It is most important when making up the desired compost formulation to achieve a uniform product and, commercially, it must be undertaken with a minimum labour input. The ingredients of the compost must be as near as possible to the specification for the chosen formulation. Materials must not be too moist when mixing because it then becomes impossible to achieve an even distribution of nutrients. There are several designs of compost mixer. Continuous mixers are usually employed by specialist compost mixing firms and require careful supervision to ensure a satisfactory product. Batch mixers of the 'concrete mixer' design are produced for a wide range of capacities to cover most nursery needs. Many of the bigger mixers have attachments which aid filling. Emptying equipment is often linked to automatic tray or pot-filling machines.
Ingredients used in loamless composts or growing modules do not normally require partial sterilization unless they are being reused, but sterilizing equipment is certainly needed to prepare loams for inclusion in loam-based composts. Where steam is used it is injected through perforated pipes on a base plate and rises through the material being sterilized. In contrast a steam-air mixture injected from the top under an air-proof covering is forced downwards to escape through a permeable base.
Storage of prepared composts should be avoided if possible and should not exceed three weeks if slow release fertilizers are incorporated. If nitrogen sources in the compost are mineralized, ammonium ions are produced followed by a steady increase in nitrates (see p366). These changes lead to a rise in compost pH followed by a fall. As nitrates increase, the salt concentration rises towards harmful levels (see conductivity). Peat-based composts can become infested during storage by sciarid flies.
The characteristics of the container affect the root environment, as does the standing-out area. There is an enormous range of containers used to meet the many different requirements of growing plants (Figure 12.2). Clay pots are porous and water is lost from the walls by evaporation. Consequently clay pots dry out quicker than plastic pots, especially in the winter and, although air does not enter through the walls, this can help improve air-filled porosity. The higher evaporation rate also keeps the clay pots slightly cooler, which can be beneficial in hot conditions. Likewise the contents of white plastic pots can be as much as 4°C lower than in other colours. Pots of white or light green plastic can transmit sufficient light to adversely affect root growth and encourage algal growth.
Biodegradable containers such as those made from paper have become popular because they can be planted directly. Some materials decompose more rapidly than others and there can be a temporary 'lockup' of nitrogen, but most peat containers are now manufactured with added available nitrogen. It is essential that these containers are soaked and surrounding soil is kept moist after planting or the roots fail to escape from the dry wall.
The air to water characteristics of the mixture in the container depend not only on the nature of the contents, but also on the characteristics of the base on which the container stands. If containers are stood out on wire mesh or on stones, relatively little water leaves so the oxygen content remains poor. It is also important to retain contact between the compost and the standing out material through adequate holes in the base, whether to help drainage or to ensure the uptake of water if irrigated from below.
Blocks are made of a suitable compressed growing medium into which the seed is sown with no container or simply a net of polypropylene. Aeration tends to be poorer than in pots, but the high surface area helps make this a successful means of growing some vegetables on a large scale. One type of block comes in the form of a dry compressed disc that expands quickly on soaking ready to receive the seed in the shallow depression in the top surface. This technique has been replaced in large measure by the use of rockwool blocks, particularly when these are to be grown on in rockwool modules (see Fig. 22.3).
Increasingly, traditional seedbed, bare-rooted or block transplant techniques have been replaced by raising a wide variety of plants in modules. A module is made by adding a loose growing medium mix to a tray of cells. The cells are variously wedge or pyramid-shaped, so designed to enable a highly mechanized transplanting process to be used. Fine, free-flowing mixes of peat, polystyrene or bark are used to fill the cells, which have large drainage holes and no rim to hold free water. Roots in the wedge-shaped cells are 'air-pruned' as they reach the edge of the cell, which encourages secondary root development. 'Plugs' are mini-modules in which each transplant develops in less than 10 cm3 of growing medium and are used for bedding plants, as well as vegetable production. The rate of establishment is largely determined by the water stress experienced by the transplant. Irrigation of the module or plug is found to be more successful than applying water to the surrounding growing medium.
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