Importance of Compost in Plant Propagation
A very important use for compost is in the propagation of plants. This may involve either seed sowing or taking cuttings and rooting them. The basic requirements of these composts are that they are free from pests and diseases, well aerated and well drained. The need for freedom from pest or diseases is obvious since at this stage both seedlings and cuttings are very susceptible to attack.
The main danger is from the damping-off fungi or the so-called “water moulds” such as Phytophthora and Pythium either before or after the seedlings emerge from the. The damage caused by these fungi is usually most severe under conditions when germination of the seed, or rooting of the cutting is slow, for example early in the season or in cold, wet soils.
Keeping the compost well-aerated and free draining will keep it warmer and combined with less saturation with water, this will discourage the diseases. There are other means of giving protection such as the use of seed dressings containing fungicides such as captan or thiram and insecticides such as gamma-HCH. A fungicide may be added to the hormone rooting powders used for cuttings. Even these precautions will be less effective or even ineffective if the compost structure is not suitable.
For seed sowing, the soil-based John Innes seed compost is suitable. The soil is sterilized and, provided that has been done effectively, the soil will be free from harmful organisms, the grit is sterile anyway and the peat will be almost or completely sterile — so the compost should be satisfactory. Improved germination and growth have been obtained by using soilless seed sowing composts in place of the soil-based seed compost. There are several formulas that may be made up such as the G.C.R.I. And Kinsealy formulae or equal volumes of sphagnum moss peat and sharp sand may be mixed (50:50 peat:sand) together with a small amount (approximately 0.75g/litre (1oz) per bushel) of a compost fertilizer and ground limestone at 4g/litre (5oz) per bushel of compost for a lime-free (acid) sand.
For rooting cuttings, there is a whole variety of recommendations, ranging from all peat through various combinations of peat and sand to all sand, even using different composts for rooting different cuttings. However, this complication seems rather unnecessary as most subjects can be rooted very successfully in the 50:50 peat-sand compost recommended above for seed sowing using one of the standard formulae or a compost fertilizer. Ready-made soilless composts can be obtained that are suitable for seed sowing and cuttings, or for growbags or bolsters. It is stressed, however, that despite many claims from manufacturers, bought-in composts can vary.
Recently there has been an increasing interest in growing cropping plants such asin compost in place of the more normal greenhouse soil. The main advantage is that the risk of pest or disease attack from the soil is prevented, as the compost is removed each year and replaced. The growing medium is known to be nutritionally good whereas there may be some doubt as to the suitability of the greenhouse soil, such as pH and fertilizer level, as well as the actual soil structure.
In the container system, the compost is placed in pots (plastic), bags or tubes which are then spaced out in the greenhouse. Plants are put individually in pots but bags or tubes may have more than one plant. It is possible to buy the bags or tubes filled with composts from various firms. The main dangers of the bag method, the modifiedand the trough method are the of water and the problems of the build-up of the salt concentration in the compost.
Watering must be done with care so that the compost does not becomeand feeding, whilst very necessary, must not be overdone in case the compost becomes toxic through excess fertilizer. One method of removing the problem is to slit the side of the growing bag or trough to allow excess water to drain away, but the disadvantage is that if the roots grow through the slits into diseased greenhouse soil, the plants will become infected.
An alternative method with the bed for trough system, is to place drainage tiles at the bottom of the trough, a system developed at the Kinsealy Institute in Ireland. A tile drain is laid in a central groove running along the bottom of the trough with a 2.5cm (1in) layer of gravel over the drain to prevent the compost entering it. The trough is topped up with not less than 15cm (6in) of soilless compost. At Kinsealy, these troughs have been used successfully for three years without replacement, but the compost is sterilized by steam through the drainage tile at the end of each season to prevent the build-up of pests and diseases.