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The importance of soil for houseplants

By Melanie Mathieson
Gardening Guru

Just like gardening outdoors, soil plays a vital role in the survival and growth of your houseplant. The potting soil or media in which a plant grows must be of good quality. It should be porous for root aeration and drainage but also capable of water and nutrient retention.
Plant roots must have air, food and water and potting soil must be porous enough to allow drainage of excess water and to admit oxygen (soil aeration) needed by the roots. Garden soil may appear ideal for potting indoor plants but actually causes problems. This soil may be wonderful for outdoor gardening under natural conditions, but after a few months garden soil becomes hard and almost rock-like in a plant pot. Plants in garden soil grow satisfactorily for a month or two, but soon the lower leaves turn yellow and the plants become unthrifty. This problem is the result of poor drainage and the lack of soil aeration due to improper soil structure. Most garden soils become compacted with time and house plants grow poorly in compacted soil.
A proper soil mixture is of utmost importance to a house plant because the roots are restricted by the pot. A good potting soil should have the capacity to retain some air and moisture and yet drain well and hold nutrients. Sand or perlite added to the soil will improve aeration and drainage. Clay or organic matter will help retain water. Organic matter, plus commercial fertilizers, will maintain adequate nutrient levels. Modern “soil-less” potting medias do not contain soil so the addition of fertilizer is required to provide all essential plant nutrients.
When you are buying potting soil make sure you purchase pasteurized or sterilized soil. Look on the bag for either of these terms. Sterilized or pasteurized soil is treated with heat and or steam to kill all of the organisms (bugs and mould and diseases) before the soil is packaged. This soil will cost a little more but it is worth every penny. My number one warning for purchasing potting soil - Do not skimp on your soil or buy the “bargain soil”!
When I bring a new pant home from the store I always repot the plant immediately. I find that many nurseries growing houseplants use a very poor quality potting soil so in order to get the plant off to the best start I repot it right away in a new soil mix I make myself.
The first step is to buy the best potting soil I can find at our local supplier. I buy as large of a bag as I think I’ll need. I also pick up one bag each of both vermiculite and perlite. Sometimes depending on the plant I have purchased I will buy a bag of sand or a bag of peat moss too. Once at home I get my soil mixing container ready. I make sure the container is clean and dry from the last time. I pour four parts of potting soil mix directly from the bag into my mixing container, then I add one part each of vermiculite and perlite. The potting soil may already contain some perlite or vermiculite but I always add more. I then mix the soil mixture very thoroughly. I do this by hand but always wear rubber gloves when handing soil in this way. I also must caution that the best place to do this is outside for both cleanliness and ventilation. Do not breathe in the dust from the potting mix, perlite or vermiculite when you are pouring it. Some of the high quality potting soil mixes often contain slow release fertilizers which are beneficial for your plants but also not good to inhale or get on your hands. If I am planting a plant that needs a drier soil I add some sand or if the pant needs a moister soil I add more peat moss. But for most plants the mixture I gave you will provide you with fail-proof results. I use a plastic container for measuring proportions of the mix and my mixing container has a tight lid in case there is any leftover soil mixture. I just snap on the lid and save it for the next time. You must have an airtight lid so bugs and spores do not get into your soil. Now you are ready to repot your pant. Detailed instructions on this will follow in a later column.
Many gardeners are unaware of what exactly vermiculite and perlite are so here are some descriptions for you.
Vermiculite This is a sterile, light-weight mica product. Vermiculite will hold large quantities of air, water and nutrients needed for plant growth. Its pH is usually in the 6.5 to 7.2 range. Vermiculite adds airspace to soil preventing it from compacting and holds water to retain moisture content of the soil. It has a pale beige metallic colour.
Perlite This is a sterile material produced by heating volcanic rock to approximately 1,800 degrees Fahrenheit. resulting in a very lightweight, porous material that is white in color. Its principal value in soil mixtures is aeration. It does not hold water and nutrients as well as vermiculite. The pH is usually between 7.0 and 7.5.
Remember that a good soil for your plat is a very important consideration if you want a healthy and lush plant over the long term. You want a soil mixture that will provide good drainage and air exchange, and has the ability to hold water and nutrients for my plant to grow. Although more expensive, it is definitely a better value for the success of your houseplants.