Plant Feeders

All plants and animals need nitrogen – this forms the ‘amino’ part of amino acids and without it we would have none of the vital proteins needed for life. About 80% of the atmosphere is made of nitrogen but as it is, animals and plants cannot make any use of it – to be of use it needs to be in a form known to scientists as ‘fixed’.

There are only three things that can turn atmospheric nitrogen into fixed nitrogen:

1)                  Lightning

2)                  Manmade methods

3)                  Bacteria

Of these, bacteria make at least half the worlds useable nitrogen, and a hundred years ago (before man-made methods) they made about 85-90% of it. Not all bacteria can make useful nitrogen – those that can are called nitrogen fixing bacteria and most can only perform this process if there is no oxygen around.

This is where plants come in. Some plants can form nodules – these are lumps on the roots that surround the nitrogen-fixing bacteria. Inside the nodules are special mechanisms for absorbing oxygen that let the bacteria get on with making useable nitrogen – this goes directly to the plant. When the plant dies or its leaves fall off, the nitrogen gets into the soil and acts as a fertiliser for other plants.

Some plants that can perform this are:

• Clover

• Soya bean

• Lentil

• Pea

These are known as leguminous plants or legumes. They can be grown specifically by farmers in poor soil and they will increase the nitrogen content, acting as a natural fertiliser, if ploughed in. This is one of the main bases for crop rotation – alternating leguminous and non-leguminous crops reduces the need for fertilisers.

Other plants that are not legumes can also harbour nitrogen-fixing bacteria and these include important plants such as:

• Leucaena (An important tree for firewood and fertilising soil in Australia)

• Alder (important in mountainous regions – its nitrogen-fixing bacteria gradually improve the generally poor soil in these regions).

• Bog Myrtle, serves a similar purpose to Alder but in heath and bog land.

This explains where plants get their nitrogen, so what about animals?

They get it from simply eating the plants, or eating the plant eaters, or eating animals that have eaten the plant eaters and so on.

Not all nitrogen-fixing bacteria live associated with plants – many live in the soil itself and many more live in the sea. The sea-dwelling ones are thought to be very important but the ones living ‘loose’ in the soil are not, contributing only about 1/100^th of the amount of nitrogen that the plant-associated bacteria do.

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