Updated: 21 Aug 2009

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Introduction

It’s become popular again to grow your own vegetables. We tested potting mixes suitable for raising a crop where space is limited.

We put 17 potting mixes through a growing trial to find the best ones for particular types of vegetables and also the best “all-purpose” mixes. Find out which mixes rated best.

Tops in the test

We wanted to find the best mixes for particular types of vegetables and also the best “all-purpose” mixes.

We tested the mixes on vegetables with a variety of needs. Leafy crops such as lettuce need plenty of nitrogen, but root crops like radishes (and carrots) shouldn’t have too much nitrogen or the tops will flourish at the expense of root growth. Tomatoes are just overall greedy feeders.

Best for a range of vegetables

The mixes that produced the best results over a range of vegetables were:

• Yates General Purpose Potting Mix (North Island version)
• Daltons Big Value Potting Mix (South Island version)
• Living Earth Potting Mix
• Kiwi Potting Mix.

Just Potting Mix was one of the best for growing lettuces and radishes. But it didn’t do so well for tomatoes, which have a longer growing period.

Just and Kiwi are sold at The Warehouse. Daltons Big Value is exclusive to Bunnings. Yates is available in garden centres and hardware stores, and Living Earth at centres in the upper North Island.

Best for specific vegetables

The lab sowed radish seed and potted up lettuce and tomato seedlings then raised them in a heated glasshouse.

• Radishes: The radish seed germinated well in all mixes. The plants were thinned and the average size was compared between potting mixes halfway through the trial and at the end. Finally the plants were harvested and the edible portion weighed.
  
  The best potting mixes for raising radishes were Kiwi, Just and Yates General Purpose (North Island version).
   
   
• Lettuces: Size was compared halfway through the trial and at the end when the plants were harvested and weighed as well.
  
  The largest lettuces were grown in Daltons Big Value (North and South Island versions), Just and Home Brand.
• Tomatoes: The lab checked the plants’ height halfway through the trial and again at the end. It then rated the plants’ health and weighed them.
  
  Living Earth, Daltons Big Value (South Island version), Results (South Island version) and Daltons (North Island) produced the best results.

 

Poorer performers

Among the poorer performers were Results (North Island version), Tui Outdoor Container Mixture and Tui Vegetable Mix.

The North Island version of Results produced very poor plants – as it did in our 2005 potting mix test. The South Island version performed better, so presumably has a different formulation.

Tui Products told us that in its autumn growing trials – which occurred after we’d bought the potting mixes for our test – it had found problems with some of its mixes.

These problems were traced to bark that was too fresh. Woody material such as bark needs nitrogen to break it down into compost that the plant can use. If it’s too fresh it uses up the nitrogen in the mix as it composts, leaving little for plants.

Tui Products informed retailers as soon as it became aware of the problem. It says it has increased its quality-control checks and no longer uses the supplier who provided that bark.

Tomato plants that we grew in the Tui Vegetable Mix also showed signs of damage that may have been caused by contamination of the compost on which this mix is based.

Distorted leaves suggested the contaminant may have been clopyralid, a long-lasting herbicide ingredient used to kill broad-leafed weeds. It can enter the composting process in lawn clippings, hay, straw or animal manure.

In New Zealand, herbicides containing clopyralid may only be sold to an approved handler. They may not be used in home gardens. Tui Products sent samples of the mix to be tested for clopyralid. No trace of clopyralid contamination was found in the samples.

Test results

 

Guide to our table

• Our test was conducted in an independent laboratory in New Zealand.
• Potting mixes
  A = North Island version is different from South Island version.
  B = Available in upper North Island only.
• Price based on a July 2009 survey.
• Scores out of 10
  Tomatoes rated on health and weight of plants.
  Lettuces rated on size and weight.
  Radishes rated on size and on weight of edible portion (root).
• Tech bits Fertiliser is slow-release; length of time (months) is what’s claimed on the pack.

 

Handle with care

Handle with care

Handling potting mix and compost carries a slight risk of legionnaires’ disease (which can be fatal).

You get the disease by breathing in contaminated dust or droplets. Common early symptoms include a sudden high temperature, loss of appetite, chills, muscle pains, headache, stomach pains and diarrhoea. A dry cough may develop into pneumonia.

To avoid catching legionnaires’ disease you should:

• Always wear gloves.
• Take care when opening bags of mix and avoid inhaling dust.
• Moisten the mix before use.
• Wash your hands thoroughly after using potting mix.

Growing tips

A bag of spuds

You can’t beat the taste of a new potato, fresh from the garden.

If your garden’s small, try growing your potatoes in a bag. You’ll need:

• a size-40 black plastic planter bag
• potting mix suitable for outdoor containers
• three seed-potatoes (Swift, Liseta and Rocket are early varieties that mature in 60-90 days).

Pre-sprout the potatoes by laying them in a warm well-lit spot indoors to encourage the eyes to grow. They’re ready to plant when shoots are 1-2cm long.

Roll the sides of the bag down until they’re about 20cm high. Half-fill with growing mix, place the potatoes on top and cover with mix.

When the shoots emerge and are 6-8cm high, roll up the sides and add more mix – but leave the tops of the shoots uncovered. As the plants grow, repeat the process until the bag is full.

Keep the bag in a well-lit but sheltered spot outdoors and water it regularly – planter bags dry out quickly in warm weather.

After 10-12 weeks cut a slit in the side of the bag and feel for the potatoes. If you’re careful you can harvest some and seal the slit again with waterproof tape.

You can also use this method to grow potatoes in old car tyres. Start with two tyres and add more tyres and soil or mix as the potatoes grow.

The trickle-down effect

We didn’t add fertilisers to the potting mixes because all but one contained “slow-release” fertilisers. These consist of granules that break down in moisture and release nutrients into the soil over a period of months.

These mixes claimed to provide enough nutrients for at least three months – some claimed up to six months’ supply. That’s plenty of time to raise a crop of radishes or lettuces.

But vegetables such as tomatoes, beans or capsicums need additional feeding with a liquid fertiliser or granules once the mix’s slow-release time is up.

Tui Vegetable Mix contained sheep pellets, blood and bone, and dolomite lime to boost plant growth. Although it didn’t contain slow-release fertiliser, there were no recommendations on the pack about adding extra fertiliser. 

More from consumer.org.nz

• Growing herbs
• Growing vegetables 

Report by Bev Frederikson