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pH and THE NUTRITION of ORCHID POT PLANTS by Bill Mather

 The grey-white crystalline incrustation or efflorescence surrounding the drainage holes of a long established orchid pot-plant using bark substrate is generally assumed to result from over-feeding – either too strong or too frequent. The usual excellent advice is to flush pots regularly with water to wash out accumulated excess salts. The "burning" of root tips in such cases is evidence that ex-osmosis has occurred - that is, water has been extracted from the whole plant via the roots by a high concentration of salts in the base of the pot, particularly when the drainage has been poor. Such a mix is typically old and "broken down". Certainly over-feeding old mix is a factor but even modest feeding levels will contribute to root loss in old mix.

In nature, epiphytic and lithophytic orchids grow on trees and rocks with their roots partially or fully exposed to the air. Roots that have died in broken-down mix have not drowned through lack of air necessarily, because the stomata in the under-surface of the foliage provide the vital pathway for respiration - the gaseous interchange of carbon dioxide from the air and oxygen as a product of photosynthesis. Remember that plant roots totally immersed in weak aqueous nutrients do not drown in hydroponic culture and that rock wool culture also involves immersion in aqueous nutrients without detriment to root tissue.

Living plants are made up of 75% by weight of water, 23% organic matter (complex carbon compounds) and 2% inorganic (non-carbon) chemical compounds. The latter represent the water-soluble mineral nutrients taken in by the root system. In pot-plant culture the substrate is almost devoid of these minerals, which must therefore be supplied by the grower.

There are 14 nutrient chemical elements (including trace elements). Four of these elements - phosphorus, calcium, magnesium and molybdenum - are insoluble in water below pH 5 (that is, more acidic than pH 5), whereas six elements (iron, magnesium, boron, copper, zinc and calcium) are insoluble above pH 8 (that is, more alkaline than pH 8). Thus the availability of nutrients to the plant varies at different pH levels and ceases beyond certain limits.

A solution at pH 7 is neutral (that is, neither acidic nor alkaline) and one at pH 6.5 is regarded as ideal for nutrition. The most important reason for the accumulation of excess nutrient salts is the development of acidity in the mix - bacterial and fungal action has lowered the pH of good freshly composted mix from pH 6.5 to pH 5 or less. Salts accumulate because they are not available in usable solution for absorption by root hairs. Worse follows by ex-osmosis - the plant dehydrates and the cells are destroyed when their cells rupture and collapse. Not only do extremes of pH affect the availability of plant nutrients but also they interfere with the delicate balance of microorganisms in the growing medium. For example, a very acidic medium can seriously interfere with mycorrhizae, beneficial fungi that make a plant's root system work more effectively.

When a potting mix has broken down, it is difficult to flush. Moreover, flushing does not change the pH; the mix remains acidic. Although it would then be logical to 'sweeten' the mix with a top-dressing of dolomite, this could provide a hit-or-miss quick-fix with the added risk of increasing the pH to such an extent that other nutrients become unavailable. Fresh repotting is always the best solution.

References:
1. The Orchid Grower's Manual by Gordon C. Morrison.
2. Fertilising by Wal Murphy.

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