John Innes Composts are a blend of carefully selected loam or topsoil, sphagnum moss peat, coarse sand or grit and fertilisers. The loam is screened and sterilised and then thoroughly mixed with the other ingredients in proportions designed to achieve the optimum air and water-holding capacity and nutrient content for different types and sizes of plants.
JOHN INNES POTTING COMPOST No.3 is a richer mixture for final re-potting of gross feeding vegetable plants and for mature foliage plants and shrubs in interior planters or outdoor containers.
Before the introduction of John Innes Composts, gardeners generally used a different compost for each species of plant. Usually the soil was not sterilised or heat pasteurised and consequently plant seedlings were often attacked and destroyed by soil-borne diseases and insects. Also the plant foods being added to the traditional composts were usually unbalanced, causing the plants to be either too "soft" in their growth and liable to diseases, or very "hard" and slow growing.
In the 1930's two research workers at the John Innes Horticultural Institute, William Lawrence and John Newell, set out to overcome these problems and to formulate composts that would give consistently good and reliable results. After six years of experiments they determined the physical properties and nutrition necessary in composts to achieve optimum rates of plant growth. They also introduced methods of heat sterilising the soil that eliminated pests and diseases, but did not cause any checks to plant growth.
The result of this work was the introduction of two standard composts, one for seed sowing and one for potting. These "John Innes" composts revolutionised not only the ways in which composts were produced, but also the growing of plants in pots. Now, after being used very widely for over 50 years, the basic formulae remain the same - tried and tested and still popular amongst discerning gardeners for growing the best quality plants with the minimum of attention. Naturally, the plant nutrients have been updated to gain the benefits of improved fertiliser technology.