Not all plant problems are due to pests and diseases. Mineral deficiencies might be to blame for symptoms that mimic those of disease. Deficiencies are present in many soils, even when added fertilizers are used. Excess of one mineral can lock up another one, making it unavailable to plant roots. Always look closely and try to determine exactly what the problem is. There might just be a quick fix.
Nitrogen
Small plants that show poor growth indicate a shortage of nitrogen. Leaves are often more yellow than green. Apply a nitrogen-rich feed like well-rotted or liquid manure.
An excess of nitrogen is indicated by plants that produce a mass of dark green leaves at the expense of flowers and fruit. Apply potash to try to lock up some of the excess.
Potassium (potash)
A shortage of potash is indicated initially by brown or yellow edges to leaves. This discolouration spreads in towards the midrib. The tops of the plants may be thin, and ripened tomatoes might have blotches of yellow and green. Poor growth and poor yields are common. Apply a potash-rich feed such as seaweed, or a sprinkling of wood ash.
Excess potash is indicated by the tops of plants browning at the growing points. Apply lime if necessary to reduce uptake of potash, or water soil well to try to wash away some of the excess.
Phosphorous
A shortage of phosphorous is indicated when leaves have a purple or reddish tinge. Plants can be small, stunted and prone to disease. Crops will be slow to mature. Apply rock phosphate or bonemeal to provide the missing mineral.
Magnesium
A shortage of magnesium will show as mottled leaves with dark veins and yellowing in between. This is particularly common in fast-growing polytunnel crops like cucumbers. Potash will lock up magnesium – especially on acid soils. Apply a solution of Epsom salts (two teaspoons in a watering can) for a quick fix. Repeat as necessary.
Liquid manure combats nitrogen shortage
Calcium
A shortage of calcium will show as leaves that brown and start to die from the stem end up. Blossom end rot in tomatoes can be partly attributed to lack of calcium. Soils that are short of calcium will be acid and sour. Apply lime to rectify the problem.