs, tennis balls and marbles, you would have pockets of both air and water. The footballs are equivalent to sand, the marbles to clay. You cannot improve a clay soil by just mixing the two together because the grains are so tiny we can never mix them sufficiently well. The thing that holds them, binds them and separates them is humic acid, forming a glue around each individual particle thus increasing the size of clay particles and allowing better air and water spaces to form and increasing the nutrients in sand. The addition of humus improves all soil structures. Worms help too, mixing the soil by eating it, secreting calcium and organic matter, and then defecating.


The pH is the acidity or alkalinity of the soil, determined by what minerals are present when the soil is formed, whether from chalk and limestone which form alkaline soils (high pH) or mostly lacking in minerals and formed from decomposing organic components forming peat which is acid (low pH). Iron, manganese, copper, boron and zinc are all more common and readily available in acid soils; calcium, magnesium and molybdenum are prevalent in alkaline soils but get ‘locked up' in acid soils. Through a chemical reaction the atoms of the minerals bond with those of the soil particles and cannot be released, thus plants which like a low pH, for example Rhododendrons, have evolved to cope without much calcium. The ideal pH of a soil for most plants is 6.5 because all the nutrients I have mentioned, plus the macro-nutrients that we all know and buy in bags (nitrogen, potassium and phosphorous - plus sulphur) are readily available at this pH. Therefore pH and nutrients are intrinsically linked.

Soil takes millions of years to form, is a living eco-system of infinite variability. Without it most plants wouldn't grow, so we would have nothing to eat. We may not own the land but we have a duty of care to look after it.

 

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