Lawns are living things, and as with all plants, they require essential nutrients to remain healthy. Although most nutrients are available in the soil, nitrogen (N), phosphorus (P), potassium (K) and iron (Fe) are often used to supplement them.

Why Apply Fertilizer?

Organic and artificial fertilizers are most commonly used to add nutrients to the lawn, and are applied in spring and autumn. Nitrogen is commonly added as it is removed by mowing, resulting in a lifeless, yellow-looking lawn. Nitrogen is also needed to produce the further growth that mowing stimulates. Iron (Fe), although not generally used on its own, is a constituent of lawn sand, which is used to control moss. Iron is particularly useful in the autumn, as it makes the grass look greener without stimulating growth.

It is important to apply lawn fertilizers evenly to avoid damaging, or even killing, areas of grass or creating variations in growth patterns. The exact amount of fertilizer that should be applied will depend on the types of grasses grown, the soil drainage, the amount of water received by the lawn and whether or not the grass clippings are removed. Light, sandy soils that receive a high amount of water need high levels of fertilizing as they will quickly lose nutrients by leaching. Conversely, heavy, clay soils that do not receive much irrigation will require less fertilizing.

Applying Fertilizer By Hand

When manually applying fertilizer to the lawn, divide up the area into square metres (or yards) using string and pegs or by laying down canes. Calculate the application rate of the fertilizer you have chosen, and measure the correct amount for the lawn out into a bucket.

Create a spreading device by putting two plastic plant pots together. Rotate them so that their holes are closed, and then measure in the amount of fertilizer needed for one of your measured areas. Mark this level on the inside of the pot. Then, slide the two pots slightly so that their holes are open a fraction and shake evenly over the first square metre. Continue in the same way until you have covered the whole lawn.

Applying Fertilizer by Machine

The easiest way to fertilize a lawn is by using a continuous belt or drop spreader, which is pushed up and down the lawn, so that each pass adjoins the previous one, without overlapping it. The fertilizer should be divided into two batches to ensure even coverage. The first half should be applied in one direction, the other at right angles to it.

If you are working on a larger area, you could use a spinning disc or broadcast spreader; however, this may produce an uneven spread. To reduce this risk, the machine should be set to half the application rate, with adjacent runs made at half the distance apart of the machine's spread. For example, if the spread is 3 m (9 ft), half the application rate and make the run 1.5 m (4.5 ft) apart.

To calculate the distribution rate for any machine, you must first calibrate the spreader. Chalk out an area of approximately 4 sq m (4 sq yards) on a flat, dry area of asphalt or concrete. Quarter fill the machine, and then set it to "Medium". Spread the fertilizer over the marked area as evenly as possible at the same speed you would use on the lawn, then brush up all the fertilizer and weigh it. Divide this weight by the area (in this case, 4 sq m / 4 sq yards) to calculate the application rate in grams per square metre or ounces per square yard. Adjust the machine and re-test until the application rate is at the desired level.

Once the machine is calibrated, weigh out the right amount of fertilizer for the lawn and divide it in half. Put the first half in the spreader, making sure that the spreading holes are closed. Apply the first half in one direction, working up and down the lawn as you would do when mowing; however, remember that each strip should join but not overlap the last. You may want to set out garden line or canes on the ground as a guide; both can be moved as you proceed. Close off the supply hole as you turn at the end of each row. Apply the second half at right angles to the first application strips and continue until you have finished the lawn.

After applying fertilizer, ensure that you wash the machine out thoroughly; fertilizers are corrosive and can damage any metal parts.