Advice
RHS Help & Advice
Nutrient deficiencies
Garden soils and compost can sometimes suffer from deficiencies in the chemical nutrients that plants require for healthy growth. From time to time, supplementation may be necessary, but in the long term regular additions of organic matter should help prevent deficiencies occurring
Are plants lacking nutrients?
When plants fail to thrive, consider whether they lack nutrients. Usually, however, garden soils contain sufficient amounts of the major nutrients phosphorus and potassium, and minor nutrients such as iron and magnesium (trace elements) are also usually found in adequate amounts.
First, then, gardeners should rule out other potential problems. The RHS Gardening Advice Service finds that symptoms gardeners consider to be nutrient deficiency more often result from insufficient moisture and poor soil structure preventing plant roots finding what they need. There may also be other causes such as pests and diseases or excess alkalinity or acidity.
Improving soil conditions
Soil problems such as waterlogging in winter and dry summer soils inhibit the ability of roots to gather sufficient nutrients. Happily, most soil problems are improved by generous use of bulky organic matter such as well-rotted farmyard manure or garden compost, either dug in before planting or applied as a mulch. This boosts soil moisture, promotes drainage and improves soil structure as well as releasing nitrogen and other plant nutrients including trace elements.
Nitrogen deficiency
If plants still fail to thrive after soil improvement, consider nutrient deficiencies. Nitrogen promotes foliage, general growth and vigour. It is released as a result of bacterial activity in warm soils, and is therefore usually abundant by the end of summer. However, soil nitrogen compounds are highly soluble and by spring this surplus may have been washed out of the soil by winter rains. Yellow, spindly plants, perhaps with pink or yellow tints are signs of shortages. Sulphate of ammonia or, for organic gardeners, poultry manure pellets will quickly remedy the shortfall. In the long term, adding organic material will maintain levels by releasing a steady trickle of nitrogen as it decays.
Potassium deficiency
A plant uses potassium to control water uptake and photosynthesis. Shortages can occur in light, sandy or chalky soils, as it is more readily washed away by winter rains from these than clay soils, where it is held within the clay particles.
Leaves with blue, yellow or purple tints with brown blotches or discoloration within or at the edges of leaves are commonly found, along with a lack of growth, flowering or fruiting. Sulphate of potash and other high-potassium fertilisers alleviate the damage. Organic gardeners are not well served for high-potassium fertilisers, but by-products of sugar-beet processing are sometimes offered.
Phosphorus deficiency
This element is used in rapidly growing parts of plants such as root tips and shoots. Phosphorus deficiency shows up as dull, yellowed foliage with slow growth. As phosphorus accumulates in the soil and most gardens have received ample phosphorus fertilisers over the years, phosphorus deficiency is uncommon, but may occur in high-rainfall districts and where the soil is heavy clay. Superphosphate and, for organic gardeners, bonemeal put right any shortfalls.
Trace-element deficiency
When other factors have been discounted or remedied, consider whether trace element deficiencies are involved. Manganese and iron deficiencies are relatively common. Yellowing of leaves (known as chlorosis) and browning on the leaf edges, extending into the tissue between the veins (left) are signs of a deficit of these elements. They occur when acid-loving plants are grown in alkaline soils, when excess lime has been inadvertently applied or hard water used for irrigation. It is possible to acidify soil with sulphur or chelated iron, or to apply foliar sprays of manganese sulphate, but choosing plants that suit the soil is a better long-term solution.
Magnesium deficiency also causes yellowing of foliage between the leaf veins (right), often accompanied by red-brown tints and early leaf fall. Acid soils, sandy-textured soils and overuse of potassium fertilisers promote magnesium shortages, but they can occur after heavy rainfall and where root growth is inhibited by soil compaction. Apply magnesium sulphate (Epsom salts), to the soil or as a foliar spray to remedy a lack of magnesium.
Calcium deficiency
Insufficient calcium can cause blackened areas at the bottom of tomato and pepper fruit (called blossom end rot), and lead to brown, bitter tasting areas of apple flesh (called bitter pit). Here the problem isn't the lack of calcium in the soil - this seldom happens - but poor movement of calcium within the plant. Increasing ventilation and providing a regular water supply helps move calcium through tomatoes and peppers, and spraying calcium nitrate every 10 days on apples in summer suppress bitter pit.
Other elements
Shortages of other elements are much less commonly seen. Elongated leaves of cauliflowers grown in soil with a high pH suggest lack of molybdenum. Boron shortages - commonest in soil with a high pH - produce stunting of growth and die-back of tips leading to stunted bushy plants, cracked celery rotten swedes and dimples pears. Borax and other boron compounds can be applied to prevent these problems.
Guy Barter