Microbes that feed the soil · Biology Class 11–12 | StemPy Lab
Course 26.3·Lesson 2 of 3
Microbes that feed the soil
Biofertilisers are living organisms that enrich the soil. Rhizobium lives in the root nodules of legumes and fixes nitrogen. Azospirillum and Azotobacter fix nitrogen as free-living bacteria. Mycorrhizal fungi such as Glomus bring phosphorus to the plant, and cyanobacteria fix nitrogen in paddy fields.
about 30 min1 simulation1 practice question
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Food for the soil
A plant needs nitrogen to make proteins and chlorophyll. The air is about four fifths nitrogen gas, but a plant cannot take it up from the air. It needs the nitrogen in a form such as nitrate or ammonium.
Farmers add fertilisers that are made in factories. Too much of them washes into rivers and pollutes the water. A biofertiliser is an organism that enriches the nutrient quality of the soil. The main sources are bacteria, fungi and cyanobacteria.
A table with four columns, Biofertiliser, Group, How it lives and What it does, and five rows. Rhizobium: a bacterium, symbiotic in the roots of legumes, fixes atmospheric nitrogen. Azospirillum: a bacterium, free-living, adds nitrogen to the soil. Azotobacter: a bacterium, free-living, adds nitrogen to the soil. Glomus: a fungus, symbiotic as a mycorrhiza on roots, passes phosphorus to the plant. Anabaena, Nostoc and Oscillatoria: cyanobacteria, free-living in water and soil, fix nitrogen, mainly in paddy fields.
Nitrogen fixing bacteria
Some microbes can fix nitrogen. This means that they change the nitrogen gas of the air into a form that plants can use. They do it with an enzyme, nitrogenase.
Rhizobium is a bacterium that lives in symbiosis with the roots of legumes. Peas, beans, groundnuts and soyabean are legumes. A symbiosis is a living together in which both partners gain.
Rhizobium cells settle on a root hair, which curls.
A tube called the infection thread carries the bacteria into the root.
The root cells divide, and the swellings that form are the root nodules.
Inside the nodule, the bacteria change into bacteroids. They fix nitrogen into organic forms that the plant uses.
The plant gives the bacteria food. A pink pigment, leghaemoglobin, in the nodule keeps the oxygen low, because the enzyme nitrogenase does not work in much oxygen. You meet this again in Mineral Nutrition.
Three panels. Panel 1, a root hair is infected: a curled root hair stands on the surface of a root made of rectangular cells, many red rods of Rhizobium lie round its tip, and a blue dashed line, the infection thread, runs from the curled tip down into the root. Panel 2, nodules on a legume root: a vertical main root with four side roots, each with three round pink swellings, the nodules. Panel 3, inside a nodule: a large circle with a pink central region full of dark red dots, the bacteroids, and a green vascular strand at each side. A label says that the pink colour is leghaemoglobin.
Azospirillum and Azotobacter are free-living bacteria. They do not need a host plant. They live in the soil and enrich its nitrogen content.
Fungi: the mycorrhiza
Some fungi live in symbiosis with the roots of plants. This living together is called a mycorrhiza. Many members of the genus Glomus form mycorrhiza.
The fungal partner absorbs phosphorus from the soil and passes it to the plant.
Plants with mycorrhiza resist root borne pathogens better.
They tolerate salinity and drought better.
Their growth and development improve in general.
The fungal threads, called hyphae, spread through a large volume of soil, much larger than the roots alone could reach. The plant gives the fungus sugars.
A drawing of a plant root, a green box of cells at the top left, above the soil, a brown layer. Brown threads, the fungal hyphae, run inside some of the root cells and out of the root into the soil, where they branch widely among blue dots that stand for phosphate. Two blue arrows lead from the soil through the fungus to the root. A note says that the fungus takes up phosphorus from a large volume of soil and passes it to the plant.
Cyanobacteria
Cyanobacteria are autotrophic microbes. They make their own food by photosynthesis. They live in water and on land. Many of them can fix atmospheric nitrogen. Examples are Anabaena, Nostoc and Oscillatoria.
In paddy fields, cyanobacteria are an important biofertiliser. They are also called blue green algae. They add organic matter to the soil and so increase its fertility.
A note on the numbers. Urea is the common nitrogen fertiliser. It is about 46 per cent nitrogen by mass. The simulation below uses round teaching values for the nitrogen that a crop needs and for the share that microbes supply. Real values depend on the crop, the soil and the season.
You should find that a bigger share from the microbes saves more urea, and that a bigger field saves more in total. A biofertiliser does not change the need of the crop, only where the nitrogen comes from.
Try it in Python
The program works out the urea that a field needs with and without a biofertiliser.
Practise with new numbers
Common mistakes
**Saying that Azotobacter lives in root nodules.** It is free-living. Rhizobium is the one in nodules.
Saying that mycorrhiza fix nitrogen. The fungus brings phosphorus from the soil.
Calling cyanobacteria heterotrophic. They are autotrophic.
**Calling Glomus a bacterium.** It is a fungus.
Forgetting that urea is not pure nitrogen. Divide the nitrogen by 0.46 to get the mass of urea.
Remember these
Biofertilisers: bacteria, fungi and cyanobacteria that enrich the soil.
Rhizobium: symbiotic with legumes, root nodules, fixes nitrogen. Azospirillum and Azotobacter: free-living nitrogen fixers.
Mycorrhiza, as in Glomus: a fungus on roots that gives phosphorus, and also resistance to root pathogens and tolerance to salinity and drought.
Cyanobacteria (Anabaena, Nostoc, Oscillatoria): autotrophic, fix nitrogen, important in paddy fields.
Check yourself
Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.
What is a biofertiliser?
Where does Rhizobium live, and what does it do?
Name two free-living bacteria that enrich the nitrogen of the soil.
What does the fungus of a mycorrhiza give to the plant?
Why are cyanobacteria useful in a paddy field?
References
NCERT (2023). Biology, Textbook for Class XII, chapter 8, Microbes in Human Welfare. NCERT (2023), Biology, Textbook for Class XI, chapter on mineral nutrition.
Vessey J. K. (2003). Plant growth promoting rhizobacteria as biofertilizers. Plant and Soil, 255, 571 to 586.
Smith S. E. and Read D. J. (2008). Mycorrhizal Symbiosis, 3rd edition, Academic Press.
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