Organic matter that’s gone through some degree of decomposition is called humus. Active humus is made up of matter rich in nitrogen, such as kitchen vegetable waste, grass clippings and young (seedless) weeds; stable humus is composed of materials that are high in carbon, such as wood chips, woody waste from garden pruning and conifer needles. When they’re blended together in roughly equal proportions, we call the resulting mixture “compost.”
Other common soil additives include gypsum (calcium sulphate) that helps to loosen heavy clay soils, and lime and sulphur, which some gardeners use in an attempt to change their soil’s natural pH. Soil pH is usually determined by the kind of parent rock that your soil is derived from: in areas where the parent rock is limestone, the pH will tend to be neutral to alkaline (7.0 to 8.0), whereas granite is apt to produce acidic soils (5.0 to 7.0). I don’t recommend attempting to change your soil’s pH; it’s much easier to choose plants that are suited to the type of soil you already have. Working with Mother Nature, rather than against her, always produces the happiest results.
Make your own soil conditioner
Mix together one part active humus and one part stable humus (or two parts compost if you have sufficient) with one part well-rotted manure (for extra nutrients) and one part shredded leaves or leaf mould (for aeration). For new beds, add a thick, 10-centimetre layer to the soil surface, and dig the conditioner in to a depth of 25 centimetres. For established plantings, rough up the soil surface with a cultivator or stiff rake to a depth of three centimetres, and spread an eight-centimetre layer of conditioner on top. You can add soil conditioner at any time from early spring to late autumn.