There are different perspectives from which the appropriateness of container media are viewed, but primarily there are two. One is from the perspective of the grower, which often goes like, "If it can last for 2 or 3 weeks w/o my having to water again, it's good". The other is from the perspective of the plant. IOW, what will best allow the plant to realize as much of the genetic potential it was endowed with by Mother Nature. When the two perspectives collide, it's much like having a discussion re what type of pot is appropriate. One person chooses pots for their pretty color or because they're plastic and therefore light, while from the plant's perspective, low fired clay or even a pond basket or plastic colander would be much more conducive to root health.
Getting back to choosing or making a medium, there is one question, the answer to which will help any grower decide if the grow medium is appropriate. The question is, "Can I water this planting to beyond the point of complete soil saturation, which would be so the medium is as wet as you can make it and water is draining from the drain hole(s), without the soil remaining wet or soggy for an interval so long root health or root function is compromised?" If the answer is anything but yes, the plant would benefit from an upgrade.
Whether or not a grower can water correctly w/o the plant having to pay an over-watering tax in the form of loss of potential and a reduction in vitality makes a very convenient point of delineation regarding whether a particular grow medium is suitable for the planting the grower intends to establish, or not.
Not only does a grow medium that can be watered correctly eliminate or significantly decrease the likelihood of over-watering and increase the margin for grower error (except for in the case of under-watering), it also makes fertilizing monkey easy and allows the grower the opportunity to retain complete control over the plant's nutritional needs; whereas, plants growing in overly water-retentive media are often watered in small sips in order to avoid over-watering, a practice that unnecessarily compromises the grower's ability to manage nutrition as effectively as in media which can be watered correctly. The typical nutritional issues that stem from watering in sips to avoid over-watering are a accumulation (build-up) of dissolved solids (salts) in the soil that can inhibit water and nutrient uptake, and a badly skewed ratio of nutrients, each to the others, that causes assimilation of some nutrients to be deficient, even though there is an ample supply of nutrients in the medium. Example: The build-up of phosphorous from using 'bloom-booster' fertilizers for containerized plants can cause deficiencies of potassium, calcium, iron, copper, and zinc. It can also cause excessive uptake of magnesium at the expense of calcium, which must be in the nutrient stream at adequate levels at all times if cells are to form normally. The takeaway here is, there is an optimum balance (ratio) of nutrients in the soil and watering in sips to avoid over-watering can quickly skew that ratio to levels that limit the plant's potential.
What primarily drives water retention in container media is the size of the particles that make up the ingredients. If you build/buy a medium based on all or a very large fraction of fine materials (peat, coir, compost, sand, composted forest products, .....) you'll almost certainly be battling the medium. If, however, your medium is based on a very large fraction of coarse material (pine bark, pumice, Haydite, Turface MVP, scoria, crushed granite, ....) and only includes enough fine material to bring water retention up to a point where watering intervals are acceptable, the medium will be working FOR you.
You'll notice I didn't recommend any particular medium or provide a recipe, this, because it's easier to agree on the concept of what makes a good soil as opposed to agreeing on a recipe. I do have recipes, but let's see if we can embrace a concept first.
Al
