Fertilisers

Fertilisers can be confusing. How do you choose the right one? Is it all marketing? What if you don’t bother? JUDY HORTON explores the options

Most plants we grow in our home gardens enjoy a bit of a feed. But what, how much and how often varies widely. Roses and citrus are examples of ‘gross feeders’, meaning they enjoy generous amounts of fertiliser, regularly. Productive plants such as vegetables are likely to grow better if they have access to extra nutrients, while indigenous plants grown in their natural environment won’t need fertiliser to perform at their best.

The nutrients that plants require in the greatest quantities are nitrogen (N), phosphorus (P) and potassium (K), and a fertiliser containing all three is described as a complete fertiliser. Many other nutrients – even minute trace elements – are just as essential for growth, although they are required in lesser amounts.

Most of these nutrients are absorbed in the water taken up by the plant’s roots.

Generally, it’s best to monitor your plants and fertilise only when they are in active growth or about to start growing. Container plants must be fertilised regularly, though, because some nutrients leach out easily, and the ongoing breakdown of organic matter in potting mix competes with the plant for nitrogen.

choosing a fertiliser

Plants such as shrubs, trees and ornamental grasses can be fed with a general-purpose fertiliser. This can also be used for slower growers, such as orchids, succulents and indoor plants, but at much-reduced levels and frequency.

If you’re unsure, choose a fertiliser that’s labelled for a particular plant type. Lawn foods, for example, are relatively high in nitrogen because nitrogen promotes leafy growth, which is exactly what your lawn wants. Citrus and rose foods promote growth and production, but rose food often has added potassium because it enhances disease resistance.

A number of Australian native plants have evolved in low-phosphorus soils, so most native fertilisers stay on the safe side and contain little or no phosphorus.

Camellia and azalea foods are formulated to suit the acidic conditions favoured by these plants.

Fertilisers are either derived from natural sources or synthetically manufactured, and some are a combination of both. If you want to grow an organic garden, look for organic certification on the pack. Although these products are more expensive, you can be sure that organic certification is strictly controlled in Australia.

Here are the different types of fertilisers available and how they work:

Compost and manures make wonderful soil conditioners, as they encourage microbial activity, improve soil structure and increase the soil’s moisture- and nutrient-holding capacity, but they are relatively low in nutrients. Some manufacturers process manure into pellets, thereby slowing the nutrient release, and chemical nutrients are sometimes added at this stage.

Blood and bone releases very slowly and generally doesn’t contain a measurable amount of potassium, so extra potassium is often added to this natural fertiliser.

Inorganic fertilisers are sometimes termed chemical or synthetic fertilisers, and come in granular or slow-release forms. They tend to be good sources of nutrients but are based on fertiliser salts, so can damage soil and plants if not applied with care and according to the label.

Slow-release fertilisers can be either organic or inorganic. They allow nutrient release over a longer period of time, reducing application frequency. However, if growing conditions are unsuitable and the plant can’t make use of the fertiliser, it can dissipate before it does any good.

Controlled-release fertilisers are the most technically advanced. Their prills have permeable outer coatings that allow water to penetrate and dissolve the nutrients within. The nutrient-laden water then moves back out into the soil or potting mix. As temperatures rise, the coating expands, allowing release of more fertiliser just when plant growth is increasing (hence the ‘control’). As they are relatively expensive, these products are most often used for feeding container plants.

Liquid fertilisers give quick results, moving rapidly into the roots and, to a limited extent, the foliage. Available in liquid or soluble powder form, they need to be reapplied every couple of weeks or even weekly at half strength. Some liquids are high in nitrogen, to promote leafy growth, while others are formulated to promote flowering, or for other specific applications. Liquids are a good choice for feeding seedlings and leafy vegetables. The popular seaweed extracts that often come in liquid form are useful for promoting plant performance, but they can’t be regarded as a substitute for fertilisers.