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Indoor Plant Bulletin #5

A publication of Horticultural Help

48-16 30th Avenue, Astoria, NY 11103

Potting Soil Basics

Good quality potting soil can make a big difference in the way that your plant grows. Poor potting soil can cause a multitude of problems: Pests, disease, root rot, and nutrient deficiency. As you know, it is in the soil that a plant’s roots grow. The root-growing environment is critical to the health of the roots and healthy roots are vital to the health of the plant. Plant roots are like the human circulatory, digestive, and nervous systems all rolled into one.

The good news is that repotting plants is not usually necessary so you won’t have to think about potting soil very often. The bad news is that it is not easy to find good quality potting soil.

Professional Secret: Do not repot unless it is absolutely necessary. Nearly all potted plants will grow more vigorously and bloom more profusely if their roots are “tight” in the pot. This means that there are lots of healthy roots growing throughout the interior and around the outside of the rootball. Optimally, root systems should occupy three quarters of the volume of the pot. A moderately potbound or rootbound plant usually dries out within 2 to 7 days following a thorough watering and that is a good sign. If your plant can go two days or more without needing water, then your plant does not need to be repotted. Just because a plant “looks too big for its pot,” is NOT a good reason to repot.

When is it okay to repot? The short answer is: Infrequently. The negative answer is: Not right after you buy a new plant. The more complete answer is: Repot only when your plant is so rootbound that there is no longer enough soil to absorb and retain water for at least two days. At this point, repotting your plant is an option, but not a necessity. If you don’t mind watering your plant every day or two and fertilizing every week or two, then you can keep your rootbound plant in the same pot without any harm to your plant.

However, you may find daily watering more than you care to put up with. Then it is time to repot. Remove the plant from its pot. If it is stuck, bang the sides and bottom of the pot good and hard or slide a long knife along the inside of the pot. You should see lots of healthy roots wrapped all around the outside of the rootball. In fact, it may be hard to see any soil except at the top. If you see more soil than roots, then put it back in its pot, give it a good soak and don’t repot it.

Finding a Good Potting Soil

Unfortunately, there are no quality or ingredient standards for potting soil. Every nursery seems to have their own blend that they swear by. Bagged potting soils sold to consumers are just as variable. It is never a good idea to use garden soil from outside. So what can you do? The first thing to do is learn about soil structure so you will understand what plant roots need from the soil.

Potting Soil Structure and Composition

Soil structure refers to the size and percentage of air pockets in the potting soil. Most plants prefer a porous soil structure – one that is made up of coarse particles and has many, good-sized air pockets throughout. A dense soil structure has very fine closely packed soil particles that have very tiny air spaces. Porous soil is more efficient at providing oxygen to plant roots; dense soil is better able to retain water for roots. Which is better? Most plants require a soil that can both retain water and also provide air spaces. For plants such as ferns, water retention is more important. For cacti and succulents, porosity is more essential. That is why there are different soil mixes made for different plant groups.

A porous soil must have large particle material to create air spaces in the soil. Perlite, vermiculite, sand, lava rock, and bark chips are the most commonly used particle materials used in potting soil. Of these, perlite may work the best because it creates air spaces between particles and is also made up of tiny air spaces. Perlite is also very hard and does not decompose or collapse readily. However, its fluoride content does not make it a good choice for fluoride sensitive plants such as peace lilies, dracaenas, and spider plants.

To make water available to the roots, potting soil needs material that will absorb and hold water. Soil and peat moss perform this function. Peat moss is particularly absorbent, is lightweight, and also retains some air pockets even when wet. Peat moss is also an excellent visual aid to proper watering: When it is wet, peat moss is dark brown and as it dries it becomes a noticeably lighter shade of brown.

Finally, a good potting soil should provide the nutrients that plants need. Potting or garden soil is composed of decayed organic matter (compost) that has decomposed into fine particles. This organic material is rich in all the nutrients that most plants require. This is the stuff that plants grow in naturally where there are no man-made fertilizer supplements. Every time a potted plant is watered, minute amounts of the soil nutrients are released and made available to plant roots. Good potting soil will have nutrients in sufficient quantity to last for several years. That is why the addition of plant food or fertilizer is unnecessary for plants in fresh soil.

To sum up: Good potting soil should have air spaces to hold oxygen, material to retain water, and nutrients to supply the plant. It should be a mix of porous material, peat moss, and decayed organic matter (soil).

Packaged Potting Soil

There are no uniform standards for packaged potting soil. The labels on these bags make wonderful claims, but almost never tell you anything about the actual composition of the soil inside. Some have just garden soil; some add a little peat or bark chips or perlite or sand; some have added fertilizer; others add moisture-retaining chemicals; some are wet and other are dry. None of them are labeled to give you a complete list of all the additives and their respective quantities. A few years ago, an independent researcher did a lab analysis of a dozen popular brands of packaged potting soils. He found that the only thing that most of them had in common was excess soil salts (too much fertilizer). They have you at their mercy – buyer beware!

When you buy packaged potting soil make sure that it states on the label that it is sterilized. That means that it is weed and insect free. It does not mean that it has no nutrients in it. If it isn’t labeled sterilized, don’t get it.

Packaged Soilless Potting Mixes

There are many peat-based potting mixes available that do not contain any soil. They are composed of mostly peat moss with vermiculite and perlite added for porosity. Most will also have some fertilizer added to provide nutrients. Remember, it is the soil that provides most of the essential nutrients and these mixes do not contain soil, so fertilizer must be added. These soilless mixes are usually more expensive than potting soils, but they have many advantages. First, they are sterile. Second, they are light in weight. Third, they are very clean to handle. Fourth, they are ready to use straight from the bag. If you use these soilless mixes, remember that they should be fertilized with a complete plant food regularly. Also, these mixes should not be used as is for cacti and succulents because they retain too much water for too long.

Making your own Basic Potting Soil

This is not hard to do. In fact, it is kind of fun. The recipe for a standard potting mix is simple: 2 parts peat moss; 1 part sterilized potting soil; 1 part perlite. Mix these ingredients together and you will have a clean, porous, moisture retaining, nutrient filled potting soil. By diluting the packaged potting soil with the peat moss and perlite, you will also reduce the excess soil salts that I mentioned above. Including garden soil in your potting mix is risky because it is too heavy and probably contains bacterial and fungal pathogens that can infect plant roots.

Cacti and Succulent Potting Mix

Succulents have fine root systems that have evolved in nature to absorb moisture quickly and efficiently move that moisture to the plant for storage. This allows them to withstand drought conditions in their native habitats. Their roots are not adapted to stay in constantly moist soil. Thus, they must be potted in a very porous mix that will not retain water for very long. A good succulent mix will have lots of sand or perlite in proportion to soil or peat moss. You should notice that the water runs through this mix easily and quickly. There are packaged cactus potting mixes available or you can mix your own.

Fern Potting Mix

Ferns are the opposite of succulents. Ferns grow in areas that are constantly damp for most of the year. So they need a soil mix that has more moisture retaining matter and less porous material. This mix should have little or no perlite in it. The perlite can be replaced with vermiculite. Many people like to add some leaf mold to this mix, as well.

Epiphytic or Orchid Potting Mix

Epiphytes or air plants (orchids, bromeliads, anthuriums) don't grow in the ground like most houseplants. In nature their roots attach themselves to tree branches. Why is this important? Because many epiphytes cannot be grown in regular potting soil or their roots will rot. They should be grown in a mix of very porous, lumpy material such as sphagnum moss, bark chips, lava rock or vermiculite & perlite. These mixes do not retain water very long so the epiphytic roots will not rot. Epiphytic potting mixes are often available in packages labeled as Orchid Mix.

Conclusion

Unnecessary repotting is one of the most common plant care mistakes made. Plants flourish when their roots are tight and struggle when surrounded with excess soil. Never repot unless absolutely necessary and never repot an ailing or recently acquired plant. On those infrequent occasions that repotting is necessary, move your plant into the next sized pot and use only a good quality, sterilized potting mix. That is the root of the matter.

Copyright: William E. Creed, 2000

Indoor Plant Bulletin Index

Bulletin #1 - Selecting A Quality Plant

Bulletin #2 - What to Do With Your New Plant

Bulletin #3 - Light for Your Plants

Bulletin #4 - How to Water Your Plants

Bulletin #5 - Potting Soil Basics

Bulletin #6 - Caring for Holiday Plants

Bulletin #7 - Plant Pests

Bulletin #8 - Fertilizing Indoor Plants

Bulletin #9 - Low Light Plants

Bulletin #10 - Moving Plants Outside

Bulletin #11 - Plant Propagation

Bulletin #12 - Ficus Trees

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William Creed

48-16 30th Avenue, 4E

Astoria, NY 11103

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