Not many of us are naturally blessed with a good rich loam soil that is ideal for growing all plants. Luckily, if you have poor soil, it is reasonably easy to improve it, so that nearly all plants will thrive.
The two soil extremes are heavy clay and light, sandy soils. Both can be improved by adding lots of bulky organic matter to improve the structure and widen the range of plants that will thrive.
Digging will help to improve the drainage of clay soils but is unnecessary on loam or sandy soils. While you are digging, incorporate as much bulky organic matter as you are able. Your compost heap will provide well-rotted material made from vegetable peelings, grass cuttings and other plant material such as fallen leaves, dead annual bedding plants and annual weeds. Alternatively, you can make leafmould from fallen tree and shrub leaves which will take a year or two to decompose to use.
If you don’t have enough material from your garden compost for your needs, then you will need to buy in suitable materials. These can include well-rotted manure, mushroom compost, composted bark, all-purpose compost, tree and shrub planting compost and soil conditioners.
Also remember to dig in any compost from spent growing bags, patio pots and hanging baskets once they are finished.
Improving heavy clay soil
Clay soils are usually cold, wet and sticky for most of the year but in dry weather, they dry out and can turn solid with surface cracks appearing or the surface cakes over. On the positive side clay soils are innately fertile and hold a lot of moisture and plant food which is not leached away by rain. Good clay soil will grow all plants well - a poor soil will only grow poor plants!
Dig any unplanted areas in early autumn and add a generous amount of organic matter as you go. Leave the clods rough so that frost can break down the structure. A dressing of gypsum and even sharp sand or horticultural grit will also help in this process of producing a crumb structure. Repeat the process each autumn to help produce crumbly textured soil.
Soil in between plants can be gradually improved if bulky organic matter is forked into the top 15cm (6in) of soil each autumn. A mulch layer of material applied each spring around established plants will also help improve the structure and the amount of worm and micro-organism activity.
Improving light sandy soils
Light sandy soils soon run short of nutrients and water quickly drains out of them which means watering is required frequently during summer. Plants will probably establish a shallow root system.
The way to improve this type of soil is to add bulky organic matter in spring. Use plenty of farmyard manure, garden compost or organic soil conditioner when planting to give moisture-holding material at the root level. Mulch all over in late spring to reduce evaporation and use ground cover plants to shield the soil.