A good gardener takes a lot out of their patch in the form of healthy, nutritious food – so it’s important to put something back in. A feed of fertiliser ensures plants are strong, disease-resistant, and productive – yet there are good reasons to avoid the usual chemical products.
Few people realise that healthy topsoil is teeming with fungi and bacteria – a cubic metre of healthy soil contains more than 1kg of fungal threads – and the fungi in particular help plants by attaching themselves to their roots. These fungal threads penetrate deep into the soil, drawing up nutrients and water which they give to the plants in exchange for a feed of carbs. Essentially they greatly extend a plant’s root system.
However synthetic fertilisers can inhibit the reproduction of these helpful microorganisms, making your soil less fertile over time – and dependent on its next dose of chemicals for its vitality, instead of being naturally self-sustaining.
Over-application of fertilisers is also one of the main reasons rivers and lakes in New Zealand are turning toxic. If you use more nitrogen and phosphorous than your plants can take up it will simply wash away in the rain and find its way into waterways. There it encourages rampant algae growth which kills other aquatic life, turning the water into a slimy wasteland.
What are the options?
The good news is you can replenish your soil and feed your plants with a range of more natural fertilisers. These all contain the three basic nutrients, nitrogen (N), phosphorus (P) and potassium (K) found in regular products, but may also deliver micro nutrients such as silica, sodium and selenium. These and other micro nutrients aren’t often displayed on fertiliser packages despite arguably being just as important. Ultimately a natural product should promote a healthy, living soil system that will be more fertile in the long run.
There are three main elements that plants need for growth: nitrogen (N), phosphorus (P) and potassium (K). Put simply, nitrogen is needed for leafy growth, phosphorus for good roots and flower formation, and potassium is needed for overall health plus good flowers and fruit.
The packaging of any fertiliser product will display how much of each element is present as a percentage of the total volume. So for example something that says NPK: 8-3-6 has 8 per cent nitrogen, 3 per cent phosphorus and 6 per cent potassium.
In New Zealand most soils are low in available potassium, so it is essential to supply it regularly to the garden. A minimum of 3.5 per cent to 6 per cent K is required for healthy plant growth.
A newly established garden will also benefit from added nitrogen. However, once established, a healthy soil system with lots of fungi – and worms to aerate it – should draw enough nitrogen for its needs from the air. You should only need to add nitrogen at times of very rapid growth.
N, P and K are only part of the picture though – dozens of other elements are needed in smaller quantities. If you’re a serious gardener, farmer or orchardist then consider getting your soil analysed to work out the precise requirements of the property.