What should potting soil contain for different types of plants?
How to choose potting soil mix ingredients for various types of plants
Growers of both ornamental and edible plants often wonder whether commercially available potting soil mixes are suitable for growing specific types of plants. This is a valid question, as different types of potting media contain different materials in different ratios, and are therefore suitable for different plant types.
It’s useful to know what purpose the individual ingredients in a potting soil mix serve and how to assess whether the potting mix is suitable for growing a particular plant. In this article we’ll examine the ingredients that are most commonly found in potting soil mixtures, their advantages and disadvantages, and which types are suitable for different types of plants.
The most commonly used ingredients in potting soil mixes
Peat: Peat has long been one of the main ingredients in potting mixes due to its exceptional properties. It is a purely natural material extracted from peat bogs, where it has been naturally formed over hundreds of years by layering and subsequent decomposition of plants (moss, herbs, grasses) without access to air. The advantage of peat is its excellent permeability and its ability to retain moisture. However, its low nutritional content is a disadvantage, so it’s often combined with compost or fertilizers to increase its nutritional value. Another advantage of peat is its low pH, as some plant species (blueberry, rhododendron, maple, heather…) thrive in acidic soil. This is why we see specialized substrates for acid-loving plants in garden centers, the main component of which is peat. Peat is particularly recognizable due to its fibrous structure and its lighter brown color.
Compost: Compost is one of the most common ingredients in potting soil mixes because it’s a nutrient-rich material in a usable form for plants. Compost is decomposed natural organic material, which is highly permeable and provides easily-accessible nutrients for plants. However, because compost can be prepared by decomposing different materials (wood, sawdust, organic waste materials such as straw, coffee waste or vegetable kitchen waste), so the nutrients it contains can vary in composition and concentration. The best quality compost can be made by home growers by composting plant residue and kitchen waste. Although compost is full of nutrients, it doesn’t have high moisture retention, so it is advisable to combine this material with other ingredients. You can recognize compost in potting soil by its dark brown color.
Perlite: Perlite is a volcanic rock with a porous structure, which makes the material airy and lightweight. Therefore, perlite is often added in small quantities to potting mixes to increase drainage capacity. Perlite does not retain water. On the contrary, it promotes the drainage of excess water out of the medium. Perlite can be seen in the form of small white balls in the potting soil. This material can be seen in larger quantities in succulent soil mixes and in potting media for arid-loving plants.
Sand: Sand also increases permeability and improves the drainage of excess water from the soil, but it is less often used in potting soil because of its high weight. An exception is turf substrate, which normally contains sand in a higher ratio. Sand is also popularly used by growers to increase the drainage capacity of large areas (e.g. heavy, clay soils or to improve drainage prior to turf laying) because of its low cost. Sand does not contain any nutrients, so this material needs to be combined with compost or fertilizer.
Tree bark: Although bark is more commonly used as a mulch, it can be found in crushed form in soil mixes because of its water-retaining properties. Bark is like a sponge that holds onto water, and is also a natural, organic material that releases nutrients into the soil as it decomposes. Pine bark can also be found in bigger pieces in potting mixes that are designed for acid-loving plants with high water requirements, such as rhododendrons. In the case of epiphytic plants such as orchids, bark may be the only component of their potting medium.
Vermiculite: This natural mineral is used to aerate the soil, and it also has the ability to retain water and release it into the soil, keeping it moist longer. Because of its water retention properties, it’s also used as one of the main components for soilless potting mixes. It’s found in potting media used for cacti, succulents and rock-dwellers, and for green roofs. Vermiculite has similar properties to volcanic tuff, zeolite and crushed bricks.
Garden soil: This is basically a mixture of different minerals (clay, sand, limestone..), organic matter and dust, so its composition and quality can be variable. However, soil in smaller quantities can be a useful part of a potting mix, especially if it is crumbly, permeable and rich in organic matter.
Coconut fiber: Coconut fiber or coconut pulp is made by grinding up the shells of coconuts. Like tree bark, coconut fiber is used to increase the water-holding capacity of a potting mix. This natural material also releases nutrients into the soil as it decomposes.
Humus/Vermicompost: Earthworms consume organic material and convert it into this natural, nutrient-rich fertilizer. As it is a more expensive material, it’s less common in commercially available potting mixes. However, vermicompost is a popular additive to organic potting soil mixes for home-growers who want to grow without synthetic fertilizers.
PH of potting soil mixes
Most potting soils have a slightly acidic to neutral composition (pH 5.5-7), which suits most plants, but there are species such as blueberries, rhododendrons, heathers, cranberries and other species that thrive in extremely acidic soils (pH 4-5) and therefore require a potting soil that’s specifically formulated for acid-loving plants. Conifers, maples and other ornamental or edible plants also prefer more acidic soils. Such specialized acidic potting mixes contain mainly peat, which has a low pH. More alkaline soils, on the other hand, are well tolerated by rock-dwelling plants, lavender, leafy vegetables and legumes. The pH of potting soil can also be influenced by other additives, such as crushed limestone or wood ash, which raise the pH, or sulphur or fertilizers, which lower it. In this way, producers adjust the pH of the potting mix to the desired level. Home growers can also use additives to adjust their commercial mix to suit their plants’ needs.
1. Find out How to test soil pH and adjust soil acidity
2. Find more about Using wood ash as a fertilizer