You can use a wide variety of soil additives to improve the soil, although choice is often governed by price and availability. Fertilizers come in many forms - both organic and inorganic.
All fertilizers are labelled to show their nutrient content in terms of nitrogen (N), phosphorus (P), and potassium (K). These are in even quantities for general purpose fertilizers such as blood & bone; bone meal and superphosphate is high in phosphorus; tomato feeds and sulphate of potash are high in potassium; and chicken manure and sulphate of ammonia are high in nitrogen.
Bulky organic material
Unlike in organic fertilizers, bulky organic materials improve soil structure and texture. They are also often rich in nutrients, which inorganic fertilizers can lack, and release their nutrients slowly. It is advisable however, to compost bulky organic material before use, particularly animal manures, which must never be used fresh as they can scorch young or tender plants due to the freely available nitrogen compounds they contain. Horse manure is the richest, followed by pig, cow, and poultry.
Garden compost is another useful source of bulky organic material that is nutrient rich and can be used as a soil conditioner or a mulch. Spent mushroom compost - usually a mixture of manure, loam and lime and straw - is a by product of the mushroom industry and makes an excellent soil conditioner or mulch. It can be used in all soils but avoid using it around around lime sensitive plants.