Home Lawn Care Lawn Repairs Lawn Fertiliser: What, Why, When and How to Feed Your Lawn

Lawn Fertiliser: What, Why, When and How to Feed Your Lawn

By Tim Stephens

Your lawn will never look its best without being fertilised. So having a basic understanding of lawn fertiliser is essential if you want your lawn to look fantastic.

Soil science and fertiliser can be an incredibly complicated subject. But unless you want to be a greenkeeper at a world-class golf course, understanding the basics will be all you need.

In this article, we’re going to take a look at everything you need to know about lawn fertiliser to keep your lawns at home lush and green.


What is Lawn Fertiliser and Why is it Important?

granular lawn feed

Lawn fertiliser, also known as lawn feed is often thought of as food but this isn’t strictly true.

In fact, all fertiliser products, whether granular or liquid are nothing more than compounds which contain the chemical elements plants need to make food for themselves. These chemical elements include;

Nitrogen (N)

  • Important for forming cells and grass growth
  • Helps create chlorophyll (what makes the grass green) which is used in photosynthesis
  • Promotes flowering and seeding
  • Improves root development and overall lawn health

Phosphate (P)

  • Needed for the production of food
  • Important for grass growth
  • Encourages strong root development

Potassium (K)

  • Assists in the creation of plant cells
  • Plays an important role in photosynthesis and other plant functions
  • Affects the drought tolerance of the plant
  • Can be lost from the soil and need regular top-ups

Grass and other plants use photosynthesis (remember that from school?) to make food from water, carbon dioxide and the chemical elements that come from lawn feed.

Water in the soil dissolves the fertiliser which enables grass and other plants to consume these chemicals via their roots.

The amount of sun, water, carbon dioxide and fertiliser your lawn receives has a huge impact on the health and look of the grass.

We have no control over the sun, carbon dioxide or water because they’re supplied naturally by the earth. Yes, we can turn the hosepipe on when it’s dry but we can’t turn off the rain.

So the only thing we really have control over is the nutrients in the soil, which, might often be in short supply.

Not all nutrients will be lacking; the amount of sand, clay, silt and living content (bacteria, insects, fungi, etc.) will all impact nutrient levels.

One thing is for certain though…

Every time you cut the grass you take away water, the primary nutrients that grass uses to make food AND food that is being stored in the leaf.

That’s Why You Need to Feed Your Lawn

Think about it, your grass is the only plant in your garden that you ‘prune’ 20 times or more every year. If you did that to other plants there’d be nothing left.

There is generally plenty of sunlight, carbon dioxide and water but these nutrients need to be replaced because you take them away every time you cut the grass.

Feeding your lawn with a good quality fertiliser puts these nutrients back into the soil. This enables the grass to recover from not only cutting but from being walked and rolled about on.

It also prevents certain weeds from growing. Yarrow, Woodrush, Clover, Self Heal and Thistles all thrive in nutrient-deficient soil. Applying a fertiliser prevents these kinds of weeds from taking over.


Understanding Lawn Fertiliser Labels

lawn fertiliser label

Oftentimes people get confused when looking at the labels on lawn fertiliser products. The big numbers and words like ‘typical analysis’ all sound a bit technical.

In fact, they’re very simple to understand.

There are normally 3 numbers on a bag of fertiliser. These numbers describe the chemical properties of the bag’s contents.

For example, on the label above the number looks like this: 29-6-12.

The first number (29) is the Nitrogen (N) content. The second (6) is the Phosphate (P) content and the third number (12) is the Potassium (K) content.

If you look at the Typical Analysis information you will notice that these numbers represent the percentage of the bag’s content;

  • Nitrogen (N): 29% or 29 grams in every 100
  • Phosphate (P): 6% or 6 grams in every 100
  • Potassium (K): 12% or 12 grams in every 100

You’ll also see that there is a letter after each chemical compound. (N) represents Nitrogen, (P) represent Phosphate and (K) represent Potassium on the periodic table.

So if you ever hear someone talk about NPK fertiliser, they’re referring to fertiliser that contains Nitrogen, Phosphate and Potassium.

But let’s get back to the percentages.

If you add 29% to 6% and 12% you get 47%, or 47g in every 100. So what’s in the other 53%?

Well if you think about it, Nitrogen is a gas so it’s not just made up of Nitrogen. If it was it would escape. So it comes in the form of a chemical compound, usually a salt which sometimes has a slow-release coating.

Some lawn fertilisers also include organic matter to help improve the soil, secondary nutrients like Calcium, Magnesium, and Sulphur, as well as trace elements which, when added up, make the 100%.


The Different Types of Lawn Fertiliser

types of lawn fertiliser

There are hundreds of different lawn fertilisers and feed and weed products on the market. Trying to figure out which lawn feed you should use is often a very confusing process.

I actually wrote a guide to tell you which product to use and when, which you can read here.

For now, read the following section, we’re going to look at the different types of lawn fertiliser and how they work.

Granular vs. Liquid Fertiliser

Dry, granular fertilisers are generally used as ‘base fertilisers’. They should be applied routinely as they are the only way to give your lawn the nutrients it needs in one go.

These types of fertilisers can be quick-release or slow-release. Either way, they dissolve and add nutrients to the soil which the grass consumes via its roots.

Liquid fertiliser should only be applied when needed. As a result, this is sometimes called ‘demand feeding’.

Again, liquid fertilisers can be quick-release or slow-release. They are sprayed onto the leaves of the grass plant and consumed by the leaf itself, not the roots. This means you will often notice the effects almost immediately.

Quick-Release vs. Slow-Release Fertiliser

Quick-release is actually an inaccurate name for a fertiliser. It should actually be called ‘ready to use’ fertiliser as the nutrients are ready for the plant to use. They typically last between 4-6 weeks, depending on the temperature and available moisture.

The warmer and wetter it gets, the more fertiliser the grass needs and so the more it uses. The more it uses, the quicker it runs out.

Because the plant uses these type of fertilisers almost immediately, the grass benefits and you’ll often see the effects on your grass very quickly.

Slow-release fertilisers, on the other hand, release their nutrients over a much longer period of time. Generally between 2-6 months.

Different slow-release fertilisers have different mechanisms for releasing their nutrients slowly.

Some come with a coating around them which breaks down slowly over time and then release their nutrients as the temperature increases. Others contain a chemical that prevents the nutrients being taken up by the plant all at once. Instead, they ‘drip feeds’ over the course of several months.

Lawn Weed and Feed

Lawn weed and feed products are marketed as ‘all in one’ kind of products. Their big sell is that they save you both time and money.

They claim to save time because they kill moss and broad-leafed weeds like DandelionsDaisies, White Clover and Black Medic while feeding the lawn at the same time. And they claim to save money because instead of buying a separate moss killer, a dedicated weed killer and fertiliser, you essentially have it all in one product.

As a result, they’re very popular.

That said, professionals don’t use all in one weed and feed products and for good reason.

Weed and feed products contain pesticides to kill moss and weeds. They come as a fine powder to ensure good contact with moss and weeds. The problem is that this means the pesticides also have very good contact with the grass, resulting in dead grass and areas of your lawn looking scorched.

I’m not saying don’t use weed and feed products, just be very careful if you do.

Iron Sulphate

Most lawn fertilisers contain Nitrogen (N), Potassium (P) and Phosphate (K). But there are other elements that your grass needs in trace amounts. These include;

  • Molybdenum
  • Boron
  • Copper
  • Chlorine
  • Zinc
  • Magnesium
  • Iron

Most soils contain these elements and only the poorest soils lack them. That said, all of the elements listed, Iron is the one that typically lacks the most.

Iron Sulphate (or Ferrous Sulphate) not only kills moss, it’s also a component of Chlorophyll which is what makes your grass green. It also hardens the grass to the harshness of winter.

However, you should use Ferrous Sulphate with caution as it can permanently stain. Some fertiliser manufacturers add it to their weed and feed products to give the grass an extra boost of green so be very careful when using them not to stain pathways, driveways or patios.


A Typical Annual Lawn Feeding Regime

lawn feeding program

We’ve covered the different types of lawn fertiliser and how they work. Now let’s look at how they fit into a typical annual lawn feeding regime.

Spring Lawn Feed

Choose a spring fertiliser like Scotts Spring/Summer Lawn Builder Lawn Food. Plan to apply this in April once you’ve given your lawn its first couple of cuts and when the Daffodils are fully open.

A good spring fertiliser will help your lawn recover from the stresses of winter and bring it back to life.

Summer Lawn Feed

Around 8-12 weeks after applying a Spring lawn feed, apply Scotts Spring/Summer Lawn Builder Lawn Food again.

It’s an anti-scorch fertiliser so it’ll give your lawn the nutrients it needs to get through the summer without scorching the grass.

If the weather is hot and dry, wait for rain and apply after a good downpour. Alternatively, give your lawn a good soak prior to applying.

Autumn Lawn Fertiliser

Late August and through September is the ideal time to apply and Autumn/Winter Lawn Food. Again, let the weather dictate when you apply it but wait until the rains start.

You should be fine to fertilise right up until October but not beyond that. Don’t apply fertilisers with high nitrogen content in the autumn and it can result in the onset of diseases like Fusarium as the weather gets colder.

Winter Lawn Feed

To protect your lawn through the winter months, give your lawn a dose of Iron Sulphate in November and then again in January.

This will;

  • Harden the turf and protect it against the stresses of Winter
  • Give your grass a nice green boost
  • Kill any moss that starts to creep in

WARNING: Applying Ferrous Sulphate will blacken any moss in your lawn. If you apply it in hot, dry conditions, it could also blacken the grass. Therefore only apply Ferrous Sulphate in cool, wet conditions.


Understanding Application Rates

Most lawn fertiliser products come with a recommended application rate.

For example 35g per m2, or 35 grams of fertiliser per square meter of lawn. So if your lawn is 50 square meters, you’d need 1.75kg of fertiliser to cover the lawn.

That’s not to say you should always apply the full amount. It is possible to overfeed your lawn which will result in the grass taking on a bluey-green colour. It also grows the leaf at the expense of good, strong roots.

Here are two things to consider:

Grass Type

Depending on the type of grass your lawn is made up of you might want to spread less fertiliser. For example, an ornamental lawn that is made up of fine fescue grasses could be ruined by too much fertiliser. On the other hand, a lawn that is made up of mainly Ryegrass will be happy with a full dose.

To stay on the safe side, I’d never recommend you apply the full dose, especially the first time you use a fertiliser.

Instead, apply a little less and see how the mowing rate and colour improves.

For fine fescue, ornamental lawns, I’d recommend you apply 20g-25g per m2.

If you have lots of Ryegrass in your lawn, try 25g-30g.

If need be, you can always apply a little Miracle-Gro Liquid Lawn Food a few weeks later on.

The Weather

The weather will also play a big role in the amount of fertiliser you apply to your lawn.

If there is plenty of rain and growing conditions are good, apply it at the higher end of the application rate.

So 25g per m2 for an ornamental lawn and 30g per m2 for a regular lawn.

If ground moisture is available but not much rain, stick to the lower end of the scale.

20g per m2 for an ornamental lawn, 25g per m2 for a regular lawn.

If it’s hot and dry, don’t apply fertiliser at all as it could result in scorched, brown grass.


How to Apply a Lawn Fertiliser

There are several ways to apply fertiliser to your lawn and depending on whether you’re applying a liquid fertiliser or a granular one.

Hand application is the most inaccurate way and often results in some areas becoming greener and growing at a faster rate than others. In worst cases, parts of your lawn could become scorched and brown.

So, if you’re spreading a granular lawn fertiliser, use a spreader such as a drop spreader, a rolling rotary spreader, or shoulder spreader. For small lawns, use a hand-held spreader.

For spreading a liquid fertiliser, use a watering can with a sprinkle bar for smaller lawns or a knapsack sprayer for larger areas. You can also use a knapsack sprayer for applying moss killer and weed killer.

The easiest way to learn how to spread fertiliser is to actually watch the process.

The Lawnsmith has videos describing exactly how to do it;

How to Spread Granular Lawn Fertiliser

Applying Liquid Lawn Fertiliser With a Knapsack Sprayer

How to Apply Liquid Fertiliser With a Watering Can and Sprinkle Bar


Now it’s Over to You

Like I said at the beginning of this article, you could make your life really complicated when it comes to lawn fertiliser.

But for a nice, lush, green lawn at home, keep it simple. Read the instructions and stay on the side of caution. Remember, you can always add more later but you can take it away.

If you have any questions, please leave a comment below.