Replacing potting soil

gcotterl(9)October 11, 2010

How often should potting soil be COMPLETELY removed and replaced with new potting soil?

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tapla (mid-MI z5b-6a)

A copy/paste from another thread:

In my estimation, the only case to be made for reusing container soils is one of economics, and you'll never find me argue against making that decision. If you can't afford, you can't afford it. That said and setting economics aside, you might decide to reuse soil for reasons other than economical. Perhaps the effort involved with acquiring (or making your own) soil is something you might not wish to go through or be bothered with.
In any case, it would be difficult to show that soils in a more advanced state of structural collapse can somehow be preferred to a soil that can be counted on to maintain its structure for the entire growth cycle. So, if the economic aspect is set aside, at some point you must decide that "my used soil is good enough" and that you're willing to accept whatever the results of that decision are.
All soils are not created equal. The soils I grow in are usually pine bark based & collapse structurally at a much slower rate that peat based soils, yet I usually choose to turn them into the garden or give them over to a compost pile where they serve a better purpose than as a container soil after a year of service. Some plantings (like woody materials and some perennials) do pretty well the second year in the same bark-based soil, and with careful watering, I'm usually able to get them through a third year w/o root issues.

Watering habits are an extremely important part of container gardening. Well structured soils that drain well are much more forgiving and certainly favor success on the part of the more inexperienced gardeners. As soils age, water retention increases and growing becomes increasingly difficult. If your (anyone's) excellence in watering skills allows you to grow in an aging medium, or if your decision that "good enough" is good enough for you, then it's (your decision) is good enough for me, too.

The phrases "it works for me" or "I've done it this way for years w/o problems" is often offered up as good reason to continue the status quo, but there's not much substance there.

I'm being called away now, but I'll leave with something I offered in reply on a recent thread:
"... First, plants really aren't particular about what soil is made of. As long as you're willing to stand over your plant & water every 10 minutes, you can grow most plants perfectly well in a bucket of marbles. Mix a little of the proper fertilizers in the water & you're good to go. The plant has all it needs - water, nutrients, air in the root zone, and something to hold it in place. So, if we can grow in marbles, how can a soil fail?

Our growing skills fail us more often than our soils fail. We often lack the experience or knowledge to recognize the shortcomings of our soils and to adjust for them. The lower our experience/knowledge levels are, the more nearly perfect should be the soils we grow in, but this is a catch 22 situation because hidden in the inexperience is the inability to even recognize differences between good and bad soil(s).

Container soils fail when their structure fails. When we select soils with components that break down quickly or that are so small they find their way into and clog macro-pores, we begin our growing attempts under a handicap. I see anecdotes about reusing soils, even recommendations to do it all over these forums. I don't argue with the practice, but I (very) rarely do it, even when growing flowery annuals, meant only for a single season.

Soils don't break down at an even rate. If you assign a soil a life of two years and imagine that the soil goes from perfect to unusable in that time, it's likely it would be fine for the first year, lose about 25% of its suitability in the first half of the second year, and lose the other 75% in the last half of the second year. This is an approximation & is only meant to illustrate the exponential rate at which soils collapse. Soils that are suitable for only a growing season show a similar rate of decline, but at an accelerated rate. When a used soil is mixed with fresh soil after a growing season, the old soil particles are in or about to begin a period of accelerated decay. I choose to turn them into the garden or they find their way to a compost pile.

Unless the reasons are economical, I find it difficult to imagine why anyone would add garden soils to container soils. It destroys aeration and usually causes soils to retain too much water for too long. Sand (unless approaching the size of BB's), has the same effect. I don't use compost in soils because of the negative effect on aeration/drainage. The small amount of micro-nutrients provided by compost can be more efficiently added, organically or inorganically, via other vehicles.

To boil this all down, a container soil fails when the inverse relationship between aeration/drainage goes awry. When aeration is reduced, soggy soil is the result, and trouble is in the making.


    Bookmark   October 11, 2010 at 9:05PM
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My question should've been "How often should a commercially-available potting mix (i.e., a soil-less growing media) be COMPLETELY removed and replaced with new commercially-available potting mix?". (I didn't mean "dirt").

    Bookmark   October 11, 2010 at 10:01PM
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rhizo_1 (North AL) zone 7

Commercial potting soils begin to lose their structure in a very short time. Since a porous and coarse textured medium is desired for a healthy root system (and plant), I'd strongly suggest that a repot be done on an annual basis.

IF you amend your commercial medium with very slow to decompose particles, then repotting process can wait a couple of years, or more.

You'd be surprised at some of the ingredients I use, without ever touching any peat moss. But, still, a perfectly wonderful mix can be formulated (by you) with the addition of a few simple ingredients.

    Bookmark   October 11, 2010 at 10:26PM
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What are "very slow to decompose particles"?

What "few simple ingredients" do you use?

    Bookmark   October 12, 2010 at 11:30AM
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tapla (mid-MI z5b-6a)

When the topic is container soils, structure is key. One of the most popular ingredients for soil on this forum is pine bark. There is a lot of benefit to be had from STARTING with larger particles and building a soil that has superior aeration and good water retention, as opposed to starting with small particles (peat/coir/compost) and trying to add enough of this and that to add aeration and reduce water retention.

You can read about how to use pine bark to build a soil, or even to amend what you have, if you see that in your future, by following the link below. It will cover the 'slow to decompose' question as well as the 'few simple ingredients'.


Here is a link that might be useful: More about soils

    Bookmark   October 12, 2010 at 2:51PM
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