There are a few different approaches on what's good potting soil. From what I've researched. There are some criteria to potting soil that should be met to be considered good potting soil.
1. Good Drainage
2. Water and nutrient retention
3. Allows aeration for your plants roots
(Fertilization is another topic)
Let's start with #1 Drainage:
One of the biggest mistakes you can make is use hard clay top soil in your plant pots. It's got terrible drainage. So most of us should turn to soil purchased from the store or make or own compost.
Many bagged potting soils are made up of peat and some from coco coir. It doesn't really matter because we want to amend the soil any ways.
Below is a list of available soil amendments out there that you can use to make the soil drain better.
4. Pine Bark
Sited from the National Garden Association: Adding Pine Bark to your soil is a great way to amend your soil as well as keeping it aerated and loose for good drainage.
What about what I've read about pine bark or wood sucking out the nitrogen from my soil?
Well it's true but only to a limited extent. Read this:
"All over the Internet ,when you go looking for information about using wood or wood chips in gardening, you read about how certain items will suck nitrogen out of the soil. When you start looking for factual evidence though, it seems very hard to find any scientific study that backs this up. Why is that?
Wood, when it comes in contact with soil, activates mechanisms in certain bacteria which then consume nitrogen as food so they can decompose the wood. If this wood is sitting on top of the soil (fallen tree, dropped tree branch and so on) then these mighty bacteria will become active, but only to a depth of around 5mm. This means that the answer to the nitrogen "robbing" wood is only involved in a minute area under and around the piece of wood. Bury that piece of wood and you will be able to measure nitrogen depletion about 10mm from the wood (in all directions since the wood is under the soil). This brings the "nitrogen loss" caused by wood into the magical realm of "Urban Myth" since such quantities of nitrogen are small (minute really) when put into proper comparison of ava. This can be shown simply by looking at the soil in a forest floor, dead wood fall, leaves, dead undergrowth are all present and yet new growth is always present, even next to dead wood you will find nitrogen needy ferns growing. If the myth about wood was true, these plants should not be there, or at least not thriving as they do.
Most of us use wood chips for paths and mulching around trees, areas where the small amounts of nitrogen being used by the bacteria are truly negligible in significance to the growth rate of the plants near by. Keep in mind that this is because in nature, almost all the available N is in compound forms and slow release is the norm. Trees feeder roots are normally located from half the distance from the trunk to the outer edge of the canopy drip line (some species go out past this drip line further than others). What does this mean to us as growers? It means we don't have to worry so much about nitrogen loss when it comes to bushes and trees, unless we want to cover an area from the magical 6" diameter away from the trunk all the way out, 3 or 4 feet past the outer edge of the canopy drip line. Tree roots (the feeder ones we are most interested in) live from half the distance trunk to drip line out to around 3 or 4 feet past the drip line. These "important" (and they really are) roots are found at a point of 1-2 cm below the soil surface down to around 30-40 cm deep. The holding roots (tap root and large spreading main roots) go deeper since their job is to hold the tree in place under the stresses of wind events. How deep these roots penetrate is dependent on how far down bed rock is found. That is why my state (Arkansas) has such a large number of downed trees in heavy storm events, the bed rock is close to the soil surface, no deep roots means the trees aren't well anchored. On my farm we are lucky, there is up to five feet of soil depth and the bedrock is highly fractured, so tree roots can anchor really well (and these roots keep adding to the fracturing of the bedrock).
The gardens, where we should be worried about nitrogen being bound up by wood (especially chips) are the perennial and annual vegetable and herb gardens, these plants don't have roots (for the most part) that go deep and widely spread, usually their roots are very near the surface and within a 1 meter diameter circle from the main stem.
Squash and other vining plants put roots out all along their vine leaf nodes, but these are still shallow roots, so they are vulnerable to nitrogen binding by any wood chip mulch we might put down.
Nitrogen, the kinds we plant growers are most interested in, as I mentioned earlier, is a slow release nutrient when provided in natural forms. For faster access to plant roots, synthetic forms are needed, not what we want to hear!
Natural Nitrogen comes to plants in large chain molecules, Nitrites, Nitrates, Ammonia salts are the normal, natural forms we can put into soils via composts, manures, urine and teas made from mixtures of these along with greenery. Compost is a very long term additive, it actually takes five years for it to give up all the soil and plant goodness it contains. Which makes it very much an ideal additive in gardens.
Now that you know more about nitrogen forms found in nature, it should be easier to go about using woodchips for a mulch. They really don't cause any problems by "robbing Nitrogen" from your plants, especially if you use compost around or over them. The nitrogen from the compost will leach through those wood chips and still find the soil beneath your mulch chips. If you build a hugel and use greens and or compost as part of your filler ,then most likely you have made up for any Nitrogen loss that buried wood might cause. And if you top dress with compost or do a chop and drop of cover crops, like most people do, then you have added more slow release nitrogen than the wood might take up. Mother Nature loves to use wood to build soil, so there really isn't any reason we should not follow her lead and do the same. We just don't need beavers to make chips out of trees like she does."
Water & Nutrient Retention:
It's a fine line between well draining and cactus mix. When I water a pot I want that water to feel moist for atleast a day or 2 depending on the outdoor conditions. If it drains through and completely dries within hours it's too loose and if it doesn't drain through after a few seconds of watering it's too dense.
I do not ever just use bagged soil straight out of the bag. I like to come up with my own cocktail to create the perfect mix. Keep in mind if you're buying a bag of soil, the manufacturer is trying to maximize their profits so they will not have everything that you would want in the soil. It's like making your own burger vs going to McDonald's.
So what goes into all of my pots?
Bagged soil 60%
Small bark 10%
Osmocote slow release - handfuls
Lime is optional - handfuls
Keep in mind that the anything you grow in a pot should be regularly fertilized. But that's a whole other article in itself.
By having nice fluffy loose soil for your pots, as a result of amendments for better drainage you allow oxygen to get to the roots of the plant.
It's the same thing as when you thatch and aerate your lawn. Which allows for nutrients, water and oxygen to easily reach the roots which results in a much healthier and thicker growth.
Seattle is unique in that we have an extremely wet fall - spring and one of the driest summers in the country. So the soil in our pots have to be able to account for drainage of lots of water and strong retention in the summer. Just be aware of you climate and create the best soil according to your climate.