Conventional wisdom on potting mixes is that the best mix is lightweight. The lighter the better. I reject that notion. Potting mixes need to be judged on seven things:
- the ability to store plant nutrients (the so called “Cation Exchange Capacity” or “CEC”), otherwise known as the “richness” of the soil
- the ability to store water for use by the plant
- the ability to remain open and free draining as to allow oxygen to get to the roots of the plant, i.e. the ability to drain, remain porous and not become waterlogged
- the ability to supply needed neutral Calcium and Magnesium to the plants
- permanence, the ability to remain part of the media for a long length of time
- if it decomposes to slowly decompose in a way that does not rob nitrogen from the plants
- cost; unless absolutely necessary, one doesn’t want to spend too much on a soil mix
The exact mix of these elements depends on the situation and the plants being planted in the container. If a planter is going to be outside during the torrential downpours of a Florida rainy season then drainage will rank number one. Plants vary considerably in their needs and that has to be taken into account when coming up with a mix. One size does not fit all unless price is no object. If price is no object then you can pot pretty much everything in the desert rose mix.
I like heavy container mixes. I don’t like perlite. Nurseries like to use perlite as its light weight cuts down on shipping charges and makes containers lighter to handle. It can be used in place of better media to reduce weight. That is its only function, to reduce weight. It is of no more use to a plant than pea gravel as it is totally inert. Perlite provides no “cation exchange capacity”. Perlite provides no water storage. It provides no Calcium or Magnesium. It does help keep a media open and porous to the same extent as pea gravel does. It also tends to separate from the mix and float to the top of the container where it will spill over the side in a heavy rain and create a mess.
Vermiculite is good. It does provide “cation exchange capacity” even when compressed. It initially stores water and initially it helps with aeration and drainage but it does compress considerably over a period of a few years just like organic materials. It doesn’t rob the plants of Nitrogen as it compresses. Vermiculite supplies needed neutral Calcium and Magnesium to the plants. Because it holds water it doesn’t significantly lighten planter or nursery containers so it isn’t used in nursery mixes. It is much better than chopped up bark or wood but is more expensive so it isn’t in most “soilless” mixes purchased premade.
Some organic materials such as composted cow manure, coir (“coco fiber”, the ground up husk of a coconut) and peat moss have excellent cation exchange capacity, excellent water storage, and excellent porosity. As they slowly decompose they don’t rob plants of needed Nitrogen. They have one shortcoming, with time they disintegrate and pack down, losing their porosity and becoming waterlogged. This can be a real problem with something like long lived perennials, succulents, bonsai or desert roses.
Milorganite and Scotts All Organic Lawn Fertilizer are interesting fertilizers. In small amounts in any mix they provide well composted organic materials with considerable amounts of nitrogen in the mix. They are useful in small amounts in any mix. They are organic fertilizers which won’t attract critters. They provide huge amounts of cation exchange capacity with very small amounts of fertilizer. All potting mixes should have about 5% Milorganite 5-2-0 organic fertilizer or Scotts 11-2-2 All Organic Lawn Fertilizer mixed into the mix to get the plants off to a good start and to add a very high cation exchange material to the mix (both are available from home improvement stores).
Other organic materials such as fine bark, chopped up wood, and most commercial “soilless mixes” (largely chopped up wood) aren’t as good as one might imagine. These materials initially have good aeration and drainage. But these materials don’t hold much water or have a good cation exchange capacity until they start to decompose, when they then pack down and stop aeration. They have no Calcium and Magnesium in them. These materials will rob Nitrogen from fertilizers as they decompose and result in stunted plant growth. I generally recommend never using a pre-mixed “soilless mix” (other than for bromeliads) as they all have too much wood and bark in them. Make your own “soilless mixes” using composted cow manure, homemade compost, coir and/or peat moss as the organic materials of choice.
If you examine so called “potting soil mixes” you will generally find they consist of sand and ground up partially composted wood which has been wetted down to make them heavy. They have the same problems as “soilless” mixes. As the wood in them continues to decompose it will rob Nitrogen from the plants. They also tend to be so finely ground that drainage and aeration are poor from the get-go. I’ve had these mixes go anaerobic in large containers and give off the infamous “rotten egg” smell which is death to any plant. I have little use for them except as a minor portion of a mix.
Pea gravel, sand and aquarium gravel have no cation exchange capacity and store no water. Like perlite their only function is to keep a media open and porous over time. The larger the particle size the more aeration the media will provide. Unlike perlite they are heavy but they also don’t float to the top of mixes and create messes.
Expanded calcined clay pellets (“Grow Stones”,”Hydroton”, “ViaStone”) are used as growing medias which hold water well but which drain very well and hold a lot of air (20% to 30% air). They also have excellent “cation exchange capacity” or ability to hold nutrients. These media are used in growing prized bromeliads and orchids and used in hydroponics. They are expensive and typically costs between $2 and $5 per pound. While they would be great in a soil mix the cost tends to make them too expensive. Also they are difficult to find in stores.
What people don’t realize is that there is another expanded calcined clay product which also holds lots of water and has good “cation exchange capacity” but which only costs $0.14 per pound, namely cheap cat litter from various “Dollar Stores” (Dollar General and Family Dollar), where it comes in 7 pound $1 bags. By itself it holds too much moisture and too little air for all but swamp plants but in a mix with pea gravel, compost, peat moss, vermiculite and/or other ingredients it makes up a very good soil mix. It won’t disintegrate and breakdown with time. This cat litter supplies needed neutral Calcium and Magnesium to the plants.
It is important to only use the really cheap, unscented, non-clumping clay cat litter. More expensive stuff doesn’t work. I recommend using this expanded calcined clay cat litter in most container mixes. Note there is a commercial product, “Turface” sold as a soil conditioner for growing grass in baseball and football stadiums which is simply cat litter, probably made by the same manufacturer (all the calcined clay comes from one big deposit of pure Montmorillonite clay in the panhandle of Florida). The ad for Turface (identical to cat litter) says “Turface granules have 74% pore space which allows them to hold water and oxygen in a nearly perfect balance. They also store nutrients vital to plant growth.
- Stable particles have less than 5% degradation in 20 years.
- High nutrient storage (CEC of 33 meq/100g).
- Nearly perfect balance between air and water porosity.”
The same holds true for cat litter. “Oil adsorbent” from the automotive section of Walmart is the same material and can be used in place of cat litter in all the formulas below.
Soil Mix for Annual Container Plants
A mix of organic and nonorganic materials is called for:
- 1/4 peat moss (or coir)
- 1/4 vermiculite
- 1/4 compost (can be homemade compost or store purchased composted cow manure)
- 1/4 cheap unscented non-clumping cat litter bags from a Dollar Store ($1 for seven pounds).
Because of its high organic content and vermiculite this mix will pack down over time and needs to be replaced every few years (or just added to). It is excellent for growing annuals or vegetables which only last a year or so. It has excellent cation exchange capacity, excellent drainage, stores a lot of water and will not rob the plants of Nitrogen. The vermiculite and the cat litter both supply Calcium and Magnesium. When wet this mix is heavy. Note this is an excellent mix for using in raised beds.
Soil Mix for Perennial Container Plants (Hibiscus, Ixoria, Blueberries, etc.)
A mix of organic and nonorganic materials is called for:
- 1/4 peat moss (or coir, homemade compost or composted cow manure)
- 1/4 vermiculite
- 1/4 sand
- 1/4 cheap unscented non-clumping cat litter bags from a Dollar Store ($1 for seven pounds).
Because of its high inorganic content the mix will not need to be replaced frequently. When wet it is heavy. The cat litter and sand give permanence and good drainage and aeration. The organic material, vermiculite and the cat litter give good cation exchange capacity and good water storage. The cat litter and vermiculite provide Calcium and Magnesium. And the organic materials won’t rob Nitrogen from the plants as they slowly decompose. Vermiculite will only compress and lose water storage capacity. In its compressed form it still gives good cation exchange capacity forever. If weight is a concern the sand can be replaced with perlite with no loss to the mix. This mix is excellent for growing roses in raised beds (the best way to grow roses in LaBelle)
Soil Mix for Epiphytic Bromeliads
Epiphytic bromeliads (most bromeliads with the exception of the pineapple) do best in a very lightweight media with little cation exchange capacity. They absorb nutrients and water best though their cups, not their roots, so foliar feeding is the preferred feeding method. They don’t need Calcium or Magnesium. It is very important that their soil media remain open, airy and well drained. This is one occasion when I find chopped wood, perlite and commercial soilless mixes useful:
- 2/3 chopped wood or bark (Fine half inch pine bark is perfect)
- 1/3 perlite
Alternatively just buy any “soilless” planting mix from the store, especially cactus mix. “Show quality” bromeliads are typically grown in expensive expanded clay pellets or expanded slate, specialty items which are difficult to find and buy. If you want a real “show” bromeliad, use the desert rose mix below. Another mix which will grow bromeliads is simply two thirds perlite to one third cat litter. This latter mix is cheap and it never packs down and never needs repotting. All the mixes need to be covered with a neutral tone aquarium gravel (brown or grey) for a show.
Soil Mix for Cryptanthus
Cryptanthus are a bromeliad which normally grows in organic swamps in full sun. The formula I use is inexpensive and it grows Cryptanthus bromeliads well:
- 1/4 composted cow manure (can replace with “African violet soil mix” from the store)
- 1/4 peat moss
- 1/2 cheap unscented non-clumping cat litter bags from a Dollar Store ($1 for seven pounds).
- The last one inch on top is 3/4 white aquarium gravel mixed with 1/4 cat litter for aesthetics (obviously an optional item).
All of the ingredients hold water well, which is important for cryptanthus. Because of its high organic content this mix will pack down over time and needs to be replaced every few years. But because cryptanthus are naturally swamp plants this packing down doesn’t adversely affect the plant. Cryptanthus are native to wet compressed swamp conditions. If you want to avoid repotting, just use 100% cat litter. For a show a neutral tone (brown or grey) aquarium gravel can be added as a final layer.
Soil Mix for Succulents
Desert succulents and cactus like to go completely dry in between watering. Obviously this is impossible if the container is outside in Florida’s rainy season. At the very least they want a lot of air around their roots in the rainy season. This is really difficult in Florida’s rainy season, especially if the media is an organic media which packs down with time. The formula I use is inexpensive and it grows the desert succulents and cactus well:
- Most of the container is 2/3 pea gravel and 1/3 cheap unscented non-clumping cat litter bags from a Dollar Store ($1 for seven pounds). Note than I don’t add sand, during the rainy season sand won’t allow enough air to reach the roots of the succulents. Organic material of any sort holds too much moisture, especially as it decomposes and packs down over time. Vermiculite also packs down with time. This mix of 1/3 cat litter hold moisture but still drains fast and holds a lot of air, even in the rainy season.
- The last one inch on top is 3/4 white aquarium gravel mixed with 1/4 cat litter for aesthetics (obviously this is optional).
If light weight is the main objective replace the gravel with perlite. One can fill most of the container with 2/3 perlite and 1/3 cat litter and top of the last one inch with 3/4 white aquarium gravel and 1/4 cat litter for a lightweight container where rain won’t dislodge the perlite particles. Note to stay away from commercial cactus mixes. They are largely ground up wood chips and will ultimately pack down and kill the succulent, all the time robbing the succulent of Nitrogen fertilizer.
Desert Rose Mix
Desert Roses have some very demanding and exacting requirements for their growing media. Desert roses are not succulents and don’t like to dry out between watering. So water storage capability is important. By the same token desert roses demand a high amount of air around their roots at all times. This is a difficult bill to fill; water AND air, both in abundance and both for the long term. The media cannot decompose and pack down with time as desert roses are very long lived plants. The formula I use is an obsessive compulsive’s dream but it grows the best desert roses. The mix is somewhat expensive but affordable considering that a good grafted desert rose can run $25.
- One 15 lbs. bag of “plant growth” aquarium gravel (wet and dense but porous rock). The trade names are “Flourite” and “Flora Max” (PetSmart has them, Petco doesn’t have them) ($17 per bag). A lot of the weight is water.
- Two cheap unscented non-clumping cat litter bags from a Dollar Store ($1 for seven pounds).
- One container activated charcoal for aquariums ($9 per 39 oz.).
- One container ammonia absorbing zeolite for aquariums ($11 per 36 oz.).
Volume wise it turns out to be about 1/3 “plant growth” aquarium gravel, 1/3 cat litter and 1/3 the combination of charcoal and zeolite. The ingredients in the formula I use are enough to pot up three desert roses in three 12 inch 2 gallon planters (note that I use PetSmart for these last two planting media ingredients, Petco is so overpriced it is ridiculous!). The cost comes out to about $13 per planter, not bad for a long term investment which will last the life of the plant (it won’t pack down, which is essential for desert roses to survive!). Note that the charcoal and the zeolite are the important components of the mix and shouldn’t be skimped on.
This mix has it all, excellent aeration and porosity, excellent water storage and 100% of the components are permanent, they won’t decompose and pack down over time. It provides plenty of Calcium and Magnesium sources and excellent cation exchange capacity. This mix is the one to use for any plant where you want a show quality plant in several years, such as a desert rose, bromeliad or bonsai. It will grow a show quality plant without having to undertake the risky proposition of repotting. Repotting always sets a plant back a bit and can result in dead leaves, which of course is very undesirable for a show plant. The “plant growth” aquarium gravel can be replaced with cheaper and lighter perlite with some loss in performance (you lose limited cation exchange capability and limited water storage capability). The pure white perlite gives an unnatural look to the soil mix which I don’t like but some people like it.
This last mix has a commercial equivalent. It is “Bonsai Soil” 21 lbs. for $35 from Wigert’s Bonsai Nursery in North Fort Myers. This soil mix consists of lava rock, “Turface”, and pine bark in equal proportions. The lava rock in the “Bonsai Soil” is only a little different from plant growth aquarium gravel rock, with larger pores (and less cation exchange capacity). The “Turface” is calcined clay, identical to the calcined clay in cat litter, probably from the same manufacturer. I replace the pine bark with activated charcoal and zeolite (high cation exchange materials) as the pine bark has little cation exchange capacity and the pine bark will decompose with time, pack down and rob Nitrogen from the plants in the process. The price for the two mixes is similar on a dry pound for pound basis, with a slight edge to the homemade mixture, $1.40 to $1.66 per dry pound.
Seed Starting Mix
For starting most seeds in pots I use a commercial Farfard brand of “fine” seed starting mix that appears to be about 3/4 peat moss and one fourth perlite, both finely ground. Most seed starting mixes sold in stores is perfectly fine. Jiffy uses about 1/2 peat moss to 1/2 vermiculite while Miracle Grow uses almost pure peat moss with just a sprinkle of perlite. The important part for seed starting media is that they be sterile. It is also desirable that they be free draining and lightweight. Most seed starting containers aren’t very robust and can’t support a lot of weight.
Dave Bogert DLBoge2@gmail.com