Loam composts, typified by John Innes composts, are based on loam sterilized to eliminate the soil-borne fungi (see damping off) and insects that largely caused the unreliable results from traditional composts. There is a risk of ammonia toxicity developing after sterilization of soil with pH greater than 6.5 or very high in organic matter (see nitrogen cycle). Induced nutrient deficiencies are possible in soils with a pH greater than 6.5 or less than 5.5. Furthermore, loam should have sufficient clay and organic matter present to give good structural stability (the original specification identifies 'turfy clay loam'). Peat and sand are added to further improve the physical conditions: the peat gives a high water-holding capacity and the coarse sand ensures free drainage and therefore good aeration. There are two main John Innes composts, one for seed sowing and cuttings, the other for potting.
John Innes seed compost consists of 2 parts loam, 1 part peat and
1 part sand by volume. Well-drained turfy clay loam low in nutrients and with a pH between 5.8 and 6.5; undecomposed peat graded 3 mm to 10 mm with a pH between 3.5 and 5.0; and lime-free sand graded 1-3 mm should be used. 1200g of superphosphate and 600 g of calcium carbonate (lime) are added to each cubic metre of compost.
John Innes potting (JIP) composts consist of 7 parts by volume turfy clay loam, 3 parts peat and 2 parts sand. To allow for the changing nutritional requirements of a growing plant, the nutrient level is adjusted by adding appropriate quantities of JI base fertilizer which consists of
2 parts by volume hoof and horn, 2 parts superphosphate and 1 part potassium sulphate. To prepare JIP 1, 3 kg JI Base fertilizer and 600g of calcium carbonate are added to one cubic metre of compost. To prepare JIP 2 and JIP 3, double and treble fertilizer levels respectively are used.
Whilst the standard JI composts are suitable for a wide range of species, some modification is required for some specialized plants. For example, calcifuge plants such as Ericas and Rhododendrons should be grown in a JI(S) mix in which sulphur is used instead of calcium carbonate. All loam-based composts should be made up from components of known characteristics and according to the specification given. Such composts are well proven and are relatively easy to manage because of the water-absorbing and nutrient-retention properties of the clay present.
These composts are commonly used by amateurs, for valuable specimens, and for tall plants where pot stability is important; but loam-based composts have been superseded in horticulture generally by cheaper alternatives. The main disadvantage of loam-based composts has always been the difficulty of obtaining suitable quality loam ('turfy clay loam'), as well as the high costs associated with steam sterilizing. Furthermore, the loam must be stored dry before use and the composts are heavy and difficult to handle in large quantities. Many of the loam-based composts currently produced have relatively low loam content and consequently exhibit few of its advantages.
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