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Container Gardening – Selecting the Right Soil

In my last post I talked about fertilizing container gardens containing a soil-less mix. In this post I’ll look at some aspects of selecting the right soil.

container gardening - soils

Container gardening – soils

Adding Organic Matter

Should you add organic matter, such as manure and compost, to your soil-less mix?

The benefit of organic matter in the ground is two fold. It provides a slow release of nutrients as the organic matter decomposes and it provides a good environment for microbes which build better soil structure. To better understand these benefits have a look at this post; Organic Fertilizer – What is its Real Value?

Are these important benefits for container gardening using a soil-less mix?

Organic matter will continue to decompose in the pot and so it would certainly add nutrients to the soil. But any commercial fertilizer does exactly the same thing. There is no difference between organic nutrients and nutrients from commercial fertilizer.

What about building better soil? Is that important? Not really. A soil-less mix is used because it already has a good soil structure for plants. It is not the same structure you find in your garden soil, but it does hold a lot of moisture and it holds a lot of air–the key things plants need. In a soil-less mix you do not need microbes to build soil structure.

If you use garden soil instead of a soil-less mix in your pots there is certainly some benefit to adding organic matter, but if you use a soil-less mix it is easier to just use commercial fertilizer. Adding organic matter will not hurt the plants, but if you add it, then you should use less commercial fertilizer to compensate for the nutrients that will be provided by the organic matter.

Water Properly

Plants in containers need to be watered very frequently because the small amount of soil dries out quickly in the wind and sun, not to mention the loss of water taken up by plant roots. As discussed in Container Gardening – Fertilizers,  this drying results in high concentrations of fertilizer and the precipitation of dry salts. Neither of these are good for plant roots.

It is a good idea to flush the soil with a lot of water about once a week. To do this, allow the water (with no added fertilizer) to flow completely through the pot and out the bottom. A heavy rain storm may do this for you.

When fertilizing your pots, just add enough fertilizer mixture to saturate the soil with a little bit draining out the bottom. This will keep your fertilizer costs down to a minimum. If you are using a slow release fertilizer you don’t have to worry about adding more fertilizer, but you should still flush the pot regularly.

Reusing Soil

Do you need to replace the soil in container gardens every year? Soil suppliers will try to convince you that you need to do this, but it’s not necessary.

A soil-less mix consists of mostly peat moss, coir (coconut husk) or shredded wood byproducts. These materials are fairly stable and will last for many years, but they don’t add much in the way of nutrients for your plants. Their main purpose is to provide a semi-solid material to hold the roots, and to provide a lot of air spaces for the roots. As the material breaks down, it becomes finer and finer which reduces the amount of air getting to your roots. But it takes 3-4 years for this to become a problem (less time in tropical climates).

Excess fertilizer is a potential problem. I talked about the fertilizer precipitating out and forming solid salts in the soil. Too much of this can start to harm the roots.

If you are the type of person that over fertilizes and you see a lot of white crust on your soil it would best to replace the soil every year or two. Or you could fertilize less. For those who fertilize correctly, and flush the soil regularly as described above, you will be able to reuse the soil for several years.

Another alternative that works well is to scrape off the top 2 inches of soil and discard that–this is where most of the salts precipitate. Then top the soil up as needed with fresh stuff.

Mycorrhizal Fungi

Mycorrhizal fungi, also called Mycorrhizae, are fungi that create a special symbiotic relationship with plant roots. The fungi provide nutrients, especially phosphate, to plants and in exchange the plants provide sugars to the fungi. Almost all plants in nature have these beneficial associations.

You can buy mycorrhizae commercially. Adding them to your garden does not provide any benefits although manufacturers claim they do. You don’t need to add them because they are already in your garden soil– you don’t need to buy more.

A soil-less mix is different since it contains no natural fungi. Since mycorrhizae are beneficial for plants it seems reasonable to add them to potted plants. But there are a couple of problems with this.

Most people over fertilize their potted plants and most fertilizer contains lots of phosphate. In this situation the plants do not need the fungi to supply phosphate–it is readily available to roots and is probably in excess.

Mycorrhizae are killed when the phosphate levels get too high–which is common in an over fertilized container. So you buy mycorrhizae and add them to the soil, then fertilize a bit too much, and kill the mycorrhizae.

For the home owner mycorrhizae are just not required.

References:

1) Mycorrhizae – So What The Heck are They?: http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CB4QFjAA&url=http%3A%2F%2Fpuyallup.wsu.edu%2F~linda%2520chalker-scott%2Fhorticultural%2520myths_files%2FMyths%2FMycorrhizae.pdf&ei=eD4hVb-iIsHesAWg8YHoDA&usg=AFQjCNHYVR6ri–XGMLh3LryWjUlbLWsQQ&sig2=12XwAs2r_LTIuczKBka3kg&bvm=bv.89947451,d.b2w&cad=rjt

2) Photo Source: Home Design Ideas

 

Robert Pavlis
Editor of GardenMyths.com
I live in southern Ontario, Canada, zone 5 and have been gardening a long time. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

I hope you find Garden Myths an educational site that helps you understand your garden better.

8 Responses to 'Container Gardening – Selecting the Right Soil'

  1. Becky says:

    Please help! I accidentally used three times as much water-soluble fertilizer (formulation for blooming plants) on my outdoors pots and hanging baskets (half tbs instead of half tsp). What should I do?

  2. tolga erok says:

    Excellent info. Thanks

  3. True; regarding the mycorrhizal products, we even try it for cuttings (Promix with added mycorrhiza) and it didn’t proof to have any beneficial effect in comparison with regular Promix (sections through the new roots were analyzed for fungi). Total waste of money.
    Interesting results can be obtained in germination though, but I’m getting out of topic…

  4. Vincent says:

    hi, could you explain more about watering, how dry the medium is consider dry and needs to be water? thanks

    • Robert Pavlis says:

      There is no simple answer. Some plants can go quite dry and actually benefit from the drying out while others need to be quite wet. You really need to know the plants–I do plan to do a post about this in the future.

      As a general rule, I stick my finger in the soil a couple of inches. If I feel moisture, I leave it alone. If it is bone dry, I water.