Knowing and Improving Your Soil
Soil Health and Fertility
Good soil drains well but retains plenty of moisture that roots can access. It is easy to dig and full of organisms, such as earthworms, beetles, bacteria and fungi. Organic matter is a vital component of soil and is broken down by tiny organisms to release nutrients and improve the soil's water-holding capacity. Healthy, fertile soil is a rich dark brown, but whatever the color of yours, improve it by digging in organic matter, such as compost and manure, or applying it every year as a surface mulch over well-watered, moist soil.
Knowing Your Soil's pH
The pH scale measures the degree of soil acidity, which determines the availability of nutrients, as well as the presence of beneficial soil organisms and less desirable soil-borne diseases. Low numbers on the pH scale indicate acidic soil; pH 7 is neutral; higher numbers show an alkaline soil. Garden soils normally fall between pH 4.5 and pH 7.5, but the ideal for vegetables is pH 6.5. Lime can be applied to increase the pH, but it is more difficult to lower it. Soil testing kits are widely available and easy to use. Simply mix a soil sample in a test tube as directed, and compare the color of the solution to the chart to determine the pH.
Determining the Soil's Texture
The size of the mineral particles in soil determines its texture, which tells you how it should be treated during cultivation. There are three basic soil types - clay, silt and sand - which occur in varying proportions. Pick up some of your soil and squeeze; when moist, clay soil forms sticky clumps and is shiny when smoothed. Clay soil holds water and nutrients well but can have poor drainage and be heavy to work.
Light, sandy soil falls through your fingers when squeezed; and fine silt feels silky. Sandy soil is light to dig, but dries out and loses nutrients rapidly.
Digging in Organic Matter
Improve the structure of soil by digging in bulky organic matter every year. This is most beneficial on sandy soils, where it helps to retain moisture and nutrients that are otherwise quickly lost. Adding organic matter along with grit to clay soils can help open up the soil structure and improve its sticky texture.
Well-rotted garden compost is a valuable form of organic matter, but well-rotted farmyard manure, spent mushroom compost and green manures are all useful alternatives. During a dry spell in fall or winter, spread a 4- to 6-inch layer of organic matter over the soil and dig it in to a spade's depth. Alternatively, sow a green manure, such as mustard, cut it down before it becomes woody, leave to wilt, then dig into the soil.
Adding a Surface Mulch
Mulching is simply placing a layer of bulky organic matter or a plastic sheet over the soil surface. This practice is beneficial because it prevents moisture from evaporating from the soil surface, controls the soil temperature and inhibits the germination of weed seeds. Organic mulches are also drawn into the soil by earthworms, where they break down and improve the soil structure. Garden compost, well-rotted manure, spent mushroom compost and straw can all be used as mulch and should be applied in a generous layer around plants. Leave gaps around plant stems, though, otherwise rot can set in, and apply the mulch to moist soil because it is more difficult for water to penetrate the thick covering and wet dry soil.
Liming Acidic Soils
By adding lime, you can increase the pH of acidic soil to make it better suited to vegetable growing. Crushed chalk or ground limestone are the safest and cheapest forms of lime to use. Apply them at the recommended rate for your type of soil; clay soils require heavier applications than sandy soils to achieve the same effect.
Treat the soil in fall or winter, at least four weeks after adding any bulky organic matter. Divide the plot into 1-square-yard sections with string lines. Wearing gloves, long sleeves, safety glasses and a mask, weigh out the correct amount of powder to treat 1 square yard. Sprinkle this evenly onto the soil with a spade, repeat until the whole plot is covered, then rake the lime into the soil.
Tidy up weed-covered ground by skimming annual weeds off the surface and burying them in a trench. Remove any perennial weeds first, then shallowly slide the spade under the surface to lift the weeds in sections.
Dig a trench in the area that you have cleared and lift the sections of weedy soil into it, so that the weeds are upside down. This process can be repeated across a large area and the weeds will break down in the trench and improve the soil.
This is the usual way to cultivate and add organic matter to the soil. Spread a layer of organic matter over the whole plot, to be incorporated as you dig. Dig a trench about 12 inches wide, to one spade's depth, across the plot and take the soil to the far end of the plot in a wheelbarrow. Use soil from a second trench, dug next to the first and to the same dimensions, to fill the first trench. Continue the process across the plot, until the soil moved from the first trench is used to fill the last.
On a new site with deep topsoil, dig a trench 2 feet wide to a depth of two spades, and add some organic matter to it. Move the soil to the end of the plot. Dig a second trench of the same size next to the first-use the soil to fill the first trench. Add organic matter to the second trench, and repeat these steps across the whole plot. If you have thin soil, dig to just one spade's depth and fork the bottom to a further spade's depth to avoid bringing infertile subsoil up to the surface.
Achieving a Fine Tilth for Sowing
A level soil surface with a fine crumbly texture, without large stones or old plant material, gives seeds and young plants the best start. This can be achieved by lightly drawing a metal-headed rake over previously dug soil in one direction, then again at 90 degrees to the first raking.
Good soil preparation prior to planting should supply plants with adequate nutrients, but for newly cultivated or poor soil, it may be necessary to apply a top-dressing of fertilizer while plants are growing. Fertilizers contain nitrogen (N), phosphorus (P) and potassium (K) in various ratios, so check the products available and choose one that suits your needs. Wear gloves when handling fertilizers and apply them at the recommended rate-excess nutrients can be harmful to plants. Scatter fertilizer evenly over the rooting area of the crop and avoid dropping any on the leaves because it could scorch them. Incorporate into the soil surface using a push hoe.
Text copyright 2007 Royal Horticultural Society