The role of nitrogen in lawn fertilisers

By | 24/07/2016

Nitrogen is the most abundant chemical in the earth’s atmosphere, making up 78% of the air that we breathe. However the vast majority of this is in a gaseous form, which cannot be utilised by plants. Plants can only use a “fixed” form of nitrogen in the soil.

In nature, plant and animal waste deposited into the soil are broken down into nutrients by bacteria and fungi. These convert the organic matter into forms of nitrogen that are usable by plants. Plants then use this nitrogen to grow. The plant is then eaten (or dies) and returns nitrogen to the soil starting the cycle again. This process is called the nitrogen cycle.

A very small amount does fall onto the soil from the atmosphere dissolved in the rain. This is only a fraction of the amount required by plants. The majority of nitrogen on the earth stays in the form of a gas in the atmosphere.

In the situation of maintaining a nice lawn, very little waste material actually makes its way back into the soil. We collect grass clippings and don’t allow any waste material to rot down into the soil replenishing the nitrogen. So, over time, usable nitrogen in a lawn becomes depleted, which is why we need to feed lawns with an additional fertiliser to keep them looking healthy.

How does nitrogen help a plant grow?

Nitrogen is very important in helping all plants to grow. It is an essential element that is used in the manufacture of new plant cells, which build up to make the leaves and stems. It is a fundamental part of all plant DNA.

Nitrogen is “fixed” in the soil by beneficial bacteria and other organisms into a form usable by plants called amino acids. These compounds are then absorbed by the grass plant through its roots and are used throughout the plant to help it to grow and to make food for itself.

Nitrogen is very important in maintaining the health of your lawn, as all parts of a grass plant depend on there being a readily available supply in the soil. If you have too much or too little nitrogen, you are going to encounter problems with your lawn.

How do I add nitrogen to my lawn?

Nitrogen is most commonly added to a lawn in the form of fertilisers, and my lawn fertilisation programme is specifically designed to provide the correct amount of nitrogen to your lawn at the correct time of year.

The N in a lawn fertiliser usually comes in the form of a compound of a water soluble ammonia (nitrate). This enables it to make it’s way down through the soil to the roots of the plant easily.

Most often it is applied as a granule. I sometimes change to a liquid form so that it can be more safely applied to the lawn in times of environmental stress, such as drought.

When should I add nitrogen to my lawn?

It is important to vary the amount of nitrogen that you add to your lawn during the year. This is so that it coincides with the amount of growth required by the grass plants.

Spring and early summer are the key seasons for adding nitrogen, as the grass will be growing as strongly as possible to make full use of the lengthening hours of sunlight and warming soil, so the increase in available nitrogen will help it to make full use of the great growing conditions.

As we go through the year, a nitrogen fertiliser should be added to your grass periodically, but it is important to keep the application rate at a level that the grass can cope with, without allowing too much of a build up in the soil. Regular feeding is the key for fertilisation during the summer.

As autumn comes around, the amount of N in the fertiliser should decrease, being replaced by other important nutrients to help the grass harden in preparation for the colder winter months. Applications of a high-nitrogen fertiliser at this time of year will only result in soft, weak growth that will be susceptible to disease, will increase the thatch levels and generally lead to a poor-quality lawn.

The regular treatment programme is specifically designed to provide your lawn with the correct amount of nutrients throughout the growing season.

Is your lawn deficient in nitrogen?

Nitrogen deficient grass
Nitrogen deficient grass

Nitrogen is very easily absorbed by the roots of grass plants. In the situation of a lawn, the levels in the soil can quickly become low enough to start to cause problems for the grass plants. This can be made worse by heavy rain or if you have a particularly free-draining soil, “leaching” the nutrients away into the earth.

This is a big problem for the groundsmen at many high-profile sports clubs. This is because a playing surface on a sports pitch needs to be hard (often 100% sand). It retains little or no nutrients within the soil. In this situation, nitrogen needs to be regularly supplied artificially just to keep the grass alive, and so the lawn becomes “drugged” and dependant on continual feeding. As a result, the grass has a very short lifespan and needs to be completely replaced every year.

Nitrogen deficiency can be seen in lawns as a yellowing of the grass. This problem is called chlorosis and is a result of the grass plant being unable find any usable nitrogen in the soil, so it cannot build new chlorophyll cells (the green part of the plant) so the old cells die and are not replaced. The leaves then turn yellow, wither and die. This is a big problem for the plant as it is then unable to manufacture food. It goes into shock, moving key nutrients back into the core of the plant. Basically going into hibernation until the situation for it improves. If it does not, then the plant will eventually die of starvation.

The problems of over-feeding a lawn with high-nitrogen fertilisers.

A burnt lawn
A lawn burnt from over feeding

Put simply, nitrogen encourages grass plants to grow. But you can certainly have too much of a good thing when it comes to lawn fertilisers. Many “reputable” lawn care companies feed their lawns with the same, high-nitrogen fertilisers over three times a year. This results in an excessive amount of uncontrolled growth and several years down the line the lawn becomes excessively thatchy, drowning itself. After several years of over-feeding, it can become very difficult and expensive to bring the lawn back to it’s best.

Most fertiliser materials are readily soluble because they manufactured in the form of a salt. Once they are dissolved in the soil, they increase the salt concentration of the soil solution. If there is not enough moisture around the grass plant during this time, then the remaining moisture inside the plant will be draw out through osmosis and the plant will be injured, sometimes fatally. This process is called fertiliser burn, and it can take many weeks to rectify the problems caused by careless application.

Over-feeding a lawn can also have environmental consequences. Run-off of rain water can take excess nutrients into streams and rivers. These nitrates feed algae, which can then grow to unsustainable numbers. They literally suck the life out of the water. For this reason, it is very important to only fertilise your lawn with an amount that is usable by the plants, and be sure to not spill any into water courses.

In summary

To keep a beautiful lawn, you need to feed it with a fertiliser that contains nitrogen. However it always needs to be applied at the correct rate and at the correct times of the year. The best way to ensure this is to have your lawn maintained by an expert.

If you would like more information about maintaining your lawn, please don’t hesitate to get in touch and I will be happy to advise.

Kris Lord
The Lawn Man

Further reading on the role of nitrogen in your lawn:

6 thoughts on “The role of nitrogen in lawn fertilisers

  1. Robert Hastings

    What year round fertiliser would you recommend for my lawn, it was laid 4 year ago but I was laid on old soil from a local pit heap, when I first grew it was lush and green but now its patchy and dead in places, grows at diffrent stages, one side of the garden grows thick and looks strong, as you get close to the middle its weak and hardly grows, would the fertiliser give the soil balance to get and even growing lawn

    1. Kris Lord Post author

      If the lawn has deteriorated gradually over time, I would certainly look at Aerating your lawn as you may have a compaction problem in those areas. This is common and one of the most overlooked problems, but most beneficial in all lawn care.
      As for fertiliser for all year round, I would suggest something gentle and balanced. Something like Growmore or a general purpose feed applied a regular intervals can do wonders for all lawns. Feed your lawn little and often will get much better results long term than feeding it rocket fuel once a year!
      Thanks for reading.

  2. jameson

    hello im struggling to understand how to make my grass look pretty, any advice?

  3. Eamon McGlennon

    Good morning, my query is how to rid my lawn of Red Thread. The lawn was relaid some 3 years ago and was fantastic the first and second year. Red thread developed last Autumn and remains. Have been advised to increase the Nitrogen and if the red thread persists use Seaweed based fertilisers. I am hesitant in using an Antifungus spray.
    Very grateful your comments.

    1. Kris Lord Post author

      Hi Eamon,
      A persistent Red Thread infection will rarely be cured by feeding alone. This will only produce soft, weak growth which will increase the problem.
      A fungicide will sort the problem, however without treating your only solutions are a really careful, year round feeding programme, keeping the thatch levels at a minimum, aerating regularly and swishing dew off the lawn during warms days.
      Thanks for reading


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