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Plant Mineral Nutrition

Green plants take in and use carbon dioxide and water in photosynthesis, and the resulting glucose and other carbohydrates therefore contain the chemical elements carbon (C), hydrogen (H) and oxygen (O).

Other classes of organic compound are built up using other elements. In addition to C, H and O, proteins contain nitrogen (N) and possibly sulphur (S). Nucleic acids contain N and phosphorus (P).

These other elements can be obtained from chemical compounds in the soil. These are generally described as minerals, mineral ions or inorganic ions.

Green plants usually absorb mineral ions from the soil (together with water) using their roots and they incorporate them into their cells. Sometimes minerals may be absorbed via the leaves.

Various mineral ions are needed as nutrients by plants, and in some cases they differ from the mineral requirements of animals. Animal mineral nutrition (and vitamins) is now a separate topic on this website.

Since there is a limited amount of these minerals available in the soil they may act as limiting factors in plant growth. Put another way, addition of several inorganic mineral salts usually improves plant growth.

Unlike animal cells, plant cells are surrounded by a very dilute solution of mineral nutrients. This results in water entering the plant cells by osmosis, causing them to be turgid as the hydrostatic pressure acts against the plant cellulose cell walls.

Also unlike animals, plants do not permanently dispose of mineral ions. They do not have a daily requirement (for replacement). Obviously different mineral ions are required in different amounts, and this will also depend on the plant's phase of growth. The absolute amount of mineral nutrition in the soil is difficult to gauge, and extra mineral nutrients are added optimistically. An excess of minerals could conceivably have an osmotic effect.

For this reason and for comparison I have looked up the amounts - expressed as parts per million (ppm) - of the various minerals considered optimal for use in hydroponic growth systems - which obviously do not involve soil.

Mineral
Role in plant cells
Deficiency causes
Good sources
Amount required
[in hydroponic systems]
ppm
Mineral
N: Nitrogen
as nitrate ions NO3-

Other N-containing material may be broken down by microbial action to provide this
Role in plant cells
Used to produce amino acids for protein production, and for nucleic acids DNA, RNA

Hence promotes vegetative growth and development of flowers, fruits and seeds
Deficiency causes
Poor growth, leaves not so green (more yellowy green) - 'chlorosis'
Amount required
ppm

125-224






Good sources
'Chemical' sources:
calcium nitrate, potassium nitrate, ammonium nitrate - also provide Ca2+, K+, NH4+
'Organic' sources;
urea CO(NH2)2
Less defined:
'blood, fish and bone'
animal droppings etc
Mineral
P: Phosphorus
as phosphate ions:
PO43-
HPO42-
H2PO4-


Role in plant cells
Component of nucleic acids (DNA, RNA, nucleotides, ATP etc)
All cell membranes are phospholipids
Phosphate groups are involved in activation of carbohydrates in respiration and photosynthesis, and also in control of enzymic processes and gene expression (phosphorylation and photophosphorylation)

Hence P promotes growth of roots and shoots
Deficiency causes
General stunting of growth

Leaves may be smaller and darker than usual
Amount required
ppm

39-45
Good sources
inorganic phosphate fertiliser

bonemeal
Mineral
K: Potassium ions K+





Role in plant cells
Involved in the opening and closing of stomata (by regulating the osmotic potential within guard cells), hence controls intake of CO2 for photosynthesis and the transpiration stream which provides water and other inorganic ions for plants.
It is said to activate a number of enzymes involved in photosynthesis and respiration.
Deficiency causes
Leaf tips become brown and curl; leaf blades become yellow between veins.
Symptoms are more marked in older leaves.
Plant growth and development of roots, seeds and fruits is reduced.
Amount required
ppm

280-350




Good sources
Inorganic potassium salts: potassium chloride (chloride no use to plants), potassium nitrate (nitrate also provides nitrogen)

Pot ash (residue from burnt wood)
Mineral
Mg:
Magnesium ions
Mg2+







Forms (the central) part of the chlorophyll molecule

Magnesium ions are co-factors for the action of enzymes required for important stages in photosynthesis (specifically carbon fixation): Ribulose Bisphosphate Carboxylase (RUBISCO) and Phosphoenolpyruvate Carboxylase (PEPC).
Deficiency causes
Marked chlorosis (yellowing) between leaf veins, which stay green
Good sources
'Epsom salts' (magnesium sulphate)

(also provides sulphate)
Amount required
ppm

48-65
Mineral
Fe: Iron
Fe2+, Fe3+ ions
Role in plant cells
Needed for enzymes involved in chlorophyll production
Deficiency causes
Chlorosis
Noticeable in alkaline soils which reduce solubility of iron compounds
Good sources
Red soils contain iron compounds
Amount required
ppm

2
Mineral
Ca: Calcium Ca2+
Role in plant cells
Calcium ions are involved in microtubule formation as part of spindle action in cell division - especially important in meristems at tips of roots and shoots.

Calcium pectate is a constituent of plant cell walls

Calcium is distributed mostly into the leaves, less so in seeds, fruits, and roots.

Calcium may function directly in several aspects of photosynthesis. It appears to modulate activity of the phosphatase enzymes in the carbon reduction cycle.
Deficiency causes
Stunted growth of plant, especially roots
Curling of leaves, and darkened appearance
Blossom end rot of fruits, exacerbated by excess nitrate or inadequate water supply

Some plants are affected by the calcium content of soils, and its effect on soil pH and the availability of other ions:
Calcifuges e.g. Ericas (heathers), Rhododendrons, Azaleas, etc require acidic soils so they develop the symptoms of iron deficiency, i.e. interveinal chlorosis, on alkaline soils.
Calcicoles thrive in lime rich soil. In acidic soils they often develop the symptoms of aluminium toxicity, i.e. necrosis, and phosphate deficiency (reddening of the leaves) and stunting.
Amount required
ppm

170-210


Good sources
Chalk and lime is often added to soils, and hard water contains calcium ions. Nitro chalk - a mixture of calcium carbonate and ammonium nitrate (calcium ammonium nitrate?) gives nitrate in addition to calcium.

Certain leafy vegetables (Brassicas e.g cabbage etc) grow more strongly when supplemented with calcium, and may in turn make a significant contribution to the human diet.

Some calcium salts (phosphate, sulphate) are insoluble so phosphates and sulphates may lower the availability of calcium to the plant.
Mineral
S: Sulphur
as sulphate ions
SO42-
Role in plant cells
Sulphate ions provide the element sulphur which is needed to produce the amino acids cysteine and methionine which are used in the synthesis of proteins.
Deficiency causes
Poor growth, leaves not so green (more yellowy green) - 'chlorosis'
- leaf veins purple
- especially noticeable in older leaves

Sulphur deficiency can occur when the soil pH is too high, or a large amount of calcium is present.
Amount required
ppm

75


Good sources
'Epsom salts' (magnesium sulphate)

(also provides magnesium)

Other elements, (possibly) required in smaller amounts

(< 1ppm)

Manganese (Mn)
Zinc (Zn)
Boron (B)
Copper (Cu)
Molybdenum (Mo)

Inorganic fertilisers

In this context, fertiliser means something which makes ground more productive by encouraging plant growth.
Of course fertilisation is an important process in Biology, and generally necessary for the production of fruits and seeds which are the products of sexual reproduction.
Mineral supplements can be added to the soil in the form of simple mineral salts : 'chemicals' in some people's language.

Most of these are produced in chemical factories, often using ores imported from other parts of the world and their manufacture uses a considerable amount of energy - probably from fossil fuels - so these apparently simple chemical compounds have an up-front ecological cost.

Nitrogen is freely available from the air (of which it forms 78% by volume) but for use by most plants it needs to be converted into soluble salts.
{The notable exceptions to this are leguminous plants (pea and been family, including clover which is important in pasture land). These have root nodules containing nitrogen-fixing bacteria which can make use of atmosheric nitrogen.}

The reaction conditions in the Haber process for the production of ammonia (NH3) are: a temperature of 450C a pressure of 200 atmospheres in the presence of an iron catalyst.

This ammonia can be used to produce ammonium salts (NH4+) and nitrates (NO3-), both of which are sources of nitrogen.

Urea - CO(NH2)2 - is a cheap nitrogen-containing product - technically not an inorganic salt. It is an organic compound easily extracted from animal urine and forming white crystals which are easily handled. In the soil it is quickly converted by bacterial action into ammonium and nitrate ions.

Phosphorus compounds for use in fertilisers is obtained from phosphate rocks originally formed from the deposition of phosphates in an ancient marine environment. They are surface-mined from sedimentary deposits in China, the Middle East, northern Africa and the United States. These ores are are not very soluble, so they need to be reacted with acids - usually sulphuric, then the phosphoric acid is neutralised with ammonia to give ammonium phosphate.

Potassium is generally provided by the salts potassium chloride or potassium sulphate which are mined from underground deposits of evaporite - originally derived from seawater.

This also contains sodium chloride and clay minerals which can be separated out from solution by a crystallisation process involving the addition of amine reagents which coat only the potassium chloride crystals and make them float to the surface to be skimmed off when air is pumped in.

NPK fertilisers consist of mixtures of mineral salts such as potassium and ammonium nitrate and phosphate. These are supplied as granules, crystals or powder to be spread on the land or dissolved as a concentrate to be diluted before application to plants in pots and grow-bags.

These products are advertised with ratios of N:P:K, to show the contributions of these three main elements. Often they give details of the analysis methods used in relation to chemical standards.
For example, P2O5 - phosphorus pentoxide and K2O - potassium oxide - are often mentioned, although they are unlikly to be actually present, and ammoniacal nitrogen refers to ammonium salts rather than nitrate.

Many granular solid garden fertilisers are based on a product National Growmore piloted in the last war to encourage the public to grow vegetable produce and assist in the war effort - with the slogan 'Dig for Victory'!
Typically this has a NPK value of 7-7-7. Its recommended application rate is 70g per square metre, and it is usually raked lightly into the soil surface.

Liquid tomato food has a NPK value of close to 4-3-8 or 4-5-9 but it is generally diluted more than 200x or 500x..



Organic fertilisers

Here the term 'organic' is used in its original context: derived from living organisms.

Animal manure and urine, and plant products such as lawn trimmings and fallen leaves, and even seaweed, provide a number of compounds which can be used by plants for growth, but the exact composition and quantities are rather undefined. In fact inorganic ions are released by the action of bacteria in soil which decompose the organic matter. Sometimes it is subjected to a partially controlled process - composting - which results in a somewhat more acceptable product. Additionally this improves soil structure and water-holding capacity.

It is seen as an example of a slow-release or controlled release fertiliser, meaning that ions are gradually released over a longer time than purely inorganic sources.

Certain products of animal origin actually provide fairly concentrated and defined components to soil:

Blood fish and bone is presumably a by-product of fish-processing. Its NPK rating is generally close to 5.5:8:6.

Bone meal is a product derived from meat processing, and contains phosphorus (bone being principally calcium phosphate) as well as nitrogen (from protein in the bone marrow), so it is said to encourage growth of roots. Some imported products have been found to be infected with anthrax , which is concerning because this anaerobic bacterium produces spores which can remain in soil for years. Its NPK rating is typically 3:9:0.



The downside of fertilisers

Inorganic nutrients are instantly accessible to plant roots so they generally cause a marked growth spurt, and cancel out any mineral deficiencies.

However they may cause other problems depending on soil type and weather conditions. In rainy conditions or after flooding these highly soluble salts may be washed out (eluted) from the soil, especially from sandy soils, which are free draining. This leaching process is less likely with clay soils which retain more water.

A similar and more severe effect may be noticed when liquid associated with animal manure escapes into the environment.

Increase in soluble inorganic nutrients (eutrophication) in water courses (streams and rivers) are likely to increase the growth of algae in the water. As water becomes deeper and more slow moving algae will die due to reduced penetration of light. This may lead to a buildup of bacteria which use up dissolved oxygen in the water, causing death of aquatic life.

Water may be removed (abstracted) from water courses for use in domestic water supply. Elevated levels of nitrate in this water has been linked to a number of human health issues.

Methaemoglobinemia is a change in the structure of haemoglobin, caused by nitrite ions derived from nitrate ions which have been ingested. This reduces the oxygen-carrying capacity of the blood, and can be a problem with children, especially under 6 months.

There is very little evidence that nitrates (or nitrites) in drinking water cause cancer. However nitrates and especially nitrites added to meat products e.g. bacon in order to speed up the 'curing' process in addition to salting are now well known to produce nitroso compounds which are carcinogenic.

The current drinking water standard for nitrates is below 50 mg/l.


Other related topics on this site

This topic has connections with other topics on this website:

Inorganic ions

Plant mineral nutrition experiment

Vitamins and Minerals from a human /animal perspective

Web links

A Recipe for Hydroponic Success Providing all of the essential elements in the right quantity and the right proportion to each other can seem like a daunting task to even the most mathematically gifted growers. by NEIL S. MATTSON & CARI PETERS


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