Composts for Seed Sowing and Potting
Several generations of gardeners have been brought up now on the composts, which, by standardising compost mixtures into just a few different categories for seed sowing and potting, have made it easy to get good results without remembering or looking up countless compost recipes. There are two basic formulae for the composts — one for seeds, the other for potting and the three ingredients used in them in bulk are loam, peat and sand.
Medium grade loam is needed for composts, neither too heavy (clay) nor too light (sand) with a pH of about 6.5. Such loam can be obtained from turves cut with 2 to 3in. of soil attached from a good meadow or building site. The loam is stacked, grass side down, and left for up to a year if possible so that the grass and soil fibres have a chance to decay. It should be riddled through a 3/8in. sieve before being used.
Composts for Greenhouse Plants The horticultural grade peat used should be granular or fibrous and reasonably free from dust. It must be well watered and made thoroughly moist before use, the watering being repeated several times, if necessary, to obtain the desired result. The sand needed is defined as coarse and damp with particles grading up to 1/8in. in diameter. Cornish river sand is ideal for the purpose; never use builder’s sand.
This compost can be bought from garden stores and nurseries but it is as well to be sure of the source because some so composts offered by dealers bear little resemblance to the real thing. If you make your own the soil should be sterilised by standing it over boiling water in a saucepan or copper, or placing it in one of the special sterilisers which can be obtained through garden stores, etc. The temperature of the soil should be raised to 93°C. (200° F.) and kept at this level for 20 minutes.
Compost is prepared by mixing 2 parts sterilised loam, 1 part peat and 1 part sand, all parts by loose bulk. To each bushel of these combined ingredients is added 1-1/2oz. of superphosphate of lime and 3/4oz. of either finely ground chalk or limestone. If the compost is to be used for lime-hating plants substitute flowers of sulphur for the ground chalk or limestone, at the same rate. First, the loam is sieved to remove stones. Then the peat and sand are added and finally the other ingredients, carefully measured out, are scattered over the top. The whole heap must then be turned several times.
Potting Compost is prepared in a similar manner, but the proportions of the main ingredients are different: 7 parts loam, 3 parts peat and 2 parts sand. A base fertiliser is added to this. It can be purchased ready mixed or can be made with 2 parts of hoof and horn meal (13% nitrogen), 2 parts superphosphate of lime (18% phosphoric acid) and 1 part sulphate of potash (48% pure potash), all parts by weight. This is added to the other ingredients at the rate of 4oz. per bushel for No. 1 Potting Compost, 8oz. per bushel for No. 2 Potting Compost, and 12oz. per bushel for No. 3 Potting Compost. To No. 1 add 3/4oz. of ground chalk, or limestone (except for lime-hating plants) per bushel of mixture; double and treble this amount for Nos. 2 and 3. the No. 1 compost is used for all ordinary purposes, No. 2 for older plants in pots over 4in. diameter, and the No. 3 for some very strong-growing plants in 10in. pots.