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The Real Dirt on Potting Soil Mixes

I was recently drawn into an on-line garden writers’ discussion of ProMix potting soil, and now it’s suddenly turning into a quick article.
My two cents on ProMix and other potting soils:

First, let me give you a bit of brief background. I’ll be 57 in a few weeks and I’ve been growing things since I was five. I have a MS in agriculture/horticulture, taught horticulture for twenty years, owned two wholesale-retail nurseries, and I continually am trying newer, better ways to grow plants.

When I had my nursery in Minnesota I used a lot of ProMix and I liked it very much, although it was fairly expensive. In California I had a difficult time getting ProMix and used to use a great deal of Sunshine Mix, which was also pretty decent. I always thought these two products were about the best potting soils I was able to buy. There may be superior ones, but I never used them.

I see that ProMix now comes in a multitude of different blends: all organic, with fungicide, with or without fertilizer, etc.
I often added some time-release fertilizer to the ProMix, and for certain crops, a little extra lime. For time-release fertilizer I generally used a commercial blend along the lines of 20-20-20 NPK. Osmocote brand is the best known time-release product but there are now numerous other good ones as well. I like to use time-release fertilizer with a 90-day rating. By the way, you can top dress pots with time-release fertilizer, with very good results. I use a teaspoon per 4-inch pot, two for a 6-inch pot, and a tablespoon per gallon pot.

I know some will not agree with this next statement, but so be it. I don’t recommend adding organic fertilizers to potting soils. I’ve done it many a time and tried pretty near every kind of them and it brought more problems than it was worth. For my vegetable garden you bet I use organic fertilizers, but for growing top grade commercial quality ornamental nursery stock, you’ll have better luck with inorganic fertilizers…at least in the potting soil.

For many plants that I was trying to root from cuttings, I'd add a goodly amount of perlite...often using a mix of 50% perlite and 50 percent ProMix. I always advise to buy your perlite in large sacks from commercial yards that cater to bricklayers and plasterers. A large amount of perlite is used to make textured plaster ceilings, and this product is the same as that sold at nurseries, only much cheaper.


Perlite is white, light, fluffy, and it adds air spaces to any potting soil. If you have too much perlite in a potting soil (which is almost impossible to do) it will dry out much faster and you’ll have to water it more often. But adding perlite to potting soils is the quickest way to get a soil media that drains well and drains fast. Drainage is perhaps the most important key to a good potting soil. If it doesn’t drain fast, it won’t grow good roots. Fast drainage equals more air in the soil and this always means improved growth. When you pour water into a pot, if it doesn’t drain right out the bottom fast, then something is wrong!

I no longer grow commercially and can't ever find any ProMix or Sunshine Mix for sale retail. I have to buy other cheaper potting soils instead, and all
of these I amend considerably. I always add perlite to all of them as none drain as well as I’d like. I also often add some super phosphate to the mix, using a pound to a wheelbarrow full of potting soil. I then also add time-release fertilizer.

One of the keys to really good potting soil is the basic ingredient in it, the peat moss. The best peat comes from bogs in Canada. I’ve used plenty of domestic peat and it worked, but was never quite as good as most Canadian sphagnum peat moss, which is coarser, holds water better, decomposes slower, has more nutrients in it, and almost always has a consistently low pH.

If you make your own potting soil from scratch, here are a few tips: Start with a bale of real Canadian peat moss. Do add time-release fertilizer, a quart of 20-20-20 (or some ratio similar to this) time-release to a large wheelbarrow of mix. Do not steam, bake or otherwise try to sterilize this mix, as the peat is pretty near sterile as it is. The peat also has some live good organisms in it and heating it will kill these off…with bad results. Do add some lime to your mix, using about a pound of dolomite lime to each wheelbarrow of mix. Most ordinary barn lime will work too. Add several cups of super phosphate fertilizer (0-40-0) or some rock phosphate to the mix. Don’t add manure or dirt to your potting mix. The manure will encourage fungus and damp off disease. The dirt will add weed seeds.

Be sure to add plenty of perlite to the mix. I like to use 2 parts peat to one part perlite for my basic potting soil mix. Do not add any vermiculite to the mix, unless you’re going to be growing tiny seed in those tiny plug trays. In these trays a soil mix with vermiculite will hold water longer, keeping the tiny cells from drying out too fast. But, vermiculite in regular potting soil just makes it too soggy.

A word of caution here: wear a paper face mask when you mix in perlite as the dust from it is fine and won’t do your lungs any good. The dust from vermiculite is even worse, and often has asbestos in it. Actually, I like to add some water to the wheelbarrow as I’m making up a mix…the dust from dry peat moss isn’t good to breathe in either. If you don’t have a wheelbarrow, use a large empty trashcan and a shovel to make up your mix.

One last tip: The key to growing really high quality bedding plants, and most anything in a container, is to use both time release fertilizer AND liquid feed fertilizer too. I take an 8-ounce Styrofoam coffee cup, fill it with a water-soluble fertilizer like Miracle Grow, toss this into an empty 40-gallon plastic trash can and then fill it up with water to the top. You can use this fertilizer water every time you water. The mix will have about 3 to 400 ppm (parts per million) N (nitrogen) in it, which is just right for constant feeding. I keep a trashcan full of this mix handy at all times and use it on almost everything in a container.

Tom Ogren is the author of five published books, including: Allergy-free Gardening, Safe Sex in the Garden, and, What the Experts May NOT Tell You About: Growing the Perfect Lawn. His website is www.allergyfree-gardening.com

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