Many different types of compost are available. If a commercially produced compost is to be used for the growing of begonias, it must be borne in mind, that the correct compost is purchased for the different stages of growth. When sowing seeds, starting tubers and rooting cuttings, a seed compost is required. For all other stages a potting compost is used. Levington make a range of composts, which have been successfully utilised by many growers. Because begonias require a very open, free draining compost to grow in, the addition of one part coarse sand (or perlite) to three parts compost, will be required.
Many of the Societies' leading growers prefer to mix their own compost, ensuring that it is always freshly mixed. Begonias thrive in a soil based compost and although over the years many different mixes have been tried, none have improved on the original John Innes formula. To mix a John Innes compost the following ingredients will be required; 7 parts loam. 3 parts peat. 2 parts coarse sand.
For every bushel (8 gallons) of the above mixture, John Innes base fertiliser and a small amount of lime has to be added.
The amount varies depending on the type of compost required as follows;
John Innes No 1. 3 3/4oz of base fertiliser. 3/4 oz of carbonate of lime (ground chalk).
John Innes No 2. 7 1/2 oz of base fertiliser. 1 1/2 oz of carbonate of lime.
John Innes No 1 is used for first potting of tubers, pricking out seedlings and potting cuttings.
John Innes No 2 for final potting.
The mix can be improved by replacing half the carbonate of lime by dolomitic lime which will help to provide magnesium. A coarser sand or grit should also be used in the final mix, to help keep the compost open.
John Innes compost, is only as good as its principle ingredient, the loam. The proper procedure is to cut turves from old pasture and stack for six months to a year before using. Lay the first layer of turves, grass side down and give a good coating of old cow manure and lay another layer on top and give a good dusting with ground limestone. Continue with this pattern until the stack is completed and cover with black polythene. The end result is a good soil structure full of fibre. If this is not possible a viable alternative is to use molehills. It must be noted that the lime added when mixing the compost only neutralises the acidity of the base fertiliser. To remain strictly true to the John Innes formula the mix of loam, peat and sand should have a pH of 6.5 and it is advisable to have it tested before use. The loam should also be sterilised, but it has been found that this is not necessary if it is disease and pest free.
Due to the difficulties experienced in obtaining a good quality loam, more growers are turning to peat based composts. Chempak produce a base fertiliser for mixing up a peat based compost. This contains lime to correct the pH level (peat is very acidic) and the nutrients required for plant growth. Peat has the advantage of being a sterile medium and the addition of the base fertiliser will give a complete compost. A potting and seed base are made and should be mixed according to the manufacturers instructions; again sufficient sharp sand must be added to provide an open medium.