I’m having trouble finding a good planting mix for propagation and seed sprouting. I’m also hoping to get a good mix for succulents and houseplants.

The planting mixes that I have purchased have some issues, and the premium mixes-which I’m guessing are great-have a high cost that will add up if I start using a significant quantity.

The first issue with premade planting mix is too much fertilizer. I don’t want to feed any fungus or bacteria that will interfere with propagation. The soil needs to remain wet for weeks while rooting happens, and during that time I may end up growing stuff that hurts the plants. I’d like to be able to add nutrients when the plants are ready for it.

Another issue is wetting, drainage and particle size. Some of the planting mix, such as Miracle Grow, appears to have a large amount of peat moss. I’m not a fan of peat moss because when it dries it becomes hydrophobic. Then when I try to water, the water just beads up on the surface until it finds a path through, and out, of the pot. It is hard to evenly water the plant. That’s mostly an issue with succulents, not propagation, but I’m hoping to find a good solution to all of my issues.

I tried Kellogg organic planting mix. It would be great if my mix is organic, although that is not a requirement. Their mix has a large amount of wood pieces. The wood seems good in that it doesn’t become hydrophobic when it is dry. The particle size isn’t ideal for propagation or seeds. There are some large chunks. I could just screen those out. There are a couple of things that I don’t like about the Kellogg mix. Something “glues” the soil together over time. After repeated watering and drying, the soil forms a solid, cohesive “chunk”. I can’t really stir the top of the surface unless I break it apart. Not that I really need to do that, but it doesn’t seem right to me. Even when I add a large amount of pumice to the mix, it still happens. I think I want something that stays flowable. I’m guessing there are some water-soluble components that dissolve and then solidify when the water dries, gluing the particles together. The other thing I don’t really like about the Kellogg mix is that I get fungus growth. White strings form on the surface, and that makes it really difficult for water to penetrate. It doesn’t happen all the time, but I’ve had a bunch of planting cells overgrown with fungus. I’m assuming that’s from the organic fertilizer components that they include in the mix.

I definitely like using pumice as an additive to my potting mix. Pumice stays very hydrophyllic and quickly draws water into the soil when watering. I prefer pumice over perlite because pumice does not float in water.

That brings me to density. I believe the material density of each component is important. If the mix contains components of widely different densities then the components will tend to separate. For example, I’ve mixed sand in with a wood-based planting mix, and over time ended up with sand at the bottom of the pot and wood chips at the top. The density that matters is the material density - the weight of a particle divided by the volume of that particle. Bulk density is different, because that includes the open spaces between particles. Each particle’s density determines how strongly it is trying to sink to the bottom of the pot. If all of the particles have the same density then they should not be able to push other particles out of the way and work themselves downward.

Densities of various materials

Material	Density, g/cm3
Rock (basalt)	3.5
Wood (dry pine)	0.35
Bark (dry pine)	0.5
Pumice	a
Perlite	a
Lava rock	a
Vermiculite	a
Peat moss (dry)	a

more stuff here…

Particle size

Particle size is a critical attribute for a soil mix. Large particles with irregular shapes tend to pack less densely than small particles. That allows open space for air flow and water percolation downward. Smaller particles will better hold water because they will have more surface area, and more small spaces between them that can grip water with capillary effect.

Am I writing a book? This is very in-depth.

What is the surface area of a particle? Assume it is a sphere. Volume is 4/3 pi r³. Area is 4pi r² (CHECK THAT) So the ratio of area to volume is 3/r. If r is 1 (units don’t matter here), then the area is 3. If r is 2 (twice as much), then the area is 1/2 of 3. For the same volume, we get half the surface area when the radius of the particles doubles. If the radius is half, the area is 6. The surface area doubles if the radius is half.

That suggest that smaller particles are better, but there’s a limit. I don’t want to hold too much water. I want the soil to be able to dry out in a reasonable amount of time. It needs to dry out to prevent too much fungus growth.