Partners in Plant Production: Rhizobia and Mycorrhizae

by Mike Carter.

Plants, like people and animals, need feeding.  Plant nutrients (or foods) such as nitrogen, phosphorus and potassium are needed for the growth and development of crops and trees.  Farmers can add more of these nutrients to the soil by using manure, compost or artificial fertilisers.

However, some plant nutrients are present in the soil in a form that the plants cannot use.  Nitrogen is present all around the crop in the air. Much of the phosphorus in a soil may be difficult for plants to use. These nutrients are of no use to the plants unless they are first ‘processed’ by tiny micro-organisms in the soil. A single teaspoonful of soil contains millions of micro-organisms like fungi and bacteria.  Most of these organisms are helpful to the crop and farmer.  Some live together with plant roots in a partnership which benefits both the micro-organism and the crop.  Crop yields will be higher when the right micro-organisms are present in the soil.  Two examples of these are mycorrhizae and rhizobia.

Mycorrhizae

These can sometimes be seen as white threads amongst the roots of trees and crops.  If a soil has been used for growing a crop or tree for many years, then the right mycorrhizae will probably already be present in the soil.  But perhaps you want to grow a crop or trees in a soil where it has not grown before - for example, pine trees on a hillside covered with grass.

Include in the soil mix that you use to fill seedling bags or boxes, some soil taken from underneath adult trees of the same type.  Here, for example, is a mixture used in Nepal for filling seedling bags before planting pine seeds:

1 part Unsieved pine forest soil with mycorrhizae
3 parts Sieved soil
3 parts Coarse sand

In particular, mycorrhizae help plants take up more phosphorus. They will still grow without the mycorrhizae, but not as fast.

Rhizobia

Rhizobia are helpful soil bacteria that can process  (or ‘fix’) nitrogen from the air and change it into a form that crops or trees can use. Rhizobia do this in partnership with legumes - plants like peas, peanuts, soyabeans and beans that produce their seeds in pods.  Pigeon pea, Leucaena, Acacia and Prosopis are other examples of legume shrubs and trees.

Rhizobia help this legume extract nitrogen from the air

They produce small nodules (lumps) on the roots of the legume in which rhizobia live.

How do I know if a legume is fixing nitrogen from the air?

Nodules can easily be seen on the roots of crops and trees. Flowering time is the best time to examine them. Nodules vary in shape. Not all nodules effectively fix nitrogen. Effective nodules are plump - cut them open and they are pink, red or sometimes black inside.  Ineffective nodules are usually shrivelled, small and green or white inside.  Unless the right strain of rhizobia is present in the soil, then effective nodules will not form.

What can I do if a legume is not fixing nitrogen?

This can often happen if you are introducing a legume into land where it has not grown before.  The right rhizobia may not be present in the soil.  In many countries inoculant of the correct strain can be obtained, perhaps from a farmers store, the Ministry of Agriculture, from a research station or university.  Usually the inoculant is mixed with peat.  Read the packet label and follow the instructions. It should be kept cool and dry until used. The inoculant is usually mixed with the legume seed in a bowl with a little water, before planting. 

What if I cannot obtain inoculant?

Obtain some soil from a field that has well-nodulated plants of the crop or tree you want to grow.  Try mixing your seed with this soil before planting.  Include in the soil some roots with nodules.

Rhizobia and mycorrhizae are friends of the farmer.  They can help increase crop and tree production.  They improve soil fertility.  With the benefits they bring, it is important that the farmer makes use of them.

Mike Carter works for T-CORD at Bishop Burton Agricultural College.