Potting Soil

The Truth About Potting Soils and Why Plants Struggle in Them

Potting soils or mixes were never meant to replace topsoil as a primary growth medium. Over time, though, people started treating potting mixes as “dirt,” leading to countless plant-growing failures—especially with vegetables and bulbs. These mixes mainly serve as fillers that help retain water and fill space during transplanting. Most contain little to no real nutritive value for plants.

Three General Types of Potting Soil

1. Basic Potting Soil

This type usually contains sand, perlite or vermiculite, and “forest products”—bark and partially decomposed wood. Wood breaks down very slowly due to its high lignocellulose content. In nature, fungi take a long time to degrade this material. Lignocellulose (a combination of lignin and cellulose) gives plant stems strength. Think corn stalks, rice, oats, and wheat—all rely on lignin to stay upright. Without lignin, they’d collapse once they reached a certain height.

Paper companies remove lignin using sulfuric acid to produce pure cellulose for paper. Since lignocellulose waste is abundant, it’s a common filler in basic potting mixes.

As an experiment, we bought two 10–12 inch tomato plants, washed the roots completely to remove soil and microorganisms, and then planted them in bare potting soil. Both died within three weeks. If you skip washing the roots, the plant’s survival largely depends on how much quality soil remains attached during transplanting.

Bulbs perform even worse in this type of soil. They may sprout a few inches—just as they sometimes do in a moist storage box—but that’s usually where it ends. Milly once learned this the hard way when she and her daughter Carole Dreier divided an order of prized Dahlia bulbs. Carole planted hers using organic soil and grew massive dinner-plate-sized blooms. Milly used bare potting soil and her Dahlias didn’t stand a chance. She hasn’t made that mistake again.

2. Intermediate Potting Soil

Intermediate blends use a wider mix of ingredients: humus, fir bark, worm castings, peat moss, chicken manure, bat guano, kelp meal, oyster shell—you name it. Plants will grow in these, but not very fast or very well. The boost comes mainly from urea in the manure and guano. Urea, a natural nitrogen source, plays a big role in plant development. It’s the same nitrogen source used in Miracle-Gro, though that version is chemically produced—so it’s not considered organic.

3. Advanced Potting Soil

These soils include trace amounts of inorganic nitrates, phosphorus, and potassium to support plant growth. The rest of the mix? Still mostly bark, peat moss, sand, and perlite or vermiculite. And no—they aren’t organic either.

Why Plants Still Struggle in These Mixes

Potting mixes often undergo pasteurization or heat treatment to kill unwanted seeds and microbes. That’s helpful for sterilization, but it also wipes out the good microorganisms that break down organic matter into plant-usable nutrients. Without added urea or soluble minerals, these “soils” can’t support plant growth alone.

Many people forget that during transplanting, they often add a little soil from the original pot. That original soil carries vital microorganisms. So when their plants thrive, it’s often because they’ve unknowingly inoculated the potting mix with the biology needed to break down organic material.

When potting mixes consist mostly of bark and lignocellulose waste, there is not enough quality organic matter to support healthy growth. On the other hand, intermediate mixes can help if they include materials that microorganisms can break down into usable nutrients.

Remember, plants are amazing organisms, but they can’t break down complex organic material independently. They rely completely on soil microbes for that job.

Yes, plants grow beautifully in hydroponics or water-based setups—but only when the fertilizer includes simple inorganic minerals they can absorb directly. In nature, where complex organic molecules make up the “feedstock,” microbes must be present to unlock those nutrients.

Final Tip

To avoid common transplanting disasters, skip the potting soil and use compost or high-quality topsoil when possible. For a little extra help, try using Bug Juice when transplanting trees or garden plants.