The purpose of soil cultivation is to improve the texture of the soil, enable it to store water without becoming waterlogged and to stimulate those natural processes within the soil by which its food reserves are liberated. Feeding may be long term to build up those reserves, or short term to increase the immediately available supply of plant food.
The chemicals most likely to be in short supply are nitrogen, phosphorus and potash and sometimes also magnesium, manganese and iron. All these can be added as chemical fertilisers or in animal manures and animal and vegetable waste in which they are present in varying quantity. The process of decay of these bulky organic materials liberates the chemicals and also produces humus, a brown, structureless, slimy substance which is extremely valuable in improving soil texture. Peat is rich in humus and for that reason is a valuable soil dressing despite the fact that it contributes little chemical food.
Deep cultivation of the soil is only possible when it is vacant. It is never wise to dig near established trees and shrubs, since this will destroy many of their roots which are quite near the surface. Even forking pan do damage, though pricking the surface is a useful method of getting rid of weeds, letting in air and stirring in topdressings of fertiliser or manure. Hoeing will produce similar results. Deep rooted perennial weeds cannot be destroyed by such light cultivation.
When plants are moved from one place to another they inevitably suffer some check to growth, though the less the roots are injured or the soil around them disturbed the smaller this check is likely to be. That is the justification for growing young plants in containers from which they can be removed with a complete ball of soil and roots. Such plants can be moved at almost any time of the year, whereas plants dug up from the open ground can as a rule only be moved safely at certain periods of the year. Most deciduous trees and shrubs (those that lose their leaves in winter) and roses are planted between late October and March. Evergreen shrubs and trees are most safely moved in April, May or October, though with care they can also be moved during mild spells in winter. Most herbaceous plants are best planted in March or April, though some transplant quite well in October