Plants require many different nutrients, but the main three are nitrogen (N), phosphorus (supplied as phosphoric acid) (P), and potash (K), plus some trace elements such as iron, magnesium, manganese, boron.
In general, nitrogen tends to promote stem and leaf growth, phosphorus root growth, and potash fruitfulness and ripening, but one can interact with another and may result in a lack of potash that is preventing the plant making proper use of the nitrogen available in the soil.
You might find that your cause for concern is not that your plants are deficient of fertiliser/s, but that they are receiving insufficient moisture, and / or the soil structure is poor.
Any combination of these can prevent plant roots finding the nutrient they need.
Or the other hand, there might be other causes such as the presence of pests and disease, or an excess in alkalinity or acidity (pH).
In many cases, soil problems can be cured by the generous use of bulky organic matter, either dug in or used as mulch.
This will improve the structure of the soil by releasing nitrogen and other plant nutrients including trace elements into the soil, and also help to prevent water-logging in winter and drying out summer.
Most animal manures contain all necessary foods but not necessarily in the amounts plants require, meaning that feeding will need supplementing.
A balanced fertilizer is one that supplies the essential ingredients in approximately the right proportions for the crop in question.
Most chemicals usually supply only one of the ingredients required, though a few, such as nitrate of potash, and phosphate of potash supply two.
In order to obtain a well balanced chemical fertilizer, several chemicals must be mixed in the correct proportions.
By law it is necessary that the nitrogen, phosphoric acid and potash content (N P K) of mixed commercial fertilizers should be quoted on the packaging.
This is done on the basis of percentage of each of these nutrients in the complete mixture.
The analysis is always given in the order as shown above, and is occasionally abbreviated to figures only.
Thus a fertilizer described as 15: 15: 30 would contain 15% nitrogen, 15% phosphoric acid and 30% potash.
Trace Elements: It is highly recommended that expert advice is taken before applying any of these.
Most garden soils usually contain sufficient amounts of the major nutrients e.g. phosphorus and potassium, and trace elements such as iron and magnesium, plus small amounts of nitrogen.
If after soil improvement plants still fail to thrive, then have a soil test done.
If it is a deficiency problem this will highlight what is lacking, or indeed what is in excess as the case may be.
Dealing with Deficiencies:
Nitrogen (N) deficiency:
Nitrogen is required to promote foliage, and general growth.
The signs of deficiency can be pale green-yellow leaves, sometimes with pink tints, and spindly plants.
Nitrogen compounds are highly soluble in water, and can be readily washed out of the soil by winter rains.
Adding organic material will help to maintain levels of nitrogen.
There are also a number of organic/inorganic products on the market that will remedy this problem, products such as: sulphate of ammonia, dried blood, poultry manure pellets.
Always add any of these at the prescribed dosage as stated on the packaging.
Phosphorus (P) deficiency:
This element is used in rapidly growing parts of plants such as the roots.
Phosphorus deficiency shows up as dull, yellowed foliage and slow growth.
Phosphorus tends to accumulate in the soil of most gardens, particularly if a regular feeding regime has been performed meaning a deficiency is uncommon.
However it may occur in areas with high-rainfall and where the soil is heavy clay.
Superphosphate and / or bonemeal should make right any shortfalls when added at the prescribed dosage as stated on the packaging.
Potassium (K) deficiency:
Plants need potassium to control water uptake and photosynthesis.
As with nitrogen,potassium can be readily washed out of the soil, particularly light, sandy or chalky soils.
The symptoms can look similar to a nitrogen deficiency i.e. leaves may turn blue, yellow or purple with brown blotches or discoloration within, or at the edges of the leaves.
Other symptoms are, lack of growth, poor flowering and or fruiting.
There are a number of high-potassium fertilizers available, e.g. sulphate of potash that will cure the problem.
Add at the prescribed dosage as stated on the packaging.
Magnesium is an essential nutrient that is used by all green plants to make chlorophyll, which is vital for photosynthesis.
The first sign of deficiency appear on the lower half of the plant/s, and is recognized when a yellow discoloration appears between the leaf veins, and there is premature autumnal leaf tints and dying leaves. e
Eventually this problem will move up the plant/s if left untreated.
Magnesium deficiency is common on acid, sandy soils, with low organic matter levels, or where heavy applications of potassium rich fertilisers have been made.
In spring top-dress the bed/s with of Epsom salts (Magnesium sulphate) applied at a rate of 30g (1oz) per sq m (sq yd).
Apply the top dressing to a moist bed and wash off any that comes in contact with the plants.
If symptoms appear during the growing season, apply a foliar spray* containing Epsom salts at 20g (½oz) per litre (1.75 pints) of clean water.
*Adding a few drops of washing-up liquid to this mix will help the solution stick to leaves.
Don’t spray in bright sunlight, to do can result in scorched leaves.
Repeat the treatment three times at fortnightly intervals.
As mentioned above it is best to seek expert advice before adding trace elements.
Manganese and Iron deficiencies are relatively common, and appear as a yellowing of leaves (known as chlorosis) and browning on the leaf edges.
This can occur when acid-loving plants are grown in alkaline soils.
This alkanility can sometimes be caused inadvertently i.e. applying an excess of lime,or using hard water 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 the better long-term solution.