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I want to plant sage, which is said to grow best in well-drained "sandy soil". I bought some potting mix from the hardware store.
Whether a potting mix drains well or not depends on the characteristics of the components. We have to look carefully at the components of the mix. Many off the shelf products are basically peat and vermiculite and perlite and other stuff which is used extensively by the horticultural industry and for most purposes drains very well because at the microscopic level the peat is just a bunch of tiny twigs that don't clump together to exclude air. Water can wrap itself in a thin film around the twigs but also maintains air spaces or pores. Problems arise when the pores fill with water and drive out the air.
So mixes become problematic when they contain other materials such as regular garden soil or composts that consist of very fine matter that can clog the air spaces. Examine the list of ingredients on the bag, or perform a soil profile test with a mason jar to separate out the components visually.
For plant materials like sage we are trying to intensify the herb virtues of high essential oil content and the other stuff that makes sage a real sage and not just some overfed weed. The idea is to keep it alive but not fat; growing in peat mixes with fertilizer is fine for the early stages, but to get it to grow to a mature bush we put it where the roots will not drown and the fertilizer available is in low concentrations. So regular garden soil with low clay, high sand components is just the ticket. Like rosemary, sage is adapted to dry conditions and can survive all but a severe drought, so while mostly you can forget watering make sure to keep it alive in really dry weather.
A practical test for free draining might be to take two pots, fill one with coarse sand and the other with your potting mix. Water freely so that each achieves a stable weight. Leave for several hours, then add a cup of water to each to see how quickly the cup of water drains out the bottom.
In general adding ingredients to any mix will decrease its drainage.
Consider a jar of marbles. It has about 30% air volume, and the spaces between are large so offer little resistance to flow.
Consider a jar of BB's. Also spherical components so also about 30% air volume. The passages are smaller so their is some increase in flow resistance.
Now mix them.
The result is a system where water has to drain between the BBs that are between the marbles. There are fewer paths because the marbles take up some of the room.
To increase the drainage of a mix you have to add chunks that internally have better drainage than the mix you are starting with. This can mean that NOT mixing the two components well will improve drainage. Consider the marbles and BB's, having it in layers may have better drainage than having them separate. This works only if the smaller stuff can't move into the voids in the larger stuff.
This can be used to improve drainage in pots: Put a layer of fast draining material that covers the drainage holes, a layer of landscape cloth, and your mix. The drainage holes of the pot are no longer a choke point for drainage flow.
In general a mix of sand and clay will have poorer drainage than will either one alone.
Soil structure is important to drainage. Adding organic matter to soil improves the structure over the long haul because it will decompose leaving tiny channels between the lumps. Humic acids from that decompostion help clay maintain some structure too, due to lowered pH.
Typical ingredients depend on the plant and soil needs. Little water holding requries good drainage. That's done with Sands or lava rocks, perlite. Playground sand is acceptable but often too fine for use. That's why you get your hands on gritty sand.
"sands" ( beach, dune , play , torpedo ) will usually pack too well to give drainage and air. So , in quantity of 50 lb, I use garnet sand blast sand . It comes in a few sizes , the grit I use in aquariums and pots is about 6 to 16 grit ( larger grains than the other named sands). For smaller quantities you can get similar coarse sand in pet shops in 5 , 10, 20 lb sizes. Either "natural brown" ( about the same as garnet) or limestone ( aka. crushed coral or Aragonite ). These ,mixed with potting soil ,can give good draining mix.
