Fertilisers are materials, whether of organic or inorganic in origin, that supply nutrients to plants. The three major ingredients of a plant food are nitrogen (N), phosphorus (P) and potassium (K). Nitrogen is extremely important for leaf growth; phosphorus promotes development of roots, flowers and seeds or fruit; and potassium is necessary for the growth of strong stems and movement of water in plants, in addition to promoting flowering and fruiting.
Most inorganic garden fertilisers combine these three major elements with secondary nutrients and trace elements in a balanced form. They are known as N.P.K. mixtures. Some of the mixtures are higher in nitrogen while others contain a greater proportion of phosphorus or potassium, depending on the purpose for which they are intended. For example a fertiliser such as Thrive Complete with an N.P.K. mix of 5:7:4 is higher in phosphorus and therefore good for root vegetables, flowers, fruits, woody shrubs, roses, canes, citrus, sweet corn and legumes. A nitrogen-rich mixture of N.P.K. 10:4:6, is better for leaf and stem vegetables (cabbage, lettuce, celery, rhubarb, etc.), leafy shrubs and foliage plants.
Organic fertilisers include manures and animal and vegetable byproducts, such as blood and bone and cow manure. These contain smaller amounts of the major plant foods, so they need to be added to the soil in greater quantities. However, as they very often contain a large proportion of fibrous material they are good for building improving soil structure and texture, especially in sandy soils. Because organic manures have to be broken down by bacteria they release their nutrients slowly over a long period. As with inorganic fertilisers, some, such as chicken manure, are high in nitrogen while others, like blood and bone, contain more phosphorus. The best results in the garden come from using a mixture of both organic and inorganic fertilisers.
- Don’t apply any fertiliser until you have read the directions carefully.
- Don’t try to apply one or two year’s supply of fertiliser at the one time.It is far better to provide little and often; trying to get fast growth by a heavy application is a recipe for plant failure.
- Don’t fertilise into dry soils. To avoid damage to roots, make sure that the soil is moist by soaking before and after the application.
- Don’t apply fertiliser to a lawn and then neglect to water it in very thoroughly, especially in hot weather. It is inevitable that leaf burn will occur with careless applications of fertiliser on lawns.
- Don’t fertilise ferns and other delicate plants with strong fertilisers. Use organic based fertilisers such as fish emulsion and blood and bone or a controlled release fertiliser such as Nutricote.
- Don’t fertilise Australian native plants with fertiliser containing a lot of phosphorus. Whilst many Australian natives accept reasonable quantities of phosphorus, there are many that resent high phosphorus levels.
- Don’t continually fertilise lawns with sulphate of ammonia, as it encourages excessive top growth but reduces root development and eventually makes for a weaker lawn. It can also alter the soil pH level towards acid conditions. Apply sulphate of ammonia occasionally, supplementing with follow up applications of a complete lawn food.