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Science and the Garden
Table 4.4 Symptoms of nutrient deficiency. Nutrient
Visible symptoms
Nitrogen
Uniform pale colour with yellowing especially of lower leaves. Feeble growth and lack of branching.
Phosphorus
Reduced growth especially soon after emergence. Often there are no other symptoms though some species show purple coloration of older leaves.
Potassium
Margins of older leaves scorched and curled up or down.
Sulphur
New leaves a uniform golden yellow or cupped and deformed. Foliage frequently stiff and erect.
Calcium
Cupping and burning of leaf tips with blackening of young leaves.
Magnesium
Yellowing between the veins on older leaves to give a mottled appearance.
Iron
Yellowing between the veins of older leaves and the production of nearly white young leaves.
Manganese
Yellowing between veins of new leaves but may quickly spread to others.
Zinc
Specific to individual species.
Copper
Very dependent on species. New leaves become greyish-green, yellow and sometimes white.
Boron
Brittle tissues which crack easily. Death of growing point and production of side shoots.
Molybdenum
Rare except in cauliflowers where new leaves become progressively twisted and reduced until only the midrib appears ('whiptail'). Wilting.
Chlorine
of their low concentration, play little part in either osmotic regulation or in the maintenance of electrochemical balance. They mainly assist in the activation of enzyme reactions. Conversely, potassium and chlorine (the only nutrients that are not part of organic structures) are very important in osmotic regulation and for electrochemical balance. Nitrogen is an essential constituent of many organic components of plants including proteins, nucleic acids, chlorophyll and hormones. Similarly phosphorus in plants is present as part of many organic compounds which play a very important part in the very large number of enzyme reactions that depend on phosphorylation. For the horticulturist, though, probably the most important aspect of plant nutrition is being able to recognise symptoms of deficiency (and in some cases, toxicity), and to remedy them. Often the growth of plants will be reduced considerably
before deficiency symptoms first appear so any measures to correct a shortage of nutrients will be beneficial only to subsequent plantings. While generalisations can be made about the deficiency symptoms of the macronutrients, the symptoms are often specific to the plant species because some species are more susceptible than others. For micronutrients the symptoms are almost entirely plant specific. Table 4.4 gives some general deficiency symptoms. A gardener suspecting nutrient deficiency as a cause of poor growth would be best advised to consult a specialist textbook and to keep some plant leaves for laboratory analysis. In the UK, deficiencies of macronutrients are more common than those of micronutrients and if nitrogen, phosphorus and potassium are provided then there are usually sufficient amounts of other nutrients in soils to allow good growth. However, this is not the case elsewhere especially on sandy soils and those where leaching of nutrients by excess rain occurs.