Soil in Containers Should Be a Good Mix
Garden soil doesn’t offer enough air, water, or nutrients to container-grown plants. Fortunately, it’s easy to amend.
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I work hard to ensure that the soil in my garden is the best I can give my plants, and they reward me with robust health. Yet that same good soil if transferred to a container would cause the plants in it to languish. That’s because garden soil doesn’t offer enough air, water, or nutrients to a plant growing in a container. Potting soils are specifically formulated to overcome these limitations.
Potting soil needs to drain well but still hold moisture
One of the most important things a potting soil needs to do is provide roots access to air by letting water drain away from them. In the ground, the soil is usually deep enough to let excess water drain beyond root zones. In pots, however, water tends to accumulate at the bottom, despite drainage holes. The smaller the pore spaces of the soil in the pot, the higher that water layer will reach. Larger pores, formed by adding mineral aggregates to potting soils, readily admit water into the soil, then carry it through the medium and out the bottom. Then, all those large, empty spaces can fill with air.
Perlite, vermiculite, calcined clay (kitty litter), and sand are the mineral aggregates most commonly used in potting soils. Perlite and vermiculite are lightweight volcanic rocks naturally filled with air. I prefer perlite over the others because it does not decompose with time nor lose its aerating ability if the potting mix is compressed. Vermiculite is a valuable additive because it prevents some nutrients from leaching away, and it even provides a bit of potassium and magnesium.
A potting mix also must have ingredients that help it retain moisture. This is where organic materials—usually peat moss, sphagnum moss, or coir—come in. They cling to some of the water that the aggregates are helping to drain. Organic materials also hold on to nutrients that might otherwise wash away.
In addition to peat moss, vermiculite, and perlite, commercial mixes often contain sawdust or various grades of shredded bark. Lime may be added to help balance the acidity of the peat moss, and a small dose of fertilizer can often make up for the lack of nutrients.
Adding compost or garden soil can be beneficial
Most gardeners make potting soil by combining perlite or vemiculite with peat or sphagnum moss. Two other organic materials that you could add to your potting mix are leaf mold and compost, which offer a wide spectrum of nutrients.
Adding some garden soil to a homemade potting mix contributes bulk while buffering against pH changes and nutrient deficiencies. The reason that garden soil is rarely added to commercial mixes is because of the difficulty in obtaining a steady supply that is consistent in quality and free of toxins such as herbicide residues.
Customize your mix to suit your plants
Soilless potting mixes are relatively free of living organisms, but mixes made with soil or compost are not. Some gardeners talk about “sterilizing” their potting mixes by baking them in the oven to rid the soil of harmful organisms, limiting the hazards of damping-off and other diseases. What I hope they mean is that they “pasteurize” their mixes. Heating homemade potting mixes to sterilizing temperatures wipes out all living things, beneficial and detrimental, leaving a clean slate for possible invasion of pathogens and causing nutritional problems such as ammonia toxicity. Pasteurization, which occurs at lower temperatures, kills only a fraction of the organisms. The best way to pasteurize your soil is to put it in a baking pan with a potato embedded in the soil. Bake it at 350°F for about 45 minutes. When the potato is cooked, the potting mix is ready.
I don’t pasteurize my potting mix. I rely, instead, on healthy container-gardening practices such as timely watering, good air circulation, and adequate light to avoid disease problems. Beneficial microorganisms in compost and garden soil also help fend off pests.
Lee’s recipe for homemade potting soil
I’ve found that making my own potting soil produces better results than commercial mixes and eliminates the need to monitor my containers’ nutrient and pH levels. With plenty of good soil in my backyard, I have no trouble making this traditional potting medium. It features a mixed bag of ingredients, but I figure that plants, like humans, benefit from a varied diet. This mix can support plants for a year or two without additional fertilization.
Mix 2 gallons each of:
* peat moss
* garden soil
with 1/2 cup each of:
* dolomitic limestone
* soybean meal
* rock phosphate
* kelp powder
I place a 1/2-inch mesh screen over my garden cart and sift the peat moss, compost, and garden soil to remove any large particles. I then add the remaining ingredients and turn the materials over repeatedly with a shovel, adding water if the mix seems dry. After a few incantations, the stuff is ready to work its magic on everything from my tomato seedlings to my weeping fig.
Make your own soilless mix
Years ago, Cornell University scientists came up with a formula for a soilless potting mix, which forms the basis for many commercial potting mixes on the market today. By following this recipe, you can easily replicate what is sold in bags at the garden center.
* 1 bushel peat moss
* 1 bushel perlite or vermiculite
* 1/2 pound dolomitic limestone
* 1 pound 5-10-5 fertilizer
* 1 1/2 ounces 20% superphosphate fertilizer
Mix the ingredients thoroughly. The mix is initially hard to wet, so moisten it as you stir it. This saves the trouble of doing so each time you remove some for use.
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Garden soil doesn't offer enough air, water, or nutrients to container-grown plants. Fortunately, it's easy to amend.
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Going to the garden center to purchase potting mix can be a little overwhelming. With many types of products to choose from, it can be difficult to know which one will be best for the plants you intend to grow. Some are meant to be added to the garden or used to fill raised beds, while others are suitable for growing in containers or pots. Garden soils are typically intended for use in the ground and contain minerals and organic matter. They are not a good choice for containers because the soil can quickly become compacted and waterlogged, reducing air space around the roots. This can lead to poor or stunted growth. Garden soils can also contain weed seeds, insects and diseases if they haven’t been pasteurized.
Potting mixes (also called soilless mixes), on the other hand, are specifically made for growing potted plants. They are lightweight, retain moisture, and they supply plenty of air space around the roots. Air space is actually one of the most critical aspects of potting mix. If the roots don’t have enough air, a plant usually doesn’t survive. Although the ingredients tend to vary, good mixes always contain an organic component (peat moss, compost, bark), vermiculite or perlite (to help retain moisture), sand, nutrients and limestone. Some contain fertilizer or moisture-retaining treatments, usually indicated on the label. Knowing what is in the potting mix is key to determining whether it will be a good match for the plants you are trying to grow. General potting mixes will work fine for most annuals and vegetables grown in containers, but they may hold too much moisture for orchids, succulents or cacti. Specialty mixes are sold for these plants and, while not absolutely necessary, can provide benefits.
Potting Mix Ingredients
Peat is a special type of organic material that comes from decomposed plants in bogs. Most peat comes from sphagnum moss, hence it’s other common name, “peat moss.” Peat is a major component of almost all potting mixes because it retains moisture without becoming waterlogged, is lightweight, and does not become easily compressed.
Compost is occasionally included in potting mix for added nutrients. It can reduce air space in the soil and should be used sparingly for potted plants. Compost should make up no more than 1/3 of a potting mix.
Bark that has been ground and partially composted is often incorporated into less expensive potting mixes in place of peat. Bark provides good aeration but dries out more quickly than peat, requiring more frequent watering.
Coir is a fibrous material from coconut husks that is sometimes used in place of peat. It is similar to peat in that it retains water without becoming soggy.
Vermiculite is the product of heating mica chips. It is a gray, spongy material that increases water retention in mixes. It also holds on to nutrients and thus keeps fertilizer available for the plant roots for a longer period of time.
Perlite is a white volcanic rock that is reminiscent of Styrofoam. It is light weight and porous and is used to improve the drainage and aeration of potting mix.
Sand is another common component of potting mixes. It can improve drainage and is often added in large quantities to mixes intended for cacti and succulents.
Fertilizer is sometimes added to potting mixes, usually in a slow-release form that breaks down gradually over time when it comes in contact with water. Thus, small amounts of nutrients are released over the course of weeks. Eventually this initial source of nutrients will be exhausted, and potted plants will require additional fertilizer.
Moisture retaining treatments come with some potting mixes and are meant to reduce how often you need to water. These “hydrogels” or “water storing crystals” are polymers that have the ability to absorb large amounts of moisture and slowly release it as the soil dries. Their effectiveness diminishes over time, and eventually the potting mix dries out as any other. Potting mixes with moisture retaining treatments are suitable for potted annuals but are a poor choice for succulents or other drought tolerant plants.
The best potting mix for potted plants may vary slightly depending on what you are trying to grow. However, all quality mixes will be lightweight, fluffy and dry, and contain peat, coir, bark, perlite, or vermiculite. Avoid products that are compost-based or seem overly heavy – these won’t provide enough air space for roots. When in doubt, choose a peat-based general purpose mix, or make your own potting mix by combining the ingredients above. Many potting mix recipes exist online, and you can adjust the ratios of the added materials according to the needs of the plants you’re growing.
Going to the garden center to purchase potting mix can be a little overwhelming. With many types of products to choose from, it can be difficult to know which one will be best for the plants you intend to grow. Some are meant to be added to the garden or used to fill raised beds, while others are suitable for growing in containers or pots. Garden soils are typically intended