If plants fail to thrive, despite adequate soil preparation, watering and mulching, it may be a sign of a nutrient deficiency. Fruit and vegetables are particularly vulnerable, as are containerised plants and those growing in very acid or alkaline soils. Yellow or reddish coloured leaves, stunted growth and poor flowering are all common symptoms of nitrogen, magnesium or potassium deficiency.
Symptoms, cause and remedy
- Symptoms: Spindly yellow plants or yellow leaves, sometimes with pink tints.
- Cause: Nitrogen promotes green, leafy growth and deficiency results in yellowing and stunted growth. Nitrogen is very soluble, so is easily washed out of the soil in winter rains, leaving the soil deficient in spring, just when plants are putting on new growth. Nitrogen deficiency is a common cause of yellow leaves in spring.
- Remedy: In the long term, mulching with organic matter (such as well rotted garden compost or manure) provides a steady trickle of nitrogen to stabilise levels. In the short term, applying high nitrogen fertilisers such as sulphate of ammonia or poultry manure pellets will remedy the problem.
- Symptoms: Yellow or purple leaf-tints with browning at the leaf edge and poor flowering or fruiting.
- Cause: Potassium is needed for controlling both water uptake and the process allowing plants to harness energy from the sun (photosynthesis). Potassium promotes flowering, fruiting and general hardiness. Shortages are more likely on light, sandy or chalky soils where potassium is easily washed away. Clay soils, by contrast, hold potassium within their structure.
- Remedy: Apply high potassium fertilisers such as sulphate of potash, tomato feed or certain organic potassium sources derived from sugar beet processing.
- Symptoms: Slow growth and dull yellow foliage.
- Cause: Phosphorus is needed for healthy roots and shoot growth. Soil shortages of phosphorus are rare, but may occur in areas with high rainfall and heavy clay soil.
- Remedy: Apply fertilisers such as superphosphate or bone meal.
- Symptoms: Yellowing between the leaf veins, sometimes with reddish brown tints and early leaf fall. Magnesium deficiency is common in tomatoes, apples, grape vines, raspberries, roses and rhododendrons.
- Cause: Magnesium is needed for healthy leaves and for plants to harness energy from the sun (photosynthesis). Soil shortages of magnesium are more common on light, sandy soils. Over-use of high-potassium fertilisers (such as tomato feed) can cause magnesium deficiency, as plants take up potassium in preference to magnesium.
- Remedy: In the short term, apply Epsom salts as a foliar feed in summer. Dilute the salts at a rate of 20g of Epsom salts per litre of water (1/3oz per pint) plus a few drops of liquid detergent. Apply two or three times at fortnightly intervals, spraying in dull weather to avoid leaf scorch. In the long term apply to the soil around the roots either Dolomite limestone (calcium-magnesium carbonate) at 100g per sq m (4oz per sq yd) or Epsom salts (magnesium sulphate) at 30g per sq m (1oz per sq yd). Dolomite limestone will make the soil more alkaline, so should not be used around ericaceous (acid-loving) plants such as rhododendrons or camellias, or where the soil is already alkaline.
Manganese and iron deficiencies
- Symptoms: Yellowing between the leaf veins with browning of leaf edges on acid-loving plants.
- Cause: Manganese and iron are important for allowing plants to harness the energy of the sun (photosynthesis). Soil shortages are rare, but manganese and iron can be unavailable to plant roots in alkaline conditions. Ericaceous (acid-loving) plants are particularly vulnerable when growing in alkaline soils or potting composts.
- Remedy: Apply chelated iron and manganese treatments, such as Sequestrene, to the soil around the plant roots.
- Symptoms: Elongated twisted leaves on cauliflowers or other brassicas growing in alkaline soil. Molybdenum deficiency is normally seen in cauliflowers and brassicas, particularly when growing in insufficiently alkaline soil.
- Cause: Molybdenum is required for a variety of plant growth processes, but is needed only in tiny quantities. Soil shortages of molybdenum are rare, but it can be less available to plant roots in acid conditions.
- Remedy: Treat with fritted trace elements (see our advice on fertilisers). Liming the soil will help in the long term, as making the soil more alkaline will help to make the molybdenum more available. See our advice on lime and liming for further detail.
- Symptoms: Stunted growth and tip dieback on lettuce, brown cracks in celery; rotten swedes, turnips and celeriac; dimples in pears with brown patches underneath.
- Cause: Boron is required for healthy plant cell formation. Soil shortages are rare, but this nutrient can be less available to plant roots in alkaline conditions.
- Remedy: Treat with fritted trace elements (see our advice on fertilisers) or by applying borax (disodium tetraborate) to the soil before sowing vegetables or as a foliar spray feed applied to pear trees. Soil application rates for borax are: 35g per 20 sq m (1oz per 20 sq yd). Mix well with a large quantity of light sand before spreading so that the chemical is evenly distributed. Foliar spray application rates for borax are: 70g borax in 22-litres water (2½oz in 5 gallons), plus a few drops of detergent to act as a wetting agent, sprayed at petal fall.