Symptoms: although plants may be dark green, they are stunted. Older leaves may be
purple in colour (D).
Potassium (K)
Function: to improve plant quality, vigour, health, drought resistance, ability to store
sugars and starches, and to form chlorophyll. Also to improve stomata function.
Indicator plants: legumes, root crops, maize, cotton and tomatoes.
Symptoms: lower leaf tips and edges are yellow to brown and appear burned on the
outer edges. Internodes shorter than normal, and plants are stunted and often diseased.
Remarks: too much potassium makes plants slow to mature. Wood ash, compost and
manure are good sources of Potassium. Not deficient in tropical soils as often as P.
Calcium (Ca)
Function: component of cell walls and membranes; to balance organic anions.
Symptoms: soft dead necrotic tissue at fast growing parts of plants, which wilt.
Magnesium (Mg)
Function: to allow chlorophyll formation and growth, to assist in nodulation of legumes,
and in the utilisation of Phosphorus.
Indicator plants: most crops, including fruit trees and bushes, Irish potatoes, cauliflower
and sugar beet.
Symptoms: older leaves are chlorotic and may have light grey stripes between the leaf
veins. Leaves may have an unaffected, green edge, but these fall off prematurely (D).
Remarks: more common in light soils with high rainfall, and in soils with low organic
Sulphur (S)
Function: general plant metabolism, protein synthesis, formation of chlorophyll and
nodule development.
Indicator plants: lucerne (alfalfa), clover and rape.
Symptoms: the leaves are chlorotic, the shoots are short and the stems are stiff, woody
and thin (D).
Remarks: this is rarely a problem, and is more commonly seen in non-industrialised (ie
less smoky) regions. Can be problematic with certain Brassicas, rapeseed and Canola.
Zinc (Zn)
Function: in the relationships of plant metals with enzymes, and in the utilisation of
nitrogen and phosphorus.
Indicator plants: maize, cereals, potatoes, onions, avocado, apples, pears, citrus fruits
and castor.
Symptoms: the young leaves are a light grey colour both sides of the midrib at the base
Remarks: Zinc becomes increasingly less available to plants as the soil pH and
Phosphorus content rises.
matter content or with excessive Potassium or Calcium. Dolomitic limestone contains
26 TONY WINCH
Remarks: Phosphorus (“Phosphate”) is best applied a little and often, but there are no
problems if excessive amounts are applied. It is most available to plants when the soil
pH is 5.5– 6.5. Often deficient in tropical soils.
(D & E).
of the leaf. It causes Mottle Leaf disease of citrus. Internodes are short. Branches die
5 10% Magnesium. –