The consistency of soil is as important as its fertility, so before you add plant foods, try to develop a soil mixture of good texture, a soil in which moisture is retained long enough to benefit roots but not so long as to rot them. If the soil mixture is too thin, water runs through too fast for plants to benefit; if it is too heavy and always wet, oxygen cannot reach roots. In the intricate process of food production, oxygen and moisture in the soil, carbon dioxide in the air, along with sunlight, operate in conjunction with the green coloring (cholorophyll) in the leaves to change food elements into forms that plants can assimilate.
Loam from a well-worked flower bed may have fairly good consistency, for loam is composed of clay, sand, and organic matter, their proportions depending on where you live.
Organic matter is material that was once living, as leafmold or other decayed vegetation, manure, or peatmoss. Very-well-rotted leaves are an invaluable source; you may even put them through a coarse sieve for easier handling. You may already have a compost pile where in the fall you heap up leaves with or without layers of soil or sod between. Even a small compost pile is useful, and leaves retained long enough in a wire circle behind a big forsythia bush are bound to rot; these can be an excellent and easy source of organic material in which beneficial bacteria develop another factor in the good growth of plants.
Manure is a source of organic material, but scarce today in the fresh form. If you can obtain this, do not use it until it is well rotted, which takes at least six months. With fresh manure, there is danger of burning roots of plants. Readily available is dehydrated manure sold in bags. This can be used at once to improve the consistency of soil. Then there are the various peatmosses long-decayed plants from bogs or swamps. Peatmoss offers an excellent means of retaining moisture in a soil mixture.
Sand has an opposite use, and I refer to builder's or mason's sand, not the seashore type. Sand loosens soil to allow proper drainage after the water-holding elements present have performed their function. In heavy or sticky soils, sand is a vital ingredient, for it separates the clay particles; more sand is required with heavy clay than in a light open soil. There are also a number of sterile commercial preparations like perlite or vermiculite that are as good as sand to lighten soils.
Clay is the mineral-rich element in soil, essential to your mixture in the proper proportions. With these essential elements, then, you can produce soil mixtures to suit most plants and, if your garden loam is particularly fertile, extra plant food may not be needed. There are bound to be some nutrients in the various organic materials you use, though not a great deal. However, many a handsome plant has grown and bloomed in the old "equal-thirds" mixture, suggested here for general use:
A modification of the above is better for plants that require much moisture but good drainage:
For plants that definitely require an acid soil, use: