I want to share my soil recipes that have worked for me. I use several different recipe's depending on what I am growing/doing: seed starting (SM), best succulent soil (BM), Lophophora Mix (LM), and the least expensive 'good enough' succulent soil (GM).
I use a 20-22 oz plastic cup to measure each "part". I make soil mixes in small batches in a large metal bowl, and I thoroughly mix by hand with a trowel. I try to use what I make in week, and store it during that time in a moderately airtight container.
(SM) Succulent Seed starting mix
1 part pumice
1 part heterogeneous sand
1 part "Black Gold" Peat seed starting mix*
1 part Expanded Cocoa Coir
1/2 part diatomaceous earth
*(I like to use this (as an organic component) in addition to coir because it contains some low levels of fertilizers useful for early stages of growth. Expanded coir has few bio-available nutrients for plants, unless they have been added. Seed starting soil in general has a higher concentration of organic matter.
I mix these together in a 1 gallon zip-lock bag. I add 2 cups of boiling water straight out of the electric tea kettle. Leave some air in the bag when you reseal, and then mix this well by moving the bag around gently, up and down, in circles, and rotate. Place on microwave safe dish/bowl, with the bag slightly un-zipped, for at least 3-4 minutes twice. If you are starting with cooler (than boiling) water, obviously microwave for longer. Remove from microwave, and zip up the bag. Let it cool overnight and use a clean spoon to dispense into clean seed starting trays/containers.
This recipe makes approximately 1/2 gallon of seed starting media, which weighs 70-75 oz. For dispensing purposes, 1 pint is 16-18 oz.
I try to use up each batch within a few days, other wise you may have to re-wet and/or re-microwave. (You inevitably introduce some contaminates every time you withdraw some soil out of the bag even under the best conditions and with clean tools. The contaminants can then grow rapidly in an other wise competition free medium)
For very small seeds, I will add a fine 2-4 mm layer of pure cocoa coir (or cocoa coir potting soil) on top of this normal soil to place the seeds on/in. Then, after seeds have been placed**, sprinkle a fine ~1mm layer of baked Commercial Grade Medium Sand in a clean salt shaker. This type of sand is great for this purpose because it is very uniform, and pre-washed, so no salts. The sand layer gives the developing seeds and roots some structure. To elaborate, I see it as something the seeds can use to push up against and use as leverage in order to get the initial roots oriented and inserted properly down into the soil.
**I recommend using my small seed handling kit. It ensures the best possible placement and positioning of seeds. This helps to avoid losses due to eventual overcrowding from seeds that aren't properly distributed. Also, the kit components are sterile, so you aren't intruding any extra contaminants into your microwaved soil. Available in my Etsy store (Noteworthy Plants).
(BM) Best Succulent soil mix
For small slow growing succulents, lithops, etc. This is what I use and sell. For reference, I have separated the inorganic and organic* ingredients by classification.
*Note "organic" in this context has nothing to do with being 'natural' or not. All ingredients are found in nature.
organics
1 part "Black Gold" Peat seed starting mix (any peat based lightweight seed starter really)
1 part expanded cocoa coir (alternatively, just double up of the peat-based seed starter)
inorganics
1 part perlite
1 part pumice
1 part granulated diatomaceous earth
1/2 part powdered diatomaceous earth
1/2 part heterogeneous sand
A pinch of lime, osmocote, and 4-4-4 instant release fertilizer pellets (Jobe's all purpose)
(LM) Lophophora soil mix
For calcium/limestone loving species
2 part Expanded Coco Coir
1 part pumice
1/2 part mix of heterogeneous sand calci-reptile sand (from the pet store)*
1/2 part powder diatomaceous earth *
1 part perlite
A pinch of lime, osmocote, and 4-4-4 instant release fertilizer pellets (Jobe's all purpose)
Use this soil for mature plants, but use standard Seed Starting Mix to start seeds.
(GM) "Good enough" succulent soil mix
Larger cacti, echeveria, any robust species, etc. Least expensive.
2 part perlite
1 part pumice
1 part heterogeneous sand
1 part standard potting soil (contains some slow release nutrients already)
1 part "top soil" or "garden soil" (these two are basically the same, but I use "top soil" myself specifically, used because it is very inexpensive but works 'good enough', FYI)
1/4 part diatomaceous earth
A pinch of lime, osmocote, and 4-4-4 instant release fertilizer pellets (Jobe's all purpose)
(EM) Easiest Mix
1 part "cactus soil" from most any major garden store
1 part perlite or pumice.
This is great for those in a rush, or if you only have access to the big box garden stores.
(EC) Expanded coir
Done in 5 gallon bucket. Makes ~2.5 gallons
1 coir brick (1.4 lbs)
3 quarts boiling water
Coir prepared in this way is very clean and will have minimal micro-organisms. I store my expanded, still moist coir, in a sealed container in the refrigerator. I only make withdrawals out using clean spoons.
Tips and tricks:
Be sure to keep the bags of any peat based soils well sealed between uses. This is because when peat dries out, it is very difficult to re-wet it. This is not a problem for coir.
Diatomaceous earth Is a great additive. It is microscopically razor sharp fossils of ancient sea creatures (diatoms) ground into a powder. This razor sharp property damages insects and small pests, and thus is a great way to control for pests. Alternatively, some have proposed a different mechanism for pest control, where the powdered diatomaceous earth dehydrates any insects that may be present on contact. It is also well draining and holds nutrients well. It can be spot applied on grown plants as an insecticide.
Fertilizer tricks: Coffee (diluted 1/4 or more) is a fantastic fertilizer for alkaloid rich succulents. Coffee has very bioavailable nitrogen (mostly in the form of caffeine), that is useful for the plant in synthesizing nitrogen rich alkaloids, and for strong vegetative growth. I will often mix diluted coffee 1:1 with a good cactus liquid fertilizer. Coffee is slightly acidic, so be careful not to use too much to perturb the pH.