What is nitrogen fixing in plants and how does it work?

Nitrogen fixing is a term many of us use. We grow beans or peas because they’re nitrogen fixing plants. We can rotate our crops by following up a crop of legumes with nitrogen needy plants such as lettuce, silver beet, kale, broccoli or Brussels sprouts. But when we look at the science, we see that it isn’t only the plant that ‘fixes’ nitrogen.


Without transformation, nitrogen is unavailable

Nitrogen makes up approximately 80% of air. Strangely enough, without transformation, this nitrogen isn’t available to plants or animals (including humans). For that we need others. We obtain most of our nitrogen not by consuming plants.

When we eat other animals, we obtain nitrogen. This particularly the case where we eat animals herbivores.

Our urine contains a massive amount of nitrogen.   That’s why we see dark green patches on our lawns – it’s where our dogs have urinated. That’s the effect of nitrogen.

Without conversion, atmospheric nitrogen (in the air) is also unavailable to plants. Microbes are the key transformers. Therefore, when we ask: what is nitrogen fixing – we’re asking about a microbial process.

According to soil microbiologists Dr Elaine Ingham, bacteria and fungi contain the nutrients that plants require. This makes sense given bacteria and fungi are key agents in decomposing organic matter.  They also consume stones and rocks,  and as they do, those mineral elements become part of the body of the microbes.  Just like us, when we eat, most of the nutrition from our food stores in our bodies for a time.

When we eat, we’re feeding particular microbes.  What we eat feeds different microbes. We’re all hearing about gut health and we’ve known for a long time that plague on our teeth is a microbial process fed by sugars.

In the soil, microbial excretions become plant available nutrients. When other soil creatures such as nematodes, micro-arthropods and earthworms etc eat bacteria and fungi, excess nutrients are also excreted. These excretions are transformed in the bodies of these beings. The excretions also become plant available nutrients.

Regarding nitrogen, the plant available form is mostly ammonium (a form of nitrate).


Its not survival of the fittest – its survival of the cooperative

All plants work with microorganisms. Plants excrete exudates into the soil and onto their leaf surfaces. These excretions feed specific species of bacteria and fungi and therefore promote the growth of particular communities of bacteria and fungi.

The type of bacteria and fungi that plants support are those that are beneficial to the plant. Clearly, if a plant fed plant parasitic microbes the plant would quickly get sick and die. This means there is some form of intent: some form of consciousness when a plant releases exudates.

So plants release exudates into the soil. The areas closest to the plant roots – the rhizosphere – is therefore extremely high in plant beneficial bacteria and fungi numbers. This is the area of extremely high activity – but only in a healthy soil.

Because of this extreme activity, pathogenic microbes find it difficult to compete and are out-competed in a healthy soil. This is one reason why a healthy soil is key to a healthy plant.  My blog on how to create healthy soil explains how to do this.


Do legumes and bacteria make contractual relationships?

Legumes include plants like peas, beans, peanuts, lentils, chickpeas, clover, alfalfa, lucerne, wattles and acacia.  These types of plants form win:win relationships with a type of bacteria (called rhizobia). Rhizobia live in the soil and come to live within the roots of the legume.

Far from being unintelligent, different legumes make what appear to be contractual, quid pro quo relationships with different rhizobia. Plant exudates attract the right sort of rhizobia toward, and ultimately into the plant. It’s like advertising for a business partner or an investor.

Having been attracted to the plant by these secretions, the bacteria begin to penetrate the plant making an initial point of infection. This infection forms a thread into the roots of the plant where the bacteria begin to multiply.

Research suggests that at this point, cells divide in a similar manner to a growth of a tumour. However, far from being uncontrolled growth leading to a malicious bacterial infection or another form of malignancy: the plant controls the formation of the growth.  This growth forms nodules and ultimately the size of the bacterial colony. It’s an example of utter intelligence and social control!

The plant supplies the bacteria with shelter, organic carbon, amino compounds and sugars etc in exchange for ammonium – a plant available form of nitrogen. The bacteria convert atmospheric nitrogen into plant available ammonium (a form of nitrate). That’s what we mean by nitrogen fixing.

Therefore, nitrogen fixing is a process brought about by the intelligent cooperation of two very different species. It’s the outcome of this synergy.  Nitrogen fixing is a symbiotic dance.


Microbial diversity in the soil

In order to colonise the plant, the rhizobia need to be in the soil in the first place. They need to exist in the soil to be attracted to the plant. They don’t just appear out of thin air! Having said that, microbes can be transported to our soil through wind and rain…

This is the importance of microbial diversity. If we have a soil with quality organic matter from diverse sources then we are more likely to have a diverse population of microbes living in that compost and soil.

Therefore, successfully growing organic plants needs quality compost to build and maintain quality, plant specific soil.

Microbes also have functions that are quite specialised. For example, particular species of rhizobia function in synergy with particular species of plants. Therefore, we need microbial diversity in our soil to increase the probability that we will attract the microbes and rhizobia we need. Makes sense, doesn’t it?


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