Which fertiliser should I use?

FertiliserWhat’s the difference between organic and inorganic fertilisers, liquid and pelletised? What’s in them, and which is best for a given situation? Here are six different types, and how they work.

  1. 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.
  2. 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.
  3. 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.
  4. 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.
  5. 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.
  6. 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.

For more information about these and other types of fertilisers for your plants, including a handy ‘at a glance’ table, pick up a copy of the September 2019 issue of ABC Gardening Australia magazine.