Understanding Growing Media

Taking samples of growing media

It is essential that any samples sent for analysis are representative of the main bulk of the growing media. For a greenhouse border, several small samples should be taken with a trowel, auger or other instrument, from a uniform depth of around 13-15cm (5-6in). Bulked together the sample should weigh about 1kg(2lb).

Where batches of compost are being made up, the same basic procedure should be followed, selecting small quantities of the media from various parts of the heap and bulking them together. It is sometimes advisable to have the basic ingredients analysed before mixing, especially if there is any doubt about extreme acidity, toxicity or other factors. Samples should be placed in a strong paper or polythene bag and clearly labelled for dispatch to the laboratory. One sample of 1 kg (2lb) will generally suffice for some 100m2 (1,000 sq ft) of border. Always label samples clearly with the exact location and source, and give previous cropping history, if any, and proposed cropping programme, as this will assist the advisory body to give any corrective treatment that may prove to be necessary.


Analysis of solid or liquid fertilizers

Quality control is very strict at the manufacturing stage for all solid or liquid fertilizers, and analysis is seldom necessary, except in the case of doubtful identity. The checking of diluted liquid feeds for salt concentration (pC or CF) is useful, particularly if a doubtful dilutor/dilution method is involved.


Examination of soil for weeds, pests and diseases

understanding growing media - leatherjackets

It would obviously be folly to use a soil known to contain pests, diseases or weeds, yet it is not always possible to tell visually whether a soil or ‘clean’ or not, or for that matter to carry out sterilization. The larger pests like wireworms, leatherjackets and other pests can of course be observed when working with the soil in any way, and particularly if putting it through a riddle or screen prior to mixing up a compost. Vegetative underground portions of many perennial weeds such as couch grass, horsetail, coltsfoot and others can also be fairly readily detected by visual examination, even if not specifically.


Smaller pests and diseases, however, can seldom be detected visually, most of the disease organisms or spores being microscopic in size. It is not always feasible to examine soil for the presence of disease using the usual pathological techniques of plating and incubation, although an indicator plant could be grown and later sectioned and examined if a pathological service is able and willing to carry out this work. Kits to detect common soil borne diseases are now available. There is now also a laboratory test for detecting club root disease. Usually the previous history of a soil or growing media gives a good indication of what to look for in the way of both pests and diseases, and if in any doubt it is folly to use a doubtful growing medium unless it can be sterilized.

Where a ‘new’ soil is involved and there is some idea of the cropping pattern carried out in that soil previously, then this also gives guidance. If in any doubt about soil, or to make doubly sure that no trouble does overtake the plants which will be grown in it, the obvious answer, where possible, is to sterilize it either by heat or chemical means. Alternatively, prepared compost can be bought in.

The increasing popularity of soilless mixes owes much to the cleanliness of the basic ingredients (peat, sand or peat alternative) and to the fact that sterilization is seldom, if ever, necessary. Peats, however, can carry rhizoctonia root rots which affect a wide range of plants. Peat can also contain weeds.

27. March 2011 by Dave Pinkney
Categories: Greenhouse Gardening, Organic Gardening, Soil Cultivation | Tags: , | Comments Off on Understanding Growing Media


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