When people ask me about plants, they frequently ask about the best soil or compost to grow it in, and it's made me realise that growing mediums, or soil, can be very confusing to most of us.
On a recent visit to my local hardware store, the vast range of composts they were offering made me realise that this could be daunting for new gardeners. Working out which type your plant baby needs involves lifting and turning each bag over to read the instructions, annoying other shoppers and perhaps a little embarrassing.
So I've put together a quick overview to help to make the picking the right compost a little easier.
Sold by almost everyone, this growing medium is a mixture of ingredients to suit as wide a range of plants as possible. Every brand has their own mix, but the primary ingredient is usually peat, due to its ability to absorb and release water and feed as required by plants. Some have also been enhanced with additional nutrients, mixed throughout in the form of small yellow, white or blue balls. These are slow release granules which release feed slowly over six months.
There is a lot of debate over peat. To explain briefly, when peat is extracted from bogs, that bog will dry out and effectively die. When they do this, they can release vast quantities of carbon into the atmosphere.
For a brief introduction to the arguments for and against, I'd recommend reading this interview with Dr Alan Knight from DEFRA.
Next year (2020), the target for suppliers is that all compost sold to the public will be peat free. Therefore, more and more ranges are coming onto the market. These are based on recycled green waste as well as wood chip, fibre, coir (fibre from coconut husks) and wool. So far this has had mixed results, as poorly composted wood and fibre holds onto nutrients and plants fail to thrive and eventually die.
Unfortunately, even top brands have struggled to produce reliable mixes over consecutive years. The industry is working hard to improve their products before 2030, the year when peat will also no longer be available to commercial growers. For the last 20 years, many growers across the industry have been using a peat-free mix from Melcourt called Sylva which has recently become available to the public at individual garden centres.
Developed in the 1930s by The John Innes Horticultural Institute these are recipes for the best peat-based composts for different growing situations. Today, many different manufacturers add the mix to their compost to help control water levels and increase the mineral content.
- Seed compost can also be used for cuttings. It's open structure, and limited nutrient level will not harm the newly emerging roots.
- No.1 is ideal for potting up or pricking out young plants into but has a limited amount of nutrition and they will have to be 'moved on' again either into the ground or larger pots.
- No.2 can be used for houseplants, veg in containers and young plants that need potting on before being planted out.
- No.3 has the highest nutrient content and is ideal for mature plants that will be grown in containers for a considerable time such as fruit trees, shrubs, houseplants and hungry veg such as tomatoes.
With a pH of 5.1 to 6.0, this acidic soil is ideal for plants such as blueberries, rhododendrons, camellias, heathers, hydrangeas, magnolias, and other lime-hating plants. Quite frequently, plant labels won't state a plant needs Ericaceous soil, as retailers are worried that shoppers will be discouraged because of perceived growing difficulties. Look out for the words 'lime free soil.'
Orchids prefer an open growing medium for their roots so most commercially sold composts are made up with a lot of bark and dried plant fibre, ensuring they drain rapidly but don't decompose too quickly. This can be quite expensive, and some growers prefer to mix their own. Oakhill Gardens have written a blog with a helpful recipe.
Bonsai and cactus compost
This compost has been developed to have a low nutrient level and good drainage, helping to prevent root rot. It is also suitable for succulents and alpine plants. A mixture of peat and sand gives it a pH of 6.5 - the slightly acidic conditions that these plants prefer. It is unfortunately not suitable for all. If you've splashed out on an expensive plant, it's worth doing some reading to ensure you pick or make the right blend.
Citrus trees prefer a pH around 6.5 that is not too rich in nutrients and free draining to ensure proper growth and fruit production. This blend has been developed to provide a light soil with enough organic matter to hold on to sufficient moisture.
Rose, Tree and Shrub
This mix has added water retaining elements and nutrients that helps bare-rooted roses, shrubs and trees to establish quickly. Some brands have additional slow release feeds that will last up to four to six months, and they can be used to pot up container plants as well.
Mostly sold in connection with lawn creation and maintenance, this medium is actually the uppermost layer of soil in our gardens, and is a mixture of organic and mineral matter. There are various grades, and for lawns, it's recommended to look for something that has been sieved and sterilised. Topsoil comes from the breaking down of underlying rock through the weathering action of rain and freezing conditions. It is not a quick process and can be costly as there is a limited supply.
Don't worry if you've read this and still feel a bit confused; you're not alone. I can guarantee there will be someone stood in a garden centre (if you're reading this during the daytime) staring at the piles of compost bags on pallets trying to figure out which one they actually want. Don't ever be shy to ask for help from garden centre staff or members of the Candide community!