Which Potting Compost Is Best For Indoor Plants?

Discover what potting mixtures contain and take the guesswork out of choosing the right one to keep your house plants healthy.

At garden centres and nurseries there is often a bewildering array of bags of potting mixtures pottting-compostavailable to choose from. What do they do for your plants and how do you know which to choose to suit the needs of your plants?

Basically a potting mixture should supply your house plants with all the nutrients and water that their roots would have found if they were growing naturally in the ground. In your home their roots are in confined spaces in containers, so the nutrients and water have to be provided by you in the the potting mixture.

It is very important that the mixture is of the right texture for the plant’s roots to grow strongly; that it holds moisture, yet is free-draining; contains enough nutrients and is changed, or partially changed, at regular intervals. There are two main types of potting mixture. One has a loam or soil base and the other is peat-based.

Never be tempted to use garden soil for house plants. It would need to be sterilized to kill pests and diseases before you could use it. Home sterilizing is messy and expensive. Particular ingredients can be added to ready made mixtures depending on the needs of the plants.

John Innes potting mixtures

John-Innes-Compost

These are the best known soil-based mixtures that are commercially available. They consist of 7 parts of loam or soil, 3 parts of peat and 2 parts of coarse sand. The main ingredients are partially sterilized and a slow-release granular fertilizer is added. There are 3 main types, John Innes No. 1, 2 and 3. They each contain different amounts of fertilizer. Look for the John Innes Manufacturers’ Association Seal of Approval when buying.

Soil-based mixtures

  • Soil-based compost is rich in plant food, and is good for greedy plants which need a regular supply. Some typical uses are as follows:
  • It is much heavier than peat-based mixtures and helps to keep large’plants (e.g. strong-growing climbers) and plants with weighty leaves (such as Mother-in-law’s Tongue) from toppling.
  • Soil-based compost is also useful for bulbs such as amaryllis, nerines and vallota, and for some of the larger leafy palms which like to be left undisturbed.
  • In texture this compost closely resembles the kind of soil in which many cacti and succulents grow in the wild. With peat or grit added to it, it suits these plants very well.

There are a number of ready-mixed soil-based potting mixtures available including those in the John Innes range. All of them provide a rich growing medium for the plant and all release their nutrients to the plant over a long period of time. The nutrients are available for several months. After about 3 months you will need to supply nutrients when you water the plant. The amount you add, of course, depends on the requirements of the plant and the time of the year—whether it is a period of active growth or not.

Holding nutrients and water

The main ingredient of the mixture–the loam or soil—is the bulk which helps to hold the nutrients. The peat in the mixture is spongy and holds water, while the sand keeps the mixture open and airy. It also makes it free-draining.

The disadvantage of a soil-based mixture is that because of the nature of the main ingredient, loam or soil, the quality of the mixture may be variable. It can also be heavy to use and it is possible to overwater plants in a soil-based mixture. Soil-based mixtures are messy and generally not as easy or pleasant to handle as peat-based ones.

Peat-based mixtures

  • Peat-based compost is light and moisture-retentive, and is ideal for plants which like moist, airy conditions:
  • Plants such as African Violets, begonias, Christmas and Claw Cactus, which have fine, hairy roots are at home in this compost. The tender roots run easily through it and pick up water and nutrients.
  • Ferns and bromeliads, which are by nature forest plants, thrive in this type of compost, as its texture resembles that of their native woodland compost.
  • Peat-based compost is also vital for the healthy growth of tropical or jungle plants, which need plenty of air as well as lots of moisture, as it can be kept constantly damp.

These composts have recently been developed because of the difficulty of obtaining suitable loam at a reasonable price. In this range you can find composts suitable for sowing, potting and general purposes.

The main ingredients are peat and sand with added nutrients. The peat and sand are sterile. They are simply used as the growing medium in which the roots establish themselves. You will have to add all the nutrients the plant needs on a regular basis. The nutrients will be used up quickly by the plant and some will be flushed out of the container and become unavailable to the plant once you start watering.

Many nurserymen and commercial growers favour peat-based mixtures. The plants you buy are likely to have this mixture in their pots.

Hard to moisten

One disadvantage of a peat-based mixture is that it is very light in weight and top-heavy plants may topple over, especially if the mixture dries out. Peat on its own is also difficult to re-moisten once it has dried out. Many ready-mixed brands now include wetting agents to avoid this problem. To re-moisten compost, stand the pot in water to its rim until moisture reaches the top of the compost.

Additives

Some plants require the basic soil- or peat-based mixture but need something added to make it just right for their special requirements. There are many additives which can change the character of the mixture.

  • Perlite is expanded volcanic rock. It is very light in weight and improves aeration, drainage and water-holding in heavier mixtures. Use with peat to root cuttings.
  • Vermiculite is expanded mica. It is light in weight, improves water-holding and holds nutrients well. It is good for lightening heavier mixes and useful in rooting mixtures.
  • Grit or coarse sand is added to mixtures to make them more open and free-draining.
  • Sphagnum moss is added to mixtures for plants that need open but moist conditions.
  • Other materials used to lighten or make soils more open are polystyrene and bark chippings. Rockwool, a coarse fibrous material, holds water well but doesn’t become waterlogged.

Buying and storing composts

These mixtures can be bought weed, pest and disease-free in various quantities from garden centres nurseries, as well as from DIY and supermarket chains. Don’t buy bags that are split – by the time you get them home some nutrients may have been lost. Always buy the freshest compost available.

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