From the Ground Up

SOIL is the most important element of your garden, but soil types and treatments can be confusing. Connect's NELL WILLIAMS takes you back to the earth - soil type by soil type, treatment by treatment - and tells you how to get the best from your garden

THE organic gardener sees soil as a resource, and soil management forms an important aspect of organic and greener gardening. Managing soil fertility through the addition of manures and other organic material, and by crop rotation, along with an understanding of the basics of soil science will enable gardeners to get the best out of their land.

Soil is formed by the breakdown of rocks over millions of years through chemical or physical processes. Chemical reactions alter the mineral composition and tend to produce clay particles. Physical processes, such as freezing and thawing of water, cracks the rocks, or erosion of streams and rivers produce silt and sand particles. Most soils tend to be a mixture of these.
In addition, soil contains organic matter, living organisms, air and water. The combination of different quantities of these, along with sand, silt and clay particles, give soil its structure - its ability to be crumbly, free-draining soil, or a water-logged bog.

Most organic matter and organisms are located in the first 15-25cm of soil, known as the topsoil. Here, most of the roots grow. Below this is the subsoil, which contains fewer nutrients and living organisms, and thus fewer roots. And under this is the parent material - the rocks from which the soil was derived.

Soils are also acid or alkaline. This acidity is known as the pH of the soil, and is measured on a scale of 1-14, with 1 being very acidic, through 7, which is neutral, to 14, which is highly alkaline. In practice, most soils fall between pH3 for acidic peats to pH8.5 for soil overlying chalk. The majority of vegetables and other plants do best on soil that is neutral to slightly acidic (pH6.5-pH7).

The pH of the soil affects crops in a number of ways. First, it can affect the availability of chemical elements important for plants and soil microbes. A low pH leads to a shortage of calcium, phosphate, nitrogen and sulphur available, and an increase in metallic ions affecting nitrogen fixation in the plants. At a high pH level, iron, manganese, boron and phosphates occur in insoluble forms.

Plants are also more susceptible to fungi at certain pH levels. For example, club root of brassicas is worse on acidic soils. However, with potato scab, the fungus is less tolerant of acidic conditions than the host, so potatoes grown on more acidic soils are less likely to suffer.
The numbers and variety of soil organisms is also affected by pH levels - this affects breakdown and incorporation of leaf litter and other organic matter.

So what type of soil do you have?
There are many different ways to establish this.

Just looking at your soil can tell you a lot. Certain wild plants prefer specific conditions and can therefore be used as indicators of soil type. Dock, creeping buttercups and dandelions are all indicative of water-logged and heavy clay soils. Nettles are found on acidic soils and show high levels of fertility, as does chickweed. Scarlet pimpernel grow on sandy, light soils and bladder campion is found on neutral to alkaline soils.

Now pick up a handful of soil. Rub it between your finger and thumb. Sandy soil feels gritty, silty soil is silky and smooth. If you can roll it into a sausage and then bend it into a ring, you have clay!

pH can be measured using kits available from garden centres and by mail order. pH can vary across a site, so it is worth doing a few tests if you have a large garden or allotment.

Soil types
Clay is made of very small particles. Clay soils are heavy and cold, and take a while to warm up in spring. But they are highly fertile and hold nutrients well. However, they are sticky and difficult to work in winter - and dry out to concrete-like lumps in the summer.

Treatment: Add lime to bind particles together to create larger crumbs. This improves drainage and aeration. The addition of organic matter does the same thing. Digging over in winter allows freeze/thaw action to help break up clods of clay.

Silt is made up small particles, but they are larger than in clay soils. Again these have a high level of fertility.

Treatment: The same as for clays - but not as much hard work!

Sand warms up quickly in spring, and is free-draining. But the loss of water means nutrients are also washed (or leached) out.

Treatment: The addition of organic material helps retain moisture and binds particles together. It is especially important to keep sandy soils covered to prevent leaching of nutrients - plant green manures in the autumn to over-winter the soil.

Peat is highly fertile, but very acidic.

Treatment: Lime will need to be added for some crops.

Chalk is soil rich in calcium carbonate. It occurs where bedrock is composed of limestone or chalk. The topsoil tends to be thin and stony and loses water and nutrients easily.

Treatment: Chalk warms up quickly, like sandy soils, and should also be kept covered to prevent leaching.

The Royal Horticultural Society can provide laboratory soil testing. Contact them at: The RHS Soil Analysis Service, RHS Garden Wisley, Woking, Surrey GU23 6QB.

So now you've established your soil type, how do you make the most of it? The best way is the addition of organic matter, which will improve structure, nutrient levels etc. Organic matter can be produced in the following ways:

Compost heaps
These were mentioned in the last issue of Connect.

All organic matter will rot down eventually. The aim of the compost heap is to speed up this decomposition.

Garden waste is broken down by the actions of bacteria, fungi and worms. To work effectively, these organisms need a balance of nitrogen and carbon, moisture and aeration. Nitrogen-rich materials include grass cuttings, green vegetation, fresh manure and vegetable waste. Straw, dry leaves, sawdust, shredded newspaper and cardboard are all high in carbon.

Bins should be an optimum size of one metre square. Fill as shown in the diagram. Cover the heap to retain heat and keep the heap damp. Organic compost activators can be purchased from garden centres, or use poultry droppings, urine or comfrey leaves instead.
Compost should be ready to use after 3-6 months.

Wormeries
Dendrabaena venerta and Eisenia andreii, also known as brandling or tiger worms, can be used to break down food waste, especially cooked leftovers that would otherwise be unsuitable for composting.

Wormeries can be situated near to the house, making them easily accessible when you don't want to walk to the compost heap in the rain!

In addition, liquid is produced which, when diluted, can be fed to your plants.

Build your own in a suitable container, or purchase one from a company such as Wiggly Wigglers, Lower Blakemere farm, Blakemere, Herefordshire HR2 9PX. Tel: 01981 500391. They also supply worms for you to add to you own wormery.

Green manures
These are fast-growing crops which are sown to help prevent weeds, bind the soil and protect it from winter rains, and to improve the condition of the soil.

There are a number of different types, which include clover, grazing rye, alfalfa, Phacelia, lupins, mustard and vetch. Some are sown in the spring, while others, sown in autumn, over-winter and help stop leeching and erosion of the soil by winter rains.

All are dug back in before they seed and are then left to rot. The nutrients stored in the plants are returned to the soil, while the stems and leaves add bulk, improving aeration and drainage, which is good for biological activity.

Comfrey
This shade-tolerant perennial is a good source of minerals, with levels of nitrogen and phosphorous similar to those in farmyard manure and compost. Potassium levels are much higher, making it an ideal feed of tomato plants.

Russian comfrey is a vigorous hybrid which can be cut four times during the goring season. It doesn't set seed, so your garden will not be swamped, as can happen with other varieties. It can be established from plants or root offsets - these are produced by division of the crown into pieces of root, each with a growing point attached.

Leaves can be used as a compost activator, put into trenches when planting potatoes, used as a mulch, or to make liquid feed. To do this, put the comfrey leaves into a container and cover with water. Leave for 4-5 weeks, by which time you will have a liquid feed for tomatoes and other plants. Throw the old leaves onto the compost heap and start again. But beware - the liquid smells, so cover with a lid and site as far away as possible from your house!

Crop rotation
The idea behind this is that different crops take different nutrients out of the soil. By rotating the crops grown, you don't deplete the soil of specific nutrients. Of equal importance is the prevention of build-up of disease within the soil. Three or four-year rotations are used. See table opposite.

Tomatoes are the same family as potatoes and should thus be in the potato rotation, while onions are planted with legumes. For a three-year rotation, potatoes and roots are combined. On a small plot, rotation can be difficult to achieve, so the basic principle to remember is - don't plant the same crop in the same bed each year.

Double digging
This is a term you may have heard of before. Some gardeners do it, others don't feel it is necessary. The technique is most useful for ground that has just been cleared. By breaking up the subsoil, drainage is improved. The addition of organic matter at the same time improves soil fertility and structure. The basic technique is as follows:

Dig out a trench a spade's depth to the width of your bed. Place this soil in a wheelbarrow or on a sheet of plastic for later.

Ford over the soil in the bottom of the trench to break it up. Cover this with 5-10cm layer of compost/manure. Dig the topsoil from the next section of your bed and cover the organic matter.

Fork over the subsoil you have uncovered and add organic matter. Continue this until you reach the end of your bed. The soil in your wheelbarrow will fill the remaining trench.

Raised beds
If your soil is particularly bad - very stony or the sort of heavy clay you feel is only good for making pots - raised beds may be the best way forward.

Basically, you divide your plot into beds no more than 4ft wide, so that you can reach the middle from the other side. They may be any length, but remember you need to walk round them because they are never to be trodden on.

The beds are best edged with timber to stop the sides collapsing, but this is not essential. Remember to treat the wood with a suitable preservative first.

Either dig over the soil contained within the timber frame or, if it is really poor, you may need to purchase some good topsoil - look in the phone directory for suppliers.

Add compost or well-rotted manure each year and you will have lovely, crumbly, uncompacted soil in which your plants will thrive.

Purchasing topsoil is not very environmentally sound because it is depleting somewhere else, so buy only the minimum amount you need. Or be patient and keep adding organic material slowly to fill the beds.