Eco friendly fertilisers

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.

Understanding N-P-K

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.

What’s available?

We checked out six fertilisers or soil conditioners available in New Zealand that were made from natural materials. Their ingredients and strength vary, but all will give your garden a boost:

Yates Dynamic Lifter

2.5kg for $7.99

Value for money: 32 cents per 100g

Certified organic? No

NPK: 3.2-2.6-1.3

Made from composted chicken manure, blood and bone, fish meal and seaweed, this product comes in pellet form for easy application. Because the ingredients are attractive to animals such as dogs you need to rake some soil over the pellets and water them in well after applying – contact a vet if your animal eats some. Adequate levels of nutrients but low in potassium. Apply every second month in the growing season.

Tui Blood & Bone

2.5kg for 9.95

Value for money: 38 cents per 100g

Certified organic? No

NPK: 7-4-0

Provides slow-release nutrients, with a good dose of phosphorous for strong roots and flowers – but lacks potassium. Literally made from blood and bone, this should not be spread anywhere that grazing animals such as sheep, cattle, deer or goats could eat it. Apply every second month in the growing season.

Fodda General Fertiliser 

4kg for $16.50

Value for money: 41 cents per 100g

Certified organic? Yes

NPK: 1.5-4-5

Made from ground-up phosphate-bearing rock, fermented fish waste and seaweed, plus charcoal, coffee grounds and sawdust, this product encourages soil biology, while delivering a good dose of phosphorous and potassium for healthy roots and fruit. Also lists levels of important secondary nutrients such as calcium, and says 80 per cent of the ingredients are diverted from landfill. Apply 100g per metre-squared, with extra at the base of trees, and repeat after 8 weeks. 

Biofeed

500ml for $9.99

Value for money: $2.00 per 100ml

Certified organic: Yes

NPK: Not stated

Brewed from hot composted animal manure and hay, this ‘tea’ fertiliser should be diluted (50ml per litre of water initially, then 20ml/litre thereafter) before being applied to the soil around plants. No information on nutrient levels is given. Liquid fertilisers should be applied more regularly than solid products – fortnightly-to-weekly – as plants absorb them more readily, and rain and watering wash them away.

Seasol

500ml for $11.95

Value for money: $2.39 per 100ml

Certified organic? Yes

NPK: 7-0.8-5.4

A concentrate brewed from seaweed, this Australian product is designed to be added to the watering can by the capful – about 3ml for every litre. Provides a quickly absorbed feed, with good levels of the major nutrients, especially potassium for disease resistance. Once mixed, should be used within 24 hours as it will begin to break down. Apply every 2-4 weeks. 

Yates Nitrosol

500ml for $16.98

Value for money: $3.40 per 100ml

Certified organic? No

NPK: 8-3-6

A concentrated liquid made from animal blood and bone, this delivers high levels of all the major nutrients for rapid uptake. Should not be spread anywhere that grazing animals such as sheep, cattle, deer or goats could eat it. Dilute in water at 5ml per litre and soak into the soil around plants every 7-10 days. Once mixed should be used within 24 hours as it will begin to break down.