Mostly because it's cheaper, holds water/nutrients longer, and puts up with neglect.

Soiless container mediums generally promotes better growth, but it requires more constant labor/care.

I use a mix for most houseplants I don't want to constantly baby. I mostly use soiless mixes for things I plan on transplanting and need to grow quickly.

Also, If I started with 'promix' which is a soilless peat/pelrite mix, and added compost to that, would that be then considered a soilless mix still? What makes "potting soil, soil? I noticed a bagged potting mix with 55% composted forest material and peat and perlite be labled potting soil. I have noticed the anotherbagged mix with the same ingredients be called 'potting mix'.

So when I added compost to the soilless mix does that now make it a potting mix/soil?

People can sell anything they want and call it soil. Read the the ingredients. Don't go by what it says on the outside of the bag. I can't say why one soil is and another is a mix. In general organic compost would make it a soil and just things like peat moss and perlite is a sterile mix you may want for seed germination is a soilless mix.

A lot of potting soils are forest/wetland topsoils and/or peats that sometimes have "nonsoils" combined. Some, such as the horrid Hyponex, are almost 100% horrible quality muck sedge peat. Some have nutrient additions mixed in.

Any time you're messing with "good" peats, vermiculite, perlite, coconut coir, pine bark, etc...you're talking soiless. Technically the sedge peats are soiless, too, but the way it's sold (such as in Hyponex) you might as well be talking about a muck soil in the way it works.

There's no across the board standard across companies, just within lines that companies sell.

Commercially avaiable potting soils will most often be soil less mixes, the commercial ones usually will not contain any mineral soil particles. The commercial potting soils will be made of peat moss, coir, or finely ground bark, or be a combination of those materials, plus either perlite or vermiculite to promote drainage.
You would use potting soils when you grow plants in pots rather than garden soil which tends not to drain as well as it should in pots. I have not seen a potting soil being sold in a store that contains any real soil in many years, although there are, on line, recipes for making potting soils from garden soil.

This post was edited by kimmsr on Sat, Jan 12, 13 at 7:48

Thanks for the responses everyone!

I may have got off subject a little.

"Why use potting soil? Is there any reason? Lighter soilless grow media has higher air porosity allowing for larger yields. So then why would anyone ever use potting soil?"

Why use potting soil if a soilless media gives larger harvest?

I have heard so many good things about the popular soilless media you see on here without any real rational view on the cons of soilless mix.

This post was edited by TheMasterGardener1 on Thu, Jan 10, 13 at 19:23

I'd like to suggest that we are splitting hairs if you are trying to distinguish between "potting soil" and "soilless mix".

Here's my take on how the industry uses the terms...

Potting soil: Anything you'd use to fill a container.
Soil-less mix: Medium to grow a plant in that does not contain soil from the garden.

These meanings are functionally equivalent, since the use of garden soil in a container is a non-starter in most cases.

The term "composted forest products" is very vague. It could refer to pine bark fines or to leaf mold. I take exception to the statement that the bag contains 55% composted forest products. There should be a qualifier, like "not less than" or "not more than". Do they guarantee that it's not 60% or 50%? And is that by weight or by volume? What happens if some of it decomposes in the bag and it drops to 50% - now they're out of spec! Not that anyone would measure such things, but the statement itself is ridiculous.

The cheaper "potting soil" often is of questionable origin, packs hard over time, and is filled with weed seeds. Much better to make your own or buy the more expensive mixes.

It's just wrong to assume that potting soil contains any (mineral) soil. In most cases, the only mineral content in "potting soil" is dolomitic lime and perlite - possibly a light fertilizer charge in isolated cases.

I've been discussing soils in forum settings and many other venues for years, and rub elbows with a lot of people who work intimately with container media. A number of terms are used interchangeably. "Container media", "container soil", "potting media", "potting soil", "soilless media" .... are all used regularly by those who discuss container media regularly. All are acceptable, and no one ever gets corrected for his/her choice of phraseology.

What's important isn't the label on the baq, it's what's IN the bag that counts; and what's on the bag usually gives little in the way of hints at what it might contain (unless you go the the fine print describing ingredients) or how it will actually perform.

Please explain this: "Well, one reason I could come up with is the compost in the soil acts as an even higher ph buffer. [than what?] In turn, the availability of nutrients are higher. [how so?] Ph swings will not take place. [container media changes pH by the hour, with a number of influences driving the change, so how can you support that claim?] when PH of water will not be as much as an issue. [??]

Al

Yea, can we agree using an inert soilless media such as "promix" is a form of hydroponics? Drain to waste hand watered....

With that said, After growing in all different kind of mixes I have found soil to be easy for me.

Yes, soil has a higher "buffer" than hydroponics or these mixes with no soil or compost and only lime to act as the buffer in the mix.

I am trying to look at this rationally. I like soil but I still understand soilless gives higher yields, less chance of root rot, ect...

Are you saying there is NO disadvantages of growing soilless over soil? So there is only advantages of growing in soilless media?

This post was edited by TheMasterGardener1 on Thu, Jan 17, 13 at 12:59

To clarify what MG said: our water has a 9.2 pH, while rain has a pH of 4-5 (less when air pollution is high). I can grow my seedlings in anything so long as it is compost or leaf mold. Same for houseplants, many of which are 15+ years old. In fact, for houseplants, having more flexibility, I put them out in May, dig out some composted material, and replace it with leaves and kitchen scraps. The worms will then process the material over the summer.

Have you tryed using that water, with such out of range ph, with a mix that had no compost?

This post was edited by TheMasterGardener1 on Thu, Jan 17, 13 at 13:04

Yes, though not systematically. It's been so long, I do not remember the details. But I did test potting soil, compost, garden soil (very sandy back then), perlite and leaf mold in various fractions. This is a type of water that makes plants miserable in a summer drought, but this is what the city passes. By now I make a mix 80% compost/20% perlite for everything.

City water with 9.2 pH? Yikes. That's even higher than the bicarbonate buffer point (8.3) so it almost has to be driven that high artificially with a stronger base than bicarbonate. Which leads one to think they are doing that for a reason, perhaps trying to precipitate some undesirable element from the raw water.

Where are you?

That is a high ph!

This post was edited by TheMasterGardener1 on Sat, Jan 19, 13 at 14:34

www.a2gov.org/government/.../water_treatment/.../annual%20summary.pdf

The original rock here is high in lime and may be limestone. Part of the water comes from the Huron River and part of it from wells. I had the soil analyzed at both my garden (well developed, fertile after years of organic matter) and the orchard (just started, extremely poor soil). Both had a pH of 7.6 or 7.7, 0.45% Ca (very very high) and very high Mg (about 250 ppm). The garden and orchard are separated by about three miles. The city compost has a pH of 8.3 too, and they deny that they put lime in it. I wonder if it is due to plant residue being extremely rich in Ca.

It is very poor water for plant growing and even a casual observer would be able to see the difference between irrigation and rain water.

"Well, one reason I could come up with is the compost in the soil acts as an even higher ph buffer. [than what?] "

Than soilless mixes with only lime...

"In turn, the availability of nutrients are higher. [how so?] "

Why do they have to lower the ph for hydroponics?

"Ph swings will not take place. [container media changes pH by the hour, with a number of influences driving the change, so how can you support that claim?]"

So you are saying rockwool has the same buffer as soil and ph will stay just as stable? Wow thats new to me. Last time I checked rockwoll ph jumps all over the place and soil ph does not...

"when PH of water will not be as much as an issue. [??]"

Really? lol

Nutrients will be availble at a wider ph range in meidium that have higher buffer like soil and compost.
----
My question remains...

"Are you saying there is NO disadvantages of growing soilless over soil? So there is only advantages of growing in soilless media?"

This post was edited by TheMasterGardener1 on Tue, Jan 22, 13 at 13:23