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Plant Nutrition


All living things (organisms), without any exceptions, need energy, and all living things, without any exceptions, get this energy from their food. Some organisms can make their own food from small inorganic molecules, and we call these producers, but other organisms cannot, instead they must feed on large organic molecules, molecules produced by other organisms: we call these consumers.

The difference between inorganic and organic molecules is discussed on the Page on Organic Chemistry.

Producers need a source of energy to make their food. Plants are producers and get this energy from sunlight.

At one time it was thought that plants were the only producers but now we have discovered others, for example organisms living at the bottom of the deep oceans where there is no light and which get their energy from inorganic molecules such as hydrogen sulphide welling up from vents in the Earth’s crust. But, apart from the last Section, on algae, this Page is only about plants.

Plants can make their own food by photosynthesis. They use the energy in sunlight to turn carbon dioxide and water into glucose and oxygen. This is more fully described in the page on Photosynthesis and Respiration.

Carbohydrates (sugars and starch) are molecules containing only carbon, hydrogen and oxygen, the hydrogen and oxygen atoms always being in the ratio 2:1 as in water (hence of course carbohydrate!). Glucose is a sugar with the formula C6H12O6. Other sugars are sucrose, fructose, lactose and maltose. Sugars are small molecules soluble in water but starch consists of very much larger molecules (actually condensation polymers) insoluble in water. Plants use enzymes to convert glucose into starch, and also into other sugars, for example the plant sap contains sucrose not glucose.

Photosynthesis enables plants to make glucose and starch, but to make proteins and many other substances the plant needs elements such as nitrogen, phosphorus and potassium, and these it cannot obtain only from carbon dioxide and water.

The way in which plants obtain and use these other elements is called plant nutrition.

This Page is really intended to be read through in order, but there is also an index which takes you to particular sections.

Nutrition in land plants

Photosynthesis is the process by which plants use the energy in sunlight to convert carbon dioxide and water into starch and oxygen. Land plants obtain the carbon dioxide they need for photosynthesis from the air, through their leaves, and they also get rid of the oxygen produced by photosynthesis in the same way. They obtain the water needed for photosynthesis from the soil through their roots, and they take up the other substances they need, in the form of simple water-soluble inorganic molecules, with this water.

Plants need roots to hold them in the ground (for anchorage) and to take up water and minerals. Many plants have one type of root system for anchorage, and separate root systems for taking up water and the minerals dissolved in it.

The main elements needed by plants are
There are also many other elements which plants need.

Amino-acids (and so proteins) are carbohydrates which have been modified by the addition of nitrogen and sulphur atoms, so plants need nitrogen and sulphur to make leaves and in fact every other part.

They need phosphorus to make ATP, the molecule essential for photosynthesis and respiration.

They need magnesium to make chlorophyll, and calcium to make the “gum” needed to stick the cell walls of adjoining cells together.

They need iron and potassium to make the enzymes necessary for photosynthesis and respiration.

Most of the other minerals needed, often in minute amounts, are needed to make other enzymes.

When we say plants need phosphorus we do not mean they need pure phosphorus in the form of the element. Pure phosphorus is very poisonous and catches fire spontaneously in air: plants need soluble inorganic molecules containing phosphorus such as potassium phosphate.

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Uptake and transport of minerals

Land plants take up water and minerals through special hairs on their roots. The concentration of minerals dissolved in the water inside the cells in the root hairs is normally greater than that in the water in the soil so water, and only water, passes from the soil to the cells in the root hairs by a process called osmosis. Osmosis does not require energy. In addition, water containing dissolved minerals passes into the cells in the root hairs by a process called active transport. Active transport does require energy, and this is produced by the plant’s respiration.

The water, and the minerals dissolved in it, from the root hairs is then carried up inside the plant to the leaves through long thin vessels (tubes) called xylem. The xylem vessels carry water in one direction only. The plant also contains other vessels called phloem, and these carry water and minerals and the sugars and other substances produced in the leaves around the plant. Unlike the xylem therefore they can carry water in either direction.

You can see the xylem and phloem vessels, and the difference between them, very clearly by putting a piece of celery into a beaker or jar of coloured water and leaving it for at least an hour. If you then cut across it with a sharp knife (care!) you should see the coloured water has been taken up in the xylem but not the phloem vessels.

Some of the water taken up by the plant is needed for photosynthesis and for other purposes, and some is not. The surplus water is passed back into the atmosphere from the leaves. This process is called transpiration. During the growing season (see below) plants lose very large amounts of water through transpiration.

The urine of carnivores is very much stronger than that of herbivores, and if a carnivore urinates on a plant (or if we apply incorrectly diluted fertiliser) the concentration of minerals in the soil may exceed the concentration inside the root hair cells. Then water passes by osmosis from the plant to the soil, and it may wither and even die.

DogUrine.jpeg - 535Kb

The grass round this lamp-post in a local Park has been killed by dogs.

To read about why the urine of carnivores is so much more concentrated than that of herbivores please click here Link to page on digestion

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Burning land plants

Most of the mass of a plant consists of compounds mainly of carbon and hydrogen: if the plant is burnt the carbon and hydrogen are returned to the atmosphere in the form of carbon dioxide and water; the ashes left behind are compounds of the elements in the minerals it obtained from the soil. These are very rich in potassium: the English word potassium comes from potash, and dates back to a time when most cooking was done by heating the food in pots over a wood fire.

All compounds of potassium are soluble in water, and the ashes obtained from burning plants dissolve in water to form an alkaline solution. Our word alkali comes from the Arabic al kalium (the ashes), and in most countries other than Britain and the United States the element which we call potassium is called kalium, hence its chemical symbol is K.

The ashes obtained by burning sea plants contain compounds of sodium as well as potassium, hence their common name soda ash. Soda ash is also soluble in water and alkaline. The old name for sodium is natrium, from a mineral called natron found in the Egyptian desert and used for drying mummies, hence the chemical symbol for sodium is Na.

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Woodland, Autumn and The Fall

Land plants lose water through their leaves, and this water can only be replaced by taking up water from the ground through their roots.

In order to photosynthesise and grow plants need water, sunlight and warmth. In temperate areas such as southern England there is enough sunlight and warmth for plants to grow well in the summer but not in the winter.

We normally divide the year into a growing season,when there is enough water, warmth and sunlight for plants to grow well, and a dormant season, when there is not. Deciduous trees shed their leaves in the autumn (hence the lovely North American term Fall) to minimise water loss during the winter (dormant season), replacing them all in the spring. Although this needs energy the growing season is long enough for them to be able to do this. In Scotland and Northern Europe however the growing season is much shorter and the trees do not have the time to grow new leaves every spring. Instead they retain their leaves all the year round - trees which do not shed their leaves during the winter (or dormant season) are called evergreen trees.

Most evergreen trees have special needle-shaped leaves to reduce their surface area and so minimise water loss during the winter, whereas most deciduous trees have broad leaves to maximise surface area for photosynthesis during the summer. Most needle-leaved trees also produce cones containing their seeds. Sometimes the terms broadleaved and deciduous, and needle-leaved, coniferous and evergreen, are used interchangeably, but they are not the same and, for example, holly is a broadleaved evergreen tree (hence its use as a decoration at Christmas) and yew is a needle-leaved deciduous tree.

If we plant trees such as evergreen conifers which are adapted to a short growing season in an area with a longer growing season they will grow very much more quickly than the native broad-leaved deciduous trees. If we see trees only as a resource to be harvested as quickly as possible, replacing the native trees with coniferous evergreen trees makes good sense, but such a policy has a devastating effect on all the birds, mammals, insects and other animals which rely on the native trees, and all the plants which need the light in the spring before the broad-leaved deciduous trees have grown their new leaves.

Because in most places there is a growing season and a dormant season trees show annual growth rings. These can be used to date trees, and objects made from trees such as the timbers in old buildings, using a process called dendrochronology - there is more about this on the Page on Carbon and other ways of dating.

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Decomposing organisms

In Nature almost everything is recycled. Many animals such as vultures feed on carrion, dead animals which they have not themselves killed. Animals which eat carrion are called scavengers. Many other animals eat non-living organic matter, for example earthworms eat bits of plant, and dung beetles eat cow dung. But plants cannot use non-living organic matter: apart from carbon dioxide and water they can only use simple water-soluble inorganic molecules, which they can take up as minerals from the soil.

Most land plants are, quite literally, rooted in the soil. They will very quickly use up all the minerals in the soil around them, and if the plant is to go on living these must be replaced. In Nature they are replaced by decomposing organisms: decomposing organisms (decomposers) are mainly bacteria and fungi.They feed and grow by breaking down the complex insoluble organic molecules in living or once-living organic material into the simple inorganic water-soluble molecules which plants can use.

It is important to understand that decomposers are very specifically the organisms which break down the large insoluble organic molecules into small soluble inorganic molecules. Vultures and earthworms and dung beetles do have a vital role to play in recycling organic material but they are not decomposers.

In unmanaged areas, away from human activity, growth and decomposition are usually in balance; in gardens and on farms however dead plants and animals are not usually allowed to decompose where they lie and so the nutrients the plants have removed from the soil must usually be replaced by adding natural or artificial fertilisers. The minerals most quickly depleted from the soil in farms and gardens are usually nitrogen, phosphorus and potassium - fertilisers containing all of these are called NPK fertilisers.

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The nitrogen cycle

Plants need nitrogen, but although the air is nearly 80% nitrogen plants cannot use the nitrogen in the air, they can only use simple water-soluble compounds of nitrogen such as nitrates and ammonium compounds. Decomposers do of course provide these, but during decomposition some of the nitrogen may be lost to the atmosphere.

Some bacteria and other organisms can fix nitrogen, taking it from the air and converting it into compounds which plants can use. These bacteria often grow in lumps (nodules) on the roots of peas and beans. Farmers sometimes grow these plants on poor soil and then plough them in to improve it. The nitrogen present in the urine of animals, particularly carnivores, also eventually passes back into the soil. During thunderstorms lightning can heat the air to a high enough temperature for nitrogen to react with oxygen and this is another way in which nitrogen is fixed.

It must be said that as well as nitrifying bacteria which fix nitrogen there are also denitrifying bacteria which unfix it....

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Insect-eating plants

Some types of soil are very deficient in plant nutrients (do not contain all the minerals the plant needs in the form of small water-soluble molecules). For example, soluble salts are very rapidly leached (washed out of) out of sandy soil, soil exposed to the continuous spray from waterfalls, and soil in areas of very high rainfall, while the water in some bogs is so acidic that most decomposing organisms cannot live in it and so the nutrients in dead plants and animals are released only very slowly.

Many plants, for example fly traps and pitcher plants, which grow in these very poor soils photosynthesise quite normally, but obtain the minerals they need not from the soil but by capturing and digesting insects. Many people keep insect-eating plants at home, but you must only water them with rain or de-ionised (distilled or purified) water - if you add any sort of fertiliser to the water they will grow quite happily but they will not catch insects.

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The tropical rain forests

The tropical rain forests are areas of very high rainfall, sufficient to have leached all the minerals out of the soil thousands of years ago. However they are also very warm and humid, ideal conditions for both growth and decomposition, and when a tree (or anything else) in the rainforest dies the nutrients in it are not only released almost immediately but also almost immediately taken up by plants. The nutrients therefore barely enter the soil and are in no danger of being washed away by the rain. However, if the trees are felled and the trunks are taken away rather than being allowed to decompose where they fall the nutrients in them are lost from the forest for ever, and even if the ground is immediately used for farming the soil is too poor to support crops for more than one season. Once the trees have gone the land is lost to all plant life - for ever.

The temperate forests of North America, Europe and Asia are also being felled, but these are areas of much lower rainfall and so the minerals have not been so completely leached out of the soil. Also the circulation of the air in the atmosphere ensures that soil carried away by the wind in the tropics is deposited in the temperate areas. The temperate forests are therefore capable of recovery in a way in which the tropical forests are not.

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Parasitic Plants

Some plants, such as ivy and honeysuckle, have normal roots and leaves, and photosynthesise and feed in exactly the same way as other plants, but their stems are not strong enough to support them, instead they must climb up trees or other structures: gardeners and farmers often provide wires or trellises for them to climb up. The general name for plants which climb in this way is vine. Vines include tomatoes and hops and of course grape vines. Because they do not need to use energy or nutrients to grow thick stems to support themselves, vines can grow very much more rapidly than other plants. Vines are classed as parasites although they do not usually harm the tree (the host) they are climbing on, unless they totally smother the leaves of the host.

Vines can also hang down from the branches of the trees they are growing on - hence Tarzan can use them as ropes to swing from!

Gardeners and fruit farmers do not usually like ivy growing on their trees, but it provides an important habitat for birds and other animals and so in woodland it is best not to remove it from trees.

Mistletoe grows from seeds which have been deposited, usually by birds, in cracks high up in trees. It has leaves and photosynthesises normally, but its roots go into the xylem of the host tree rather than down to the ground, so it takes its minerals from the sap rising in the host tree. Although it is classed as a parasite it does not usually harm its host.

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Mosses and lichens

Mosses and lichens are often considered together, and are often confused, but they are actually quite different.

Mosses are very tiny plants, with roots and leaves, and they feed and photosynthesise like other plants. They can grow in cracks in walls, on tree trunks, in gutters, anywhere where there is even the smallest amount of soil.

Lichens (correctly pronounced like-enns but increasingly pronounced as it is written) can grow on trees, rocks, walls, roofs, tombstones etc - they do not need any soil at all. They are very sensitive to atmospheric pollution so you may find them on tombstones in old country churchyards but not usually in town ones. They are not actually plants at all but a symbiotic relationship between a fungus and an alga. Algae are very simple organisms capable of photosynthesis. They used to be classified as plants but now they are classified as protista. The alga and the fungus which make up a lichen are totally dependent on each other. The alga photosynthesises and produces carbohydrate and other complex organic molecules both for itself and the fungus, while the fungus releases minerals from the rock (the stratum) or other surface on which it is growing, and in some cases fixes nitrogen from the air, to provide nutrients for the alga.

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Nutrition in water plants

Some plants growing in the water margins around rivers and lakes, such as alders and bulrushes, are essentially land plants which need a lot of water. They have their roots in the soil (or mud) and their leaves in the air, and photosynthesise and feed like other land plants.

Some plants, such as water-lilies, have their roots in the soil and their leaves (or at any rate their top surface) in the air. They obtain their nutrients from the soil or mud and their carbon dioxide from the air; they use the water only for support: if you take a water-lily out of water it will flop. Water lily leaves have their stomata (the pores which let carbon dioxide and oxygen in and out) only on the top surface, whereas the leaves of most other land plants have their stomata only on the bottom.

Some plants, such as duckweed and water hyacinths, float on the surface of the water with their roots in the water. They obtain their carbon dioxide from the air and their nutrients from the water. They have no anchor roots so drift with the current.

Most other water-weeds, sea-weeds and sea-grasses have their leaves under water and obtain both their carbon dioxide and minerals from the water which surrounds them. They use their roots only for anchorage, to stop them from being washed out to sea or thrown onto the land. Many sea-weeds have gas-filled bladders or sacs (little bags) to keep the leaves near the surface to obtain the maximum amount of light for photosynthesis. These plants can only live in relatively shallow water.

In the deep oceans and seas which cover two thirds of the surface of this planet the only form of plant life, and therefore the only beginning of every oceanic food chain, are the very tiny plants in the plankton layer. These plants, the phytoplankton, obtain their carbon dioxide and nutrients from the water which surrounds them. They are eaten by very tiny animals, the zooplankton, and the zooplankton in their turn are eaten by slightly bigger animals, and so on until sharks and turtles and whales. The plankton layer is only a few metres thick because the phytoplankton needs light in order to photosynthesise.

There is just one problem. For reasons explained on the Page on submarines, a shark or turtle or whale cannot stay at a constant depth in the ocean without making tiny movements with its fins or flippers. If it dies it will float to the surface or, much more usually, sink to the bottom. As it sinks its carcase will be eaten by other animals, and what is left by the time it reaches the bottom will eventually decompose. But the minerals in it will have been removed from the plankton layer.

When the Earth was young the oceans teemed with life. It is not commonly realised that today nearly 80% of the surface of the oceans, and therefore nearly half of the surface of this planet, is almost devoid of all life because the plankton layer has become depleted of minerals and now no longer contains enough iron to support the growth of phytoplankton. The result is that those areas of the oceans which do still contain enough iron to support phytoplankton, and therefore the food chains dependent upon it, are being severely over-fished. Experiments carried out in the 1990s have shown that where iron, in the form of ferrous sulphate, has been added to the surface in areas without phytoplankton, and therefore without any of the animals dependant upon it, within a few days the water has been teeming with not only phytoplankon but also all the animals as well. It may well be therefore that the key to feeding the world's ever-increasing population is as simple as sprinkling ferrous sulphate (easily made from, for example, rusty old cars and the sulphur dioxide produced as a waste product by coal-fired power stations) on the oceans.

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Algae, biofuels and eutrophication

Algae can photosynthesise but they are actually classified as protista rather than plants.

They grow in water or other very damp conditions, and exist either as single cells or very long thin filaments. As they do not have to make roots or stems or leaves or transport systems, under the right conditions they can grow very rapidly, even in water heavily contaminated with sewage or agricultural or industrial chemicals.

There is a lot of very exciting work going on to grow them as a source of biofuels such as methane and ethanol, even using the gases from coal-fired power stations to raise the carbon dioxide level in the poly-tunnels they are growing in.

If algae are growing in ponds or lakes or rivers containing large amounts of sewage or run-off from farmland containing high levels of fertiliser they may initially grow very rapidly indeed. This rate of growth cannot be sustained and they will quickly exhaust their food supply. Then they will start to die. The decomposing bacteria will feed on the dead algae and their population will increase very rapidly. Their respiration will very quickly use up all the oxygen and then both they and all the other animals in the water will die. Within a few days all that is left is a pool of lifeless green water. This is called eutrophication.

If untreated sewage is allowed to run into a river the most important first step to stop all the fishes dieing is to install pumps to oxygenate the water.

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© Barry Gray August 2008 last revised May 2016

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