Most of the nutrients essential for growing lawns are plentiful within the soil except for nitrogen (N), phosphorus (P), and potassium (K). Application of fertiliser stimulates turfgrass photosynthesis, respiration, and growth, but uses more storage reserves. Respiration increases because there is more plant material to keep alive. Overall, regular fertiliser applications ensure a vigorous healthy lawn.
Note: In acidic or alkaline soils, iron (Fe) or magnesium (Mg) may be bound up in the soil particles, and may be required as a fertiliser. Iron can also be used to control moss and some broad-leaved weeds, while darkening the grass, making it appear greener without stimulating growth.
Nitrogen (N )
Nitrogen is removed in grass clippings, and is essential for producing further growth that is stimulated by mowing. Addition of N produces deep green, rapidly growing leaves and shorter roots. N-deficient grass tends to be yellow-green in colour and lacks vigour.
However, high N levels produce turfgrass plants with thin cell walls, making it susceptible to attacks by fungi or insects. They also produce high water content in the plant tissue, increasing the requirement for irrigation and making the plant more susceptible to heat and drought stress. Furthermore, N-stimulation of leaf growth can deplete nutrient reserves in the roots, reducing the ability of the plant to survive dormant periods. Therefore, nitrogen should be applied in autumn to enhance turf density and rooting, rather than in spring or summer when it stimulates unnecessary leaf growth.
Phosphorus (P)
Phosphorus facilitates the metabolic functions of the plant plus energy storage and use. The roots are a primary organ for energy storage and are dependent on P levels in the plant. The demand for P is greatest during formation and germination of seed. Since turf plants are not maintained for seed production, they only need low levels of phosphorus. Therefore, turf fertilisers for established lawns are low in P. Because, germinating seed has a high requirement for P, along with its property of being fairly immobile in the soil, particularly clay soils where it may take years for P to move a few centimetres, a fertiliser containing a higher level of P such as an N-P-K ratio of 1:2:2 is recommended.
Potassium (K)
Potassium regulates water relations in the plant. The absence of adequate amounts of K compared to high N levels results in thin cell walls and high water content. Increasing the ratio of K to N in the plant induces thicker cell walls and higher cell moisture content, making the plant less susceptible to fungal or insect attack, and more tolerant of droughts. Furthermore, the rate of leaf growth is reduced, thereby reducing the demand for nutrients by the leaves and ensuring that they are available for stolon, rhizome and root growth. Potassium is easily leached in the soil, and may also be lost from the plant through its leaves during rain or irrigation. Thus, K should be applied to the soil at regular intervals at a constant rate.
Application of fertiliser
Two applications of fertiliser a year are sufficient. Select fertilisers that contain N, P and K with a mixture of slow and fast release nitrogen that allows the lawn to green up within 2-3 days and stay green for several weeks. Apply in late spring and early autumn. Nitrogen applied in summer may stimulate lush growth and encourage diseases. Apply a known weight of fertiliser uniformly to a known area at the recommended rate, to avoid variations in growth and damaging or killing the grass as a result of toxic doses.
leave lawn clippings on the lawn if the lawn is less than
70 mm in height, as at short lengths, the clippings decompose quickly and return nutrients to the soil. Clippings also enhance the habitat for beneficial microorganisms such as

earthworms, which increase the aeration and fertility of the soil, and maintain a thatch base that makes the lawn feel springy when walked on.
Established turf generally resists dry periods, but growth is retarded in prolonged periods of moisture stress. To maintain growth and colour in the lawn during dry spells, the lawn needs to be watered. Under drought stress, turfgrasses dry up and the leaves roll and turn a dull purplish shade, a process called dry wilt. However, too much water may induce wet wilt, which occurs when the soil becomes saturated and movement of oxygen into the soil and carbon dioxide out of the soil stops. Then the plant’s root system is unable to take in oxygen and give off carbon dioxide and the plant soon dies. As well, excess moisture can cause disease pressure from fungi such as the ones that cause leaf spot.
When?
Water any turf just before it begins to wilt. This stage can be recognised by a dull purple tinge with the leaf blades beginning to roll or fold. The grass will also not spring back up after it has been walked on.
It is better to water in the early morning; at this time of the day, there is little wind and temperatures are low, so less water is lost to evaporation. Watering in the late evening also has the benefit of reducing water loss to evaporation, but because the grass usually stays wet all night, late watering can induce disease outbreaks.
On very hot days, when ground temperatures exceed 35°C, apply 3 mm of water at midday, to reduce the leaf and soil temperature, while putting the water in the root zone where the plant needs it most.
How much?
Water to a depth of 10-15 cm. Deep watering encourages development of an extensive root system, enabling the plants to utilise nutrients and water in the soil more efficiently than with shallow root systems. Light, frequent sprinklings produce shallow root systems, making the lawn more susceptible to drought and soil-borne insect pests that feed on roots, such at grass grub ( Costelytra zealandica ), striped chafer ( Odontria striata ) and manuka beetle ( Pyronota species).
The soil type affects how much water is needed to wet soil to the desired depth. About 1.3 mm water is required to achieve the desired wetting depth if the soil has a high concentration of sand, and about 2 mm of water if the soil is a loam. For soils rich in clay, 25 mm of water is necessary to wet the soil to the desired depth.
Thatch is not just a collection of grass clippings caught between blades of grass. Thatch is primarily grass root stolons and rhizomes. In a healthy lawn, earthworms and soil microorganisms break down the thatch naturally. When thatch becomes thicker than 1.5 cm, it suppresses grass growth, prevents water penetrating the soil, and becomes a perfect medium for fungus.
Short-term control – use a mechanical de-thatcher to reduce the amount of thatch in either early autumn or spring.
Long-term control - Reduce the amount of chemical fertilisers applied. Fertilise the lawn in spring and autumn to encourage deep roots and to restore earthworms and other biological activity to your soil.
Note: For further information on lawn care refer to the fact card 1.4 Repairing Lawn Damage & Annual Maintenance
For Lawn Care, and to the – 1. Grass lawns and Turf teacher’s guide