Plant nutrition is the study of the chemical elements that are necessary for plant growth. There are several principles that apply to plant nutrition. Some elements are directly involved in plant metabolism (Arnon and Stout, 1939).[citation needed] However, this principle does not account for the so-called beneficial elements, whose presence, while not required, has clear positive effects on plant growth.
A nutrient that is able to limit plant growth according to Liebig's law of the minimum, is considered an essential plant nutrient if the plant can not complete it´s full life cycle without it. There are 14 essential plant nutrients.
Carbon, Hydrogen and Oxygen are essential for plants, but are not considered plant nutrients.
Macronutrients:
N = Nitrogen
K = Potassium
Ca = Calcium
P = Phosphorus
Mg = Magnesium
S = Sulfur
Si = Silicon
Micronutrients (trace levels) include:
Cl = Chlorine
Fe = Iron
B = Boron
Mn = Manganese
Na = Sodium
Zn = Zinc
Cu = Copper
Ni= Nickel
Mo = Molybdenum
These nutrients are further divided into the mobile and immobile nutrients. A plant will always supply more nutrients to its younger leaves than its older ones, so when nutrients are mobile, the lack of nutrients is first visible on older leaves. When a nutrient is less mobile, the younger leaves suffer because the nutrient does not move up to them but stays lower in the older leaves. Nitrogen, phosphorus, and potassium are mobile nutrients, while the others have varying degrees of mobility.
Nitrogen (N) deficiency in plants can occur when woody material such as sawdust is added to the soil. Soil organisms will utilise any nitrogen in order to break this down, thus making it temporarily unavailable to growing plants. 'Nitrogen robbery' is more likely on light soils and those low in organic matter content, although all soils are susceptible. Cold weather, especially early in the season, can also cause a temporary shortage.
All vegetables apart from nitrogen fixing legumes are prone to this disorder. Symptoms include poor plant growth, leaves are pale green or yellow in the case of brassicas. Lower leaves show symptoms first. Leaves in this state are said to be etiolated with reduced chlorophyll. Flowering and fruiting may be delayed.
Prevention and control of nitrogen deficiency can be achieved in the short term by using grass mowings as a mulch, or foliar feeding with manure, and in the longer term by building up levels of organic matter in the soil. Sowing green manure crops such as grazing rye to cover soil over the winter will help to prevent nitrogen leaching, while leguminous green manures such as winter tares will fix additional nitrogen from the atmosphere.
Phosphorus (P) deficiency is a plant disorder that is most common in areas of high rainfall, especially on acid, clay or poor chalk soils. Cold weather can cause a temporary deficiency.
All plants may be affected, although this is an uncommon disorder. Particularly susceptible are carrots, lettuce, spinach, apples, currants and gooseberries. Symptoms include poor growth, and leaves that turn blue/green but not yellow—oldest leaves are affected first. Fruits are small and acid tasting. Phosphorus deficiency may be confused with nitrogen deficiency. Undersides of tomato plant leaves, and the veins and stems, may turn purple.
It can be controlled by applying organic sources of phosphorus such as rock phosphate or a high-phosphorus fish fertilizer applied as a soil drench or foliar feed.
Plants that are naturally adapted to low levels of available soil phosphorus, however, are more likely to suffer from phosphate poisoning: the key is to provide the right level for any particular plant type, neither too high nor too low.
Potassium (K), also known as potash deficiency, is a plant disorder that is most common on light, sandy soils, as well as chalky or peaty soils with a low clay content. It is also found on heavy clays with a poor structure. Plants require potassium ions (K+) for protein synthesis and for the opening and closing of stomata, which is regulated by proton pumps to make surrounding guard cells either turgid or flaccid. A deficiency of potassium ions can impair a plant's ability to maintain these processes. The deficiency most commonly affects fruits and vegetables, notably potatoes, tomatoes, apples, currants, and gooseberries, and typical symptoms are brown scorching and curling of leaf tips, and yellowing of leaf veins. Purple spots may also appear on the leaf undersides.
Deficient plants may be more prone to frost damage and disease, and their symptoms can often be confused with wind scorch or drought.
Prevention and cure can be achieved in the shorter term by feeding with home-made comfrey liquid, adding seaweed meal, composted bracken or other organic potassium-rich fertilisers. In the longer term the soil structure should be improved by adding plenty of well rotted compost or manure. Wood ash has high potassium content, but should be composted first as it is in a highly soluble form.
Boron (B) deficiency is an uncommon disorder affecting plants growing in deficient soils and is often associated with areas of high rainfall and leached soils. Boron may be present but locked up in soils with a high pH, and the deficiency may be worse in wet seasons.
Symptoms include dying growing tips and bushy stunted growth, extreme cases may prevent fruit set.
Crop-specific symptoms include：
‧Apple- interacting with calcium, may display as "water core", internal areas appearing frozen.
‧Beetroot- rough, cankered patches on roots, internal brown rot.
‧Cabbage- distorted leaves, hollow areas in stems.
‧Cauliflower- poor development of curds, and brown patches. Stems, leafstalks and midribs roughened.
‧Celery- leaf stalks develop cracks on the upper surface, inner tissue is reddish brown.
‧Celeriac- causes brown heart rot.
‧Pears- new shoots die back in spring, fruits develop hard brown flecks in the skin.
‧Strawberries- Stunted growth, foliage small, yellow and puckered at tips. Fruits are small and pale.
‧Swede (rutabaga) and turnip- brown or grey concentric rings develop inside the roots.
‧Arecaceae (Palm Tree) - brown spots on fronds & lower productivity.
Boric acid (16.5%boron), borax (11.3% boron) or SoluBor (20.5% boron) can be applied to soils to correct boron deficiency. Typical appliations are about 1.1 kg/hectare or 1.0 lb/acre of actual boron. Borax, boric acid or Solubor can be dissolved in water and sprayed or applied to soil as a dust. Excess boron is toxic to plants so care must be taken to ensure correct application rate and even coverage. Leaves of many plants are damaged by boron; therefore, when in doubt, only apply to soil.
Calcium (Ca) deficiency is a plant disorder that can be caused by insufficient calcium in the growing medium, but is more frequently a product of a compromised nutrient mobility system in the plant. This may be due to water shortages, which slow the transportation of calcium to the plant, or can be caused by excessive usage of potassium or nitrogen fertilizers Calcium deficiency symptoms appear initially as generally stunted plant growth, necrotic leaf margins on young leaves or curling of the leaves, and eventual death of terminal buds and root tips. Generally the new growth of the plant is affected first. The mature leaves may be affected if the problem persists.
Crop-specific symptoms include：
‧Apple：
'Bitter pit'-fruit skins develop pits, brown spots appear on skin and/or in flesh and taste of those areas is bitter.
Usually occurs when fruit is in storage. Bramley apples are particularly susceptible. Has relation to boron
deficiency, "water cored" apples seldom display bitter pit effects.
‧Cabbage and Brussels sprouts
Internal browning.
‧Carrot：
'Cavity spot'-oval spots develop into craters which may be invaded by other disease causing organisms.
‧Celery
Stunted growth, central leaves stunted.
‧Tomatoes and peppers
'Blossom end rot'-Symptoms start as sunken, dry decaying areas at the blossom end of the fruit, furthest away
from the stem, not all fruit on a truss is necessarily affected. Sometimes rapid growth from high-nitrogen
fertilizers may cause blossom end rot.
Iron (Fe) deficiency is a plant disorder also known as "lime-induced chlorosis". It can be confused with manganese deficiency. A deficiency in the soil is rare but iron can be unavailable for absorption if soil pH is not between about 5 and 6.5. A common problem is when the soil is too alkaline (the pH is above 6.5). Also, iron deficiency can develop if the soil is too waterlogged or has been overfertilised. Elements like calcium, zinc, manganese, phosphorus, or copper can tie up iron if they are present in high amounts.
Iron is needed to produce chlorophyll, hence its deficiency causes chlorosis. For example, iron is used in the active site of glutamyl-tRNA reductase, an enzyme needed for the formation of 5-Aminolevulinic acid which is a precursor of heme and chlorophyll.
Symptoms include leaves turning yellow or brown in the margins between the veins which may remain green, while young leaves may appear to be bleached. Fruit would be of poor quality and quantity. Any plant may be affected, but raspberries and pears are particularly susceptible, as well as most acid-loving plants such as azaleas and camellias.
Magnesium (Mg) deficiency is a plant disorder with two main causes. Magnesium can be easily washed out of light soils in wet seasons or excessive potassium fertilizer usage can cause also Mg to become unavailable to the growing plant.
This disorder particularly affects potatoes, tomatoes, apples, currants and gooseberries, and chrysanthemums.
Symptoms include, yellowing between leaf veins, which stay green, giving a marbled appearance. This begins with older leaves and spreads to younger growth. Can be confused with virus, or natural aging in the case of tomato plants. Fruits are small and woody.
Mg deficiency can be rectified in the short term by applying a foliar feed fortnightly, with epsom salts diluted at a rate of 200g per 10 litres of water (8oz per 2½ gal). In the longer term add dolomitic limestone if soil pH allows, or other Mg containing rocks such as Kieserite. Reduce usage of potash fertilisers if this may be contributing to the problem.
Manganese (Mn) deficiency is a plant disorder that is often confused with, and occurs with, iron deficiency. Most common in poorly drained soils, also where organic matter levels are high. Manganese may be unavailable to plants where pH is high.
Affected plants include onion, apple, peas, French beans, cherry and raspberry, and symptoms include yellowing of leaves with smallest leaf veins remaining green to produce a ‘chequered’ effect. The plant may seem to grow away from the problem so that younger leaves may appear to be unaffected. Brown spots may appear on leaf surfaces, and severely affected leaves turn brown and wither.
Prevention can be achieved by improving soil structure. Do not over-lime.
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