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What is the best potting mix?

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10:56 pm
October 3, 2010



posts 263

I'm really confused about potting mix. Many I've found seem to have lots of safety warning about toxicity. Does it really make much difference? What is the best one for setting up a kitchen garden in pots on a deck?

I would really appreciate any recommendations.

1:11 am
October 6, 2010



posts 60

Hi Cindy, well I'm a mad keen gardener who has experimented quite a bit with the potting mix and here's my views, although others will have their own experiences. 

First, I'm not surprised you're confused, because potting mixes come in a range of different types and prices. Even so, they have to have a standard on them for Australia and NZ as being safe for human handling and use in your garden. This safety standard means they shouldn't carry any potential to spread weeds, or leach toxins into the soil, because a proportion of such mixes is usually a compost of some kind. So the materials should have been 'sanitised' usually with heat or steam to kill off unwanted weeds and grass seeds etc, and the compost has to be made from 'clean' materials that hasn't be tainted with toxic waste. 

That said,  all potting mixes, mulches and composts carry a risk to you when you handle them of transporting fungi and bacteria into your lungs. I'm not a medical expert, but I suspect this is no different to any soil you would work with or high concentrations of particulates in the air, so you just need to exercise some common sense. What I do is wear a cloth mask if I'm handling lots of the stuff, and I don't work in my garden on windy days, I also wear gloves and old clothes for my gardening. People who have compromised immune systems or problems with their lungs should be particularly careful. Otherwise, it's just a matter of not sticking your nose into the stuff and certainly you should make sure your hands are thoroughly clean before you touch food or put them near your mouth. That's all.

If you're like me, and suffer from hayfever you might find after working with the stuff in large amounts at this time of year, you will start to suffer the symptoms more!

As to 'what's best to buy' well that depends on what you're using it for. For example, because I have a native garden, I do not use the 'fancy' potting mixes. Some come with fertiliser already in them. These tend to be high in phosphorus, which Australian natives don't need much of. In fact, some native plants will die with too much phosphorus, because our soils are naturally low in this chemical.

Some come with water holding crystals. Some are plain. Some are very light in composition ie., lots of compost and not much sand. Some are really quite heavy with a good proportion of sand and possibly even loamy material in them. If you're trying to grow vegetables, say, then it's best to buy the heavier types that are specific for veggies. Similarly, there are special mixes for roses and other exotics that like a heavier soil. But, if you're trying to plant out some native plants that like good drainage, then the lighter less ameliorated mixes might do you quite well. The fact is, my plants do very well with the black and gold brand which costs virtually nothing, but provides good drainage for my native tubestock which don't like water collecting around their roots (it causes rot). All I do then is add some blood and bone or cow manure once a year to the natives, if they need it. You want to be careful you don't over fertilise your soil, otherwise it just ends up leaching out into our water ways. 

However, for my camellias, roses, lemon tree etc I'd use the fancier, heavier mixes. 

The other thing I note about all potting mixes is that they become water repellant really quickly. All you can do is make sure they never completely dry out and that you have lots of 'humus' in the soil. You'll know water repellancy, because it will simply flow right through the pot and out the holes without staying long enough for the roots to take the water up or, worse, float right off the mulch around the plant without even permeating the soil. I've found that adding some good quality compost helps overcome this problem as well. 

Hope this helps. 

4:24 am
October 6, 2010



posts 456

Hi Quetzal, thanks for your very informative post.

You made an interesting point about the water repellancy of potting mixes and mulches. I've noticed that happening too. It's also an issue mentioned by Janet and Andy in relation to their native garden (see climateXchange article here) where they found that "adding mulch made the problem of dry soils worse by preventing moisture from entering the soil when it rained. We should have ensured there was sufficient moisture in the soil before spreading the mulch."

4:11 am
October 7, 2010



posts 60

Hi, yes, this issue of water repellancy seems worse for pot plants than plants in the garden soil itself to me. There's a couple of ways around it. One is to buy 'wetting agents', which I've never bothered to try because I'm a cheapskate. What I've found works best is to mix potting mix with some soil from the garden, which in my case tends to be rather clayey, and holds water better than the potting mix. 

In the garden, I manually break up the soil with a hand held hoe, dig in some manure and compost around the plant, and water in well then top up with the mulch. Note, it's the humus from broken down plant matter which helps retain water in between the sand particles in soils, therefore putting in compost and manure adds humus. As mulch breaks down, it adds to the humus over time as well. 

People need to be aware of watering method. With hand held hoses, it's best to water plants deeply and less frequently, to promote deep root growth and resilience to dry periods. However, when you water the plant you should give them a little bit, stop, let the water seep through, then water the plant until the soil around it is saturated. 

12:45 am
December 3, 2010

Cindy Eiritz


Post edited 1:07 pm – December 3, 2010 by admin

Quetzal said:

Hi Cindy, well I'm a mad keen gardener who has experimented quite a bit with the potting mix and here's my views, although others will have their own experiences……


Thanks so much Quetzal :-). Your advice really helped me alot. Sorry it has taken so long to reply :-(. I'm a bit of a techno clutz and have trouble logging on as me so I'm just trying as a guest today.

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