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The Dirt on Dirt. What You Need To Know Before Buying Potting Soil

by Fern on March 17, 2009

in How To

It’s nice to talk about great plant combinations. Or how to select the right pot. But if you don’t use good dirt in a container garden, you might as well be putting lipstick on a pitbull, pigs, and probably hockey moms too. Lipstick and Vice Presidential candidates aside, it’s hard to get plants to do well if their roots are sitting in poor soil, or the wrong type of soil. So, here’s the dirt on dirt…

Photo by Toni Girl

Don’t Use Dirt in a Container Garden

What?! I must have lost my marbles right? How can my first bit of advice be to never use dirt? Simple. Dirt is the stuff that nature made all by her lonesome. You can find it in every single back yard in the country. It’s usually just fine for the needs of plants growing in the ground, but it is not right for plants growing in pots. For starters, it may have pests, diseases, weeds, etc in it. It is also likely to be too compacted for the good drainage container grown plants need. Finally, garden dirt doesn’t have enough nutrients in it for container grown plants, who can only use the tiny bit of soil in their pot for sustenance.

The stuff you want to use is called potting soil, or soilless mix. It doesn’t have any actual dirt in it at all. Potting soil is comes in bags from the nursery. It contains bits of decomposed leaves and bark, perlite (the tiny white pebbles), peat moss, and sand. Oftentimes, in addition to the ingredients I’ve already mentioned, potting soil will have time released fertilizer, bat guano, worm castings, lime, or any number of other things mixed in to make the soil richer and keep its pH balanced.

What to Look For

Good quality potting mixes will not be cheap. If you find a bag that is being sold for $1.99 I will bet my first born child that the garden center is either having a going out of business clearance sale or the bag you are looking at is not good quality. Or maybe you’re looking at a really small bag.

For starters, good quality mixes will tell you what’s used in the mix. Here are some ingredients to look for:

  • Sphagnum peat moss, sedge peat, or coconut coir (coconut coir is an environmentally friendly alternative to peat)
  • Composted, aged forest products
  • Sand
  • Vermiculite
  • Perlite
  • Charcoal
  • Wetting agent and water-holding polymer
  • Lime for balancing the pH

Just because a bag of potting mix doesn’t list every one of the above ingredients doesn’t mean that it’s bad. For example, not every potting mix will need a wetting agent or lime (both are used to correct specific problems that not all mixes have). But the ingredients I listed are common, and if you see several of them listed, the mix you are looking at is headed in the right direction.

When you look at the actual potting mix, it should be fluffy. If you see or can feel that the mix has formed hard clumps inside, put it back on the shelf. If the potting soil can do that while inside a bag, imagine what it will do when it goes through the watering/drying out cycle, or is subjected to hot temperatures in the middle of summer.

My personal preference is to pick a mix without any fertilizer. I want to be able to control the amount of fertilizer and the type used. I want to know exactly what fertilizer went where and when. But if you’re not that picky, I recommend choosing one with a time released fertilizer and then applying more time released fertilizer in about half the time recommended on the bag (so if it says that there is enough fertilizer for 3 months, apply more in about a month and a half to two months).

This info should get you started, but come back tomorrow for info on chosing specific potting mixes for specific types of plants, and show up on Thursday for the dish on keeping your potting soil in good shape with fertilizers and compost.

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{ 4 comments… read them below or add one }

Genevieve March 21, 2009 at 8:36 pm

This is great info, Fern. I get asked about potting mixes fairly often in my business and I think I’ll just email people your link. You really explain things so perfectly and clearly here!

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myna lee johnstone May 22, 2010 at 4:40 pm

Carolyn Herriot, a veteran gardener in Victoria/BC/Canada has an excellent recipe to make your own.
Book is called: A Year on the Garden Path: a 52 Week Organic Gardening Guide
This is my Bible.
She has a new book too this year, on the Victory Garden, haven’t seen it yet.
You can buy from her. Just google in the info

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Fern May 22, 2010 at 5:22 pm

Thanks Gen!

Myna–I’ll have to check that out. I prefer to buy bagged potting soil because I don’t have space or time to mix my own, but it sounds like a great resource that I might find useful.

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Erika May 11, 2012 at 5:11 pm

Thank you thank you thank you! I have been searching the Internet for a week to see if I needed to add anything to my potting soil. I just planted seedlings two days ago and have been wondering if the organic miracle gro potting mix is enough because it says it has fertilizer. I am subscribing to your website :)

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