Nitrogen. Get your fix here.........

Back in 2019 we wrote about fertilisers. Now whilst it might have sounded a bit like a chemistry lesson there were a few important things to consider - so much so we thought we'd look at it again. You can read that original article here:

Fertilisers – Part One (

This time though we're going to focus on Nitrogen a bit more. And why are we doing that? While the other day when we were clearing one of our beds we noticed these odd looking things hanging on for grim death to the roots of our old broad beans.

Now what we're looking at here are the small white nodules that have developed on the roots. What we reckon they represent are potentially valuable sources of Nitrogen and it seemed such a shame to simply pull them up if they could be of benefit to the soil. So what we did was take some time out and do a bit of research.

This is what we found out and the conclusions that we arrived at.

The first thing we thought about was what Nitrogen is and why it's important.

What is it?

  1. Nitrogen, or N, is a colourless, odourless element that plays a key role in plant growth.

  2. Nitrogen is everywhere and is the most abundant element in the atmosphere – almost 80% of the atmosphere is Nitrogen.

  3. However too little Nitrogen and plants cannot thrive – that might mean low yields. Equally too much Nitrogen can be harmful to plants and can be detrimental to the environment. It’s vital to get the balance right.

Why is it important?

Research tells us that when plants don’t get enough Nitrogen they:

  1. lose their normal colour and can start to yellow,

  2. suffer from poor and stunted growth, that can often result in

  3. lower yields - smaller fruits and flowers.

And why does that happen?

Well whilst not wanting to sound even more like a chemistry teacher – although as we said back in 2019 there’s nothing wrong with sounding like one - Nitrogen is an essential part of our DNA and RNA.

DNA is in nearly all living things and are the “instructions” that are needed for things to know how to grow. RNA is an acid that is the way in which those “instructions” are carried throughout the living thing.

If plants do not get enough Nitrogen they are unable to produce sufficient acids and proteins that the cells need to develop and grow. Without enough Nitrogen, plant growth is affected. Conversely too much Nitrogen and plants produce too much organic matter, such as stalks and leaves, and not enough roots.

It’s important to get that balance right!

How do our plants get it? The Nitrogen Cycle.

So we’ve already said that Nitrogen exists in the atmosphere. The thing is, and whilst our plants are clever things, they aren’t able to absorb that Nitrogen whilst it exists only as a gas. It needs to go through a transformation – a process called Nitrogen Fixation.

And that's what's been happening with those broad beans - we'll go into some detail about that shortly - but for now we'll simply describe the Fixation process as being the way that Nitrogen from the atmosphere is converted into a form that our plants can absorb through their root systems.

Now this is where it starts to get clever as our plants can essentially get their fix in one of two ways - naturally or with some human intervention. Weather can help as under some conditions the Nitrogen that exists in the atmosphere can be converted into a plant friendly form - Nitrogen (N) converts into Nitrogen Oxide (NO) and Nitrogen Dioxide (NO2) - that can then be absorbed into the roots through precipitation - rain or snow!

When humans get involved and when they add heat and pressure through an industrial process then fertilizers are produced. Let's add some more numbers and letters to our Nitrogen - when this occurs and Hydrogen is added to our Nitrogen, Ammonia (NH3) is formed and if that is then processed further then that results in Ammonium Nitrate (NH4NO3). Both are forms of Nitrogen that can be added to soils and used by plants.

Another natural way and why bacteria are important.

Most Nitrogen fixation occurs naturally in the soil and this is why bacteria are so important. Some bacteria attach themselves to the roots of our plants and form a mutually beneficial relationship with the plant - the bacteria gets it's energy from the plant through photosynthesis and quid pro quo they provide the Nitrogen to the plant in a form that it needs. That is then carried throughout the plant and is used by the plant to enable it to grow.

And that's what's been happening with those broad beans. But hang on. There's a bit more to talk about before we get to them..........

Let's talk about Nitrogen Mineralisation.

This is the second stage of the Nitrogen cycle and takes place when the plant has used up all of the nutrients and when it dies and starts to decompose. Microbes act on that organic material and start to convert it into a form of Nitrogen that can be used by plants. And here we'll talk about those broad beans.

With the exception of legumes - of which those broad beans are a part (along with peas and other other types of bean) - all plants get the Nitrogen they need through the soil. Legumes however get their fix through root nodules - and this is what those small white things are on the roots of those broad beans.

Now these were sown towards the end of the summer last year in the hope that we'd get a second crop to replace the ones that we lost earlier in the year when the rabbits beat us to the harvest. The thing is - and as you can see from this photo - we were also unlucky with the variety that we tried.

We think we planted them too late and the weather meant that although flowers started to form we missed the all important pollination to get the flowers to set and turn them into beans. Not to be deterred however we'll try again this year.

But all wasn't lost as what this meant was that the Nitrogen that had started to be stored in those nodules wasn't actually used in the growth of those beans and was simply left on the roots.

Now there is a view that if the plants had survived and given us a healthy crop of beans then that Nitrogen would simply have been used up and left nothing behind. Whilst that makes sense it must equally follow - if what we've said is right - that if those beans haven't provided us with a crop and haven't used up that Nitrogen then some of it must remain. And following that logic through what we've done is simply chop all of those old plants and lightly dug them into the soil - returning whatever Nitrogen was left back into the plot for the coming season.

The form of Nitrogen produced by the mineralisation process is Ammonia (NH3). That then reacts the soil with water to form Ammonium (NH4). This is then held in the soil and is available for use by those plants that don't get it through that beneficial relationship we talked about earlier.

A word of caution here though - and this is where we mentioned earlier about not having or applying too much Nitrogen. If we do have too much and particularly where it's now in the form of Ammonium then we can have the risk of it leaching through the soil or running off the surface and causing harm elsewhere. You see? There's a fine line between providing enough Nitrogen for your plants to thrive and giving them too much that they can't absorb and that then having a detrimental effect somewhere else.

Let's talk briefly now about the final stages of the Nitrogen Cycle.

The third stage is called Nitrification and also occurs within the soil. This happens when the Ammonia in the soil that's been produced during mineralisation, is converted into compounds called Nitrites (NO2) and Nitrates (NO3). Nitrites aren't usable by most plants until other bacteria has changed it into a Nitrate. The process of nitrification is important to plants, as it produces an additional source of Nitrogen that the plants can absorb through their roots.

The penultimate stage of the cycle is Immobilization. This is sometimes described as the reverse of the second part of the cycle, mineralisation, as the pair of them when they act together are able to control the amount of Nitrogen in our soil. Like our plants, bacteria in the soil require Nitrogen as a source of energy. Bacteria take Nitrogen from the soil when the they can't get it from the decomposing plants - particularly when those dead and dying plants no longer contain enough Nitrogen.

And when that happens the bacteria is using up, controlling and balancing the amount of Nitrogen in the soil that might otherwise be used by our plants. And we know that's important as too much Nitrogen can often lead to wonderfully large and leafy plants, but with all that energy put into leaf production the roots and flowers may well inevitably suffer - and that's the last thing that we want.

The final stage of the nitrogen cycle is known as Denitrification. What's happening here is when those ever industrious bacteria convert those nitrates into Nitrogen and return it back into the atmosphere.

And this takes us all the way back to the beginning of the cycle. How clever is that?

We'll talk another time about other ways in which we can help give our soils a Nitrogen fix. In the meantime and if you have any comments or questions please don't hesitate get in touch through either my facebook page or by dropping us a line to

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