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Soil is the medium in which plants grow - the earth beneath our feet. Soil is not 'dirt' until we track it all over the kitchen floor. Next to water and the air we breath, soil may be our most precious resource. Despite the wonders of modern science and technology, our survival depends on our ability to preserve a very limited amount of fertile soil.
Soil is essentially rock and the remains of once-living things. The mineral content of soil is the degraded remains of rock, reduced to minute particles over staggering amounts of time by erosion and weathering. The organic component is the remains of plant and animal life.
The nature and quality of soil, therefore, depends on the type and mixture of the minerals and organic matter, as well as the amount of air and water that it contains.
Soil gradually evolves from bare, sterile rock to the glorious stuff which is treasured by gardeners. The long history of any soil is recorded in distinct layers called horizons, which are used to classify soil types according to the relative depths of different horizons within a soil profile, like the layers of an exotic cake.
The scientific classification of soil types is quite complex. But most gardeners are happy to think of soil as being either clay, silt, sand, or loam. These are the 'primary colours' which blend to make up the subtle hues of our garden's soil.
A heavy soil with a significant, though smaller, proportion of organic matter is a clay-loam soil.
A sandy soil with roughly the same organic content is a sandy-loam soil.
A soil with very little of anything but decomposed organic matter is a loam soil.
A soil without clay, silt, or loam is a beach.
Texture and Structure
Texture is determined by the size of soil particles; and structure is the way that they are arranged. The difference affects the speed that water percolates through the soil, and the rate at which gasses are exchanged. The difference most likely to announce itself to the gardener is the way that clay soils compact more than sandy soils, and become difficult to work.
All garden soils contain a mixture of the basic components: sand, silt, clay, and loam. To decide which you have in your home garden, pick up a handful of moist soil, and give it a good squeeze...
If the soil crumbles in your hand, it can be considered a sandy soil.
If the soil forms a ball, which resists breaking but will crumble with a little persuasion, it is a silty soil.
If the soil forms a tight ball, which will not easily fall apart, you have a clay soil.
If the ball suggests that you sculpt it into an ashtray or tiny Venus de Milo, you have a problem.
Whatever form the mineral content of your soil takes, the quality of its structure can be improved by adding organic matter, such as compost or peat. Your garden will be 'greener', however, if you opt to use homemade compost rather than peat, which is a rapidly disappearing part of the natural landscape.
Adding organic matter makes gardening a much less back-breaking chore. It will also provide a much healthier home for your plants. Organic matter is, itself, an important source of food for your plants. But, more importantly, it provides valuable exchange points, on a molecular scale, for plant roots to take up dissolved nutrients. It binds sandy soils, and helps them to retain moisture, which would otherwise run straight through. And it breaks up clay soils, providing valuable pore spaces for water and for gas exchange.
Tilling and Tilth
Tilth refers to the state of a soil's structure. Tilth is how well the particles of soil bond together. Tilling is turning over the soil and mixing it up in preparation for planting.
A great deal of research is being devoted to tilth, because understanding how tilth works is fundamental to being able to control the degredation of our farms' soil, and, therefore, how long they will remain productive. This is very serious stuff.
Tilth is affected by soil type, organic content, water content, and cultural practises, such as tilling and irrigation. In healthy soil, which contains a high proportion of organic material and adequate water, the tilth generally remains good despite being subjected to tillage. Soils which are depleted of organic material or desicated degrade rapidly, and are soon lost to erosion. In the 1930's, vast amounts of farmland simply blew away in a catastrophe known as the Dust Bowl. John Steinbeck described the desperation of a nation losing its farmland in The Grapes of Wrath:
Car-loads, caravans, homeless and hungry; twenty thousand and fifty thousand and a hundred thousand and two hundred thousand. They streamed over the mountains, hungry and restless - restless as ants, scurrying to find work to do - to lift, to push, to pull, to pick, to cut - anything, any burden to bear, for food.
The stakes are obviously lower for home gardeners. But it is important to conserve your soil, from a moral standpoint as much as from the urge to grow prize dahlias.
The key to creating and conserving good soil is to think of it as a living community, a tiny city, in which all of the residents have needs. This means providing the soil with the organic matter and the moisture required by micro-organisms for their sustenance. It is these tiny organisms that govern the soil's fertility and how well your plants will grow.
Tilling the soil and routine cultivation are necessary to reduce soil compaction caused by irrigation, traffic, and time. Compacted soils don't absorb water well; they dry out quickly; and they trap gasses, which are harmful to plants' roots. Tilling and cultivation are important ways to conserve moisture in the soil and thwart weeds. They improve the appearance of your garden, and make it look less derelict. But too much tilling can quickly destroy the tilth of your soil and turn it into dust.
In the old days, when gardens were tilled with a fork or spade, this wasn't a serious problem, because any sane gardener would stop for a cup of tea long before he could work his soil to death. And only an idiot picks up a job that doesn't need improving. Nowadays, many gardeners turn over their land with power tillers, which pass quickly from a job done adequately, to a job done to excess, to one done to death, in no time. A fine, fluffy bed looks lovely. But your tiller shouldn't do the work for which the rake hanging in the shed is intended.
One of the most important demands on a gardener's time is making sure that plants are properly watered. The process of delivering water to where it is needed, whether you use an old tin watering can or a state of the art, computer-controlled system is called irrigation. How often you have to water your garden depends on the climate, what type of plants you are growing, and the state of your garden's soil.
Water enters the soil by falling on its surface and percolating through the tiny spaces between the aggregates which make up the soil's structure. It can also rise from below by capillary action. After a heavy rain most of the water may run through the soil and drain away. But a certain amount will remain in the soil, adhering to the soil particles. The amount of water that the soil retains in this way is referred to as Field Capacity. Clay soil has a higher field capacity than sandy soil, because the size of the aggregates is very much smaller and an equivalent volume has a much higher surface area. Adding organic matter, such as compost, to the soil is a good way to improve field capacity.
When so much water is absorbed by the soil that the spaces between the aggregates fill, then the soil has reached its Saturation Point. This water is in excess of the amount that the soil is capable of retaining, and will normally drain away. This is important because saturated soil can be a problem for plants. The flooded pore spaces can drown plant roots; and toxic gasses can become trapped, with lethal consequences. Salt residues on the surface of the soil indicate that there is a serious problem with drainage that requires your immediate attention.
Permanent Wilting Point
This sounds serious... and it is. Prolonged periods of drought may result in the amount of water in the soil dropping to a level which is less than that being taken up by plants' roots. When soil is so desiccated that it reaches its Permanent Wilting Point serious attention should be given to irrigation, because your garden is about to die. Permanent Wilting Point also indicates that you are a slovenly gardener who is in imminent danger of being ostracized or chased away by your neighbours.
Ideally, your garden should be watered enough that the soil remains close to its Field Capacity. This can vary quite a lot, depending on the type of plants in the garden. How to judge when your garden has had enough water is really a matter of experience. But it is better to water thoroughly and deeply than it is to sprinkle a little bit on the surface, which will evaporate before it does any good. It is also better to water early in the morning than in the heat of the day. Watering at night can lead to problems with fungi, as the leaves remain wet for a long period, promoting the growth of spores.
The Nitrogen Cycle
There are a number of ways in which animals can contribute to soil fertility. The least popular, of course, is by dying and decomposing, although this is a choice nearly all of them select in the end. The most important - and the most fun - is the nitrogen cycle.
Picture a circle. At a point on its circumference stands an animal of some sort - a landscape architect, let's say. The landscape architect eats whatever plant life happens to be close by, and passes urine1, which is absorbed by the soil. The urine (ammonia) is converted by bacteria in the soil first to nitrites, then to nitrates; which are used by plants to produce chlorophyll for photosynthesis, which promotes new growth, which in turn is eaten by other landscape architects. And so on...
Atmospheric nitrogen enters the cycle, when lightning produces nitrogen compounds, which fall to earth in rain drops. These can be used by certain plants (legumes) without the intercession of landscape architects.
A biproduct of the nitrogen cycle is nitrous-oxide, otherwise known as laughing gas. So, if you hear the micro-organisms in your soil giggling... now you now why.