This was first published by Growing Today in 2003.
A long time ago, in a garden far, far away, I made an attempt to understand “ soil”. I read gardening books, watched TV gardeners, let the dry crumbly stuff run through my fingers, and then gave up — snowed under by the welter of technical terms, detailed classifications and chemistry that had me struggling to remember the stuff I’d learned at school. “Soil” was filed under T for Too Difficult, and I concentrated on just growing things. Luckily, my living did not depend on the results.
Now that I’m, ahem, a “professional” grower, I’ve had to revisit that T file, and make some real effort to understand the difficult stuff. Botany didn’t come easy, because I gave up the plant part of biology at 16, concentrating on animals. I used to be able to do a really good dissection of a worm (my eviscerated rats were, some might say, beautiful), but got completely lost when phloem was mentioned. Latin plant names were a struggle too. None of that mattered too much in the middle of a big city, in the middle of a business career, but when I discovered a mid-life urge to plant tree crops and found myself confronted by grass covered paddocks, re-education became a priority.
My motivation in all this, as regular readers may know, is that I want to grow things that taste good. Truffles, olive oil, wine grapes, walnuts, chestnuts, mushrooms — all of these are (with luck) on the way. Each of them depends on the soil they’re growing in. The soil affects the taste of the stuff I produce, so in my version of reality it has to be important. It’s my biggest asset, and understanding something about that asset has become a priority.
Soil is a small word for a very big topic. It is the living skin of the planet, the product of a complex interaction between rocks, landscape, climate, and life in all its forms. That complexity is what makes it daunting. First, there’s the geology underlying it all, full of odd names for rocks. When the rock gets weathered down into particles, complicated mineral names are bandied around (montmorillonite, smectites, other ’ites and ’anes apparently ad infinitum). Then we get into chemistry, and finally biology, with millions of little bugs and beasties all chewing and digesting their way through the earth. If you thought soil life was all about earthworms, think again. Finally, since soil is a unique product of each place and its climatic history, there are hundreds of place names that are used to describe characteristic soil types. You can rest assured that I am not going to delve too deeply into all this complexity.
It isn’t all science, however. The politics of agriculture also get involved. How you approach your soil says a lot about your farming style. Dose it up with chemical fertilisers, inject it with tear gas as a fungicide, or shun anything artificial, focussing on creating rich, healthy stuff. The “traditional” versus “organic” fence is there to be jumped. But first, the basics.
Soil starts out as rock. As the rock weathers, it breaks down into lumps and grains, and eventually into the particles that form the basic structure of soil. Two processes are at work; physical weathering through the action of heat, water, ice and plant roots, and chemical weathering, where the rock is attacked by air and water, slowly rotting down. These are remarkably powerful processes, if a little slow for us fast-paced animals to appreciate. Freezing water can split rocks just as easily as it bursts pipes, and rock can get very hot in the midday sun. When plants such as lichens and mosses get involved, clinging to the bare rock, they start to capture dust and produce organic debris. This reacts with water, creating organic acids that chew away at the rock. It’s slow but sure. In Russia, for example, a 200 year-old limestone tower was found to have developed no less than 30cm of soil on its flat roof. In New York, 70 years exposure to pollution, rain, heat and cold all but erased the inscriptions in the granite of “Cleopatra’s Needle” — which had happily survived 3,500 years in Egypt’s dry desert climate.
Rock particles don’t always stand still. They can be washed down slopes by rain, accumulating thickly in valley bottoms, or washed miles away by rivers. They can also be blown around the place, building up dunes (called loess, and common in Canterbury). Layers of different particles of various origins can be built up by geological and climatic processes, or — as in much of the North Island — deposited as ash or lava flows by volcanoes.
Soil particles are classified into three sizes. Sand is the largest, silt lies in the middle, and clay is the smallest. Sand and silt provide the structure of the soil, the skeleton, but it’s the clay particles that determine its chemical properties. Compared with sand, clay particles are tiny (under 0.002mm). Clays are the end product of weathering on the minerals that make up rock, and there are lots of different kinds of them. They play a crucial role in determining the availability of nutrients for plants and how the soil retains water, because they have an enormous surface area. Five grams of a clay called allophane has the same surface area as a rugby pitch!
On those surfaces, clays hold nutrients that plants need — potassium, ammonium, phosphates and sulphates. Some are better at it than others: they’re said to have a high “cation exchange capacity”. The surfaces also get coated in water, meaning that clay soils will hold water long after it has drained away from coarser, sandy soils.
One key chemical characteristic of the soil is its acidity, measured in terms of its pH, and that depends on the presence of lime (or calcium carbonate). Pure water is neutral, or pH7.0. Vinegar is 3.1, and Alka Seltzer 8.2. Acid soils restrict the availability of nutrients such as potassium, nitrogen and calcium, whereas alkaline soils can be low in iron, cobalt, boron and manganese. The optimum pH for most plants is somewhere on alkaline side of neutral, which is why adding lime to paddocks is a key agricultural activity in many parts of New Zealand. My truffle paddock, on the other hand, has so much natural lime that the iron levels in the soil are very low, so low that my young oaks suffer from chlorosis. Their leaves turn yellow, and they stop growing. They don’t die, they just get stunted — so if I want to get some good growth I have to go out and given them extra iron, either as a foliar spray or soil drench.
If the physical and chemical characteristics of the soil are complex, that’s nothing when compared to the complexity of the life it contains. Almost everything I’ve read about soil life starts with some staggering numbers: here are some. One millilitre of healthy soil could contain 50 nematodes ,62,000 algae, 72,000 protozoa, 111,000 fungi, 2,920,000 actinomycetes and 25,280,000 bacteria. The fungal threads (hyphae) in one teaspoon of soil would stretch 15km. The weight of earthworms in a healthy dairy paddock is greater than the weight of the cows grazing on it. And so on.
Life not only creates the soil, it is the soil itself — a vast, interlinked web of living things eating each other, cooperating with each other, modifying the soil in a myriad ways, responding to the weather and climate and the plants growing in it. When you dig a hole or plough a field, you’re creating carnage in the soil. The fresh ammonia smell of a newly ploughed field is caused by the death and decay of billions of bacteria.
Some soil bacteria live by decomposing plant and animal wastes, others can “fix” nitrogen, taking it from the air and converting it to a form that plants can use. Actinomycetes are also decomposers, while algae, a sort of single-celled plant, make sugars by photosynthesis. Some fungi are particularly good at rotting down wood and plant material, while others form beneficial associations with plants, exchanging plant sugars for nutrients extracted from the soil by the mass of hyphal threads. My truffles are this sort of mycorrhizal fungus. Animal life kicks in with protozoans and small nematode worms, eating bacteria, and themselves being eaten by larger nematodes. Bigger still, earthworms stir up the soil, mixing layers very effectively, while the grubs and larvae of moths and beetles — such as grass grub and porina moths — feed on plant roots.
Actinomycetes and fungi are also important because they form a kind of organic glue, sticking the soil particles together to create the various kinds of crumbs and textures we find when we cultivate the soil.
Although the numbers may be vast, the biomass of life in the soil may be as little as five per cent of the total organic matter present. The rest is humus, and it’s powerful stuff. Humus is usually described as a relatively stable, dark organic substance derived from the breakdown of animal and plant residues by microbial action. The precise composition of humus is impossible to pin down, as it varies just as much as the soil itself, but it is a hugely valuable source of nutrients for both plants and soil life. In energy terms, it’s a sort of solar battery, acting as a bank for nutrients and energy. One British scientist estimated that an acre of soil with 4% organic matter contained as much energy as 20 tons of coal in its surface layers.
Humus therefore represents an enormously valuable larder for the plants and animals that grow in or on it. It effectively stores surplus energy from the sun, making it available in future years. Humus is also a key part in helping to create new soils, building up as the soil ecosystem develops. A relatively small quantity of humus can turn sandy soils into rich loam, or clay into fertile fields.
If the soil acts as a nutrient bank for the plants we grow in it, and all the animals that live in it, it is also hugely important as a water storage medium. Clay particles, as we’ve seen, can absorb a lot of water, and water is also stored in the gaps and spaces in the structure of the soil. All the soil particles are coated in a film of water, and it’s in this film that much of the soil activity goes on. If the soil becomes totally dry, all soil life suffers — not just my trees. Just as the soil stores energy across seasons, so it can store water, re-charging over winter, and releasing it gradually as plants require it. Obviously, deeper soils with good clay content will do this better than thin or sandy soils.
Soils are also an important heat store, warming up during the day and then cooling only gradually during the night. Air temperatures can oscillate from frost to sweltering in a few hours, but the soil will react much more slowly — and smoothly. In spring, dark soils may warm up sooner than light soils, extending the growing season, and warm soil surfaces can help to protect plants from damaging frosts early and late in the year.
We’ve now looked at all the basic elements that make up any soil — from rocks to earthworms. It’s time to introduce the next level of complexity. Soils have profiles, and like humans, they can vary a lot. A profile describes how the soil changes with depth. At the bottom, you have the parent rock, grading upwards through rubble and stones into finer and finer particles, getting richer in humus, with a layer of vegetation on top. For some of the soils on my property, that’s all there is. The parent limestone has only a thin layer of soil on top. In other parts, the soil formed over the limestone (technically, a rendzina) has been washed down the slope, and overlies gravels. Those gravels were laid down by the river, and themselves cover different sorts of limestone, siltstone and mudstone. Leave the farm to walk to the truffle paddock, and although you may not notice much change in the surface soil, what lies underneath changes in rock type, sub-soil, depth and drainage characteristics. When I finally get my vineyard planted, I expect to be able to see vine by vine variation in vigour, and perhaps flavour, depending on the route their roots take down through all the different stuff underneath.
Soil profiles are obviously affected by geology and the history of the land. Volcanic eruptions leave layers of ash to make new land, glaciers move rocks and soils around, wind leaves drifts of dust or sand, while rivers flood and deposit silt and gravels. But soil formation is also affected by the plants that grow in it, the weather and climate of the location, and the shape of the land. Soils formed in very wet areas such as the West Coast can have the nutrients in them washed out by the continual rain. This process, called leaching, can push the nutrients down into the deeper layers of the soil profile. Soils formed under forests are very different to those formed under pasture, while the shape of the land can determine the thickness of the soil and how it drains. North-facing slopes will be warmer than South-facing ones. It’s a complex interaction.
A good place to see soil profiles is in road cuttings, preferably new ones. A minor interest in soil can add a whole new dimension to long road journeys, though “spot the rendzina” doesn’t seem to go down too well with the children. When you’re planning to plant a new crop, especially a deep rooting crop such as vines or trees, it’s a very good idea to dig a hole to check out the profile. It will tell you a lot about the water-retaining and draining properties of the paddock, as well as revealing potential problems with hard layers that may cause water to pond or be difficult for roots to penetrate. If you’ve got a hard “pan”, you will probably have to deep rip the ground before planting any tree or vine crop.
All of this information about soil structures isn’t much help if we don’t know how the soil interfaces with the plants that we’re trying to grow. Plants take in their nutrients as minerals. They can’t taken in the complex molecules in the humus directly, they have to be broken down into their constituent chemicals first. That’s why “chemical” fertilisers work so well. You’re giving the plant the nutrients it needs in a form it can directly access. The NPK and other stuff just dissolves in the soil water, and is there for the plant roots straight away. That’s why you can grow many plants hydroponically. Put the right stuff in the water, give the plant some support, and away they go.
Out in the wild, plants can’t rely on friendly growers supplying them with food. They have to find their own. A healthy soil, rich in humus, will have plenty of nutrients available for plant growth. Bacteria and fungi will mobilise the humus, and the plant roots and their mycorrhiza fungi will transfer them into the growing plant. The roots, in return, add carbon to the soil through decay and by being eaten, helping to create new humus. It’s a virtuous circle.
Enter the grower. Our aim is take a crop from the field. That could be grass, going into sheep or cows, transforming itself into meat, wool or aged Edam, or it might be olives, apricots, lavender or herbs. Although the energy for the process comes from the sun, we are taking nutrients away from the soil, and if we don’t put anything back, we are effectively strip mining the land. Over the years, the soil’s reserves, found in its humus, become depleted, and as a result the whole complex ecological fabric begins to look a bit frayed. If we choose to address the problem by adding chemical fertilisers, we can get yields up, but as the years go by increasing doses are needed to maintain production, until eventually it becomes too difficult or expensive to grow anything at all.
In my olive grove, I take nutrients and energy out of the grove in two ways — through the wood and leaves in the prunings each winter, and in the fruit I harvest (if the blackbirds don’t get it first). A lot of the stuff that the tree has used to make structure and fruit come from photosynthesis, but a lot comes from the soil. The trees can’t move on to a nice fresh paddock when they use up their own bit of soil, so it makes sense to put back as much as you take out. This is where choice comes in. The “traditional” approach is to add fertiliser, usually in the form of NPK formulations. The “organic” approach looks to enrich the soil by adding humus, trusting that the crop plants will perform well in the rich soil.
Enter politics, stage left. I suspect (or hope) that most readers will, like me, lean towards the organic end of the spectrum. Knowing that the soil is an incredibly complicated ecosystem, it seems sensible that we should feed the whole thing, rather than just the plants. Adding humus, mulching, feeding the microbes and fungi, all ensure that the nutrient and energy bank in the soil is kept in credit. In the long term, keeping your soil thriving with life will keep your crops producing good yields. And they may even taste better…