There are so many horror stories from novice and experienced gardeners about using an incorrect soil mixture for their garden planters and containers. Here is the best advice that we have culled from many sources, especially from those growers that specialize in container plants.
Bagged commercial potting mixes are the very best choice for filling containers. These mixtures contain a variety of ingredients, including composted bark, sawdust, peat moss, perlite, vermiculite, or sometime real soil which is usually pasteurized. Some container mixes also contain fertilizers, and if you wish your planters and pots to be organic, you will want to avoid those. Although perfect loamy soil may be used as an ingredient for your homemade potting mix, most ordinary garden soil is not suitable. It is too heavy and compacts easily in pots and planters and doesn’t drain properly.
Here are some suggestions if you wish to create your own container mixture – 100% compost, 100% soil-less mix, 25% garden soil with 75% compost or soil-less mix and we have also seen 50% soil-less mix and 50% compost. There are a lot of variations; however, garden soil may contain weed seeds or pathogens that cause plant diseases.
Initially, you will need to purchase enough container soil to fill all your planters and urns, but you may cheat a little the next year and just replace half the soil from the previous year and top-up for the next year. This may be false economy and it is very good practice to replace all the soil every year and then you know that you have the best conditions for your new plants. Just add the spent soil to the compost pile or the “old” soil to pot up those plant sale donations.
Your very large planters and pots may be cumbersome to move around, so consider setting them in position before filling them with your potting mix.
Even with the lighter pots, it is often easier to fill them where they will eventually reside. Just move around with your container mix and plants and fill up your planters with those flats of beautiful new flowers, herbs and perennials.
A long lasting container mixture is the perfect medium for long-term plants such as woody plants and perennials. Use one part: peat moss, composted bark, compost, sand and perlite. If your compost is too heavy, just add more perlite to ensure good drainage. You can use extra compost for mature plants; however, it may be too rich for young plants and could damage their delicate roots and stems.
If you are considering Cacti and Succulents for your planters and containers this year, they certainly use less water; this is a soil mixture that may be considered. Start with five parts perlite, 4 parts bagged potting soil, 1 part coarse sand and a pinch of rock dust. Once your plants are in, just top dress with small rive rock, gravel, aquarium stone or a fine grade of gravel to keep the crowns of the plants from rotting. To increase the acidity (succulents thrive in acid soil), add a tablespoon of white vinegar to 5 gallons of water when watering.
To sum up, always moisten potting mixture before filling containers. Pre-moistening is essential, potting mix may be difficult to wet and you may end up with pockets of soil that stays dry. This dry soil will just pull the water right out of the plant roots and damage them. Some gardeners even consider pre-soaking their plants before filling the pots and containers.