How to choose the right compost for the right spot
As spring approaches, we’ll be sowing, planting, mulching and digging – but which compost do we need for each job?
To keep things simple, be aware that there are basically two types of compost: Soilless and soil-based.
Soilless: These types are ideal for quick-growing crops or annuals in pots, where the plants are only going to be in the pot or container for a year. These composts are mainly made up of organic matter such as bark, coir, green waste, paper, leafmould and sawdust, with added nutrients and water-retaining agents.
Soil-based: These are better for planting trees or growing permanent specimens in containers, such as shrubs or perennials. They are sold under the John Innes formula and are made from a mixture of loam, sand or grit and peat, with plant foods added.
Each John Innes number contains progressively more feed. John Innes No 1 is good for pricking out or potting up young seedlings; No 2 has more nutrients and is used when potting up small plants and vegetable plants in containers; No. 3 is ideal for mature plants, permanent plantings in pots and when planting trees and shrubs.
Here’s what you need to know about the different styles of compost available…
This is the most versatile compost, which can be used to dig in and enrich beds and borders, as well as plant up containers. There are many which contain blends of ingredients, including plant foods which can feed your plants throughout the season, water-retaining properties and added John Innes.
Many gardeners are opposed to composts containing peat because they continue to deplete British peat bogs, although historically, peat has been used in compost because it’s good at holding water and retaining nutrients.
Some gardeners argue that plants grow better in peat-based products, but others insist that if you adapt to the properties of peat-free, you should be fine.
Peat-free composts generally retain water better, which is great in hot summers, but can lead to rotting plants in wet winters. So add grit to your peat-free compost before planting to enhance drainage and water your plants little and often during the summer, rather than completely soaking them once a day.
Check the wording on the bag – if it doesn’t say ‘peat-free’ then it generally isn’t. Marketeers may use wording such as ‘environmentally friendly’ and ‘organic’, but that doesn’t necessarily mean it’s peat-free.
Acid-loving plants including camellia, azalea, rhododendron and heather are best planted using ericaceous compost. If you are planting them in a flower bed that naturally has alkaline soil, they are likely to suffer, as sooner or later the ericaceous compost will lose its efficacy and the original soil make-up will seep through.
It’s often better to choose plants that will like your existing soil, rather than trying to adapt unsuitable plants using specific composts. If you love azaleas but don’t have acid soil, consider planting them in pots using ericaceous compost.
Seed and potting compost
Seed compost has the lowest amount of nutrients, which encourages the best germination and growth of tiny roots. A seed compost is much more in tune with the needs of a developing seed than a general multi-purpose compost.
Low nutrient levels don’t affect the plant growth because individual seeds already contain a store of food to feed the developing plants.
Spent mushroom compost
This is generally cheaper than other composts and is often used as a soil conditioner or to mulch a bed. It is called ‘spent’ because it is compost left over from mushroom farming.
It has a high lime content so is ideal for the vegetable garden as veg crops, including brassicas, grow best when the soil is not acid. However, don’t place it near acid-loving plants like azaleas and rhododendron.
The RHS advises gardeners to use it in moderation, alternating with well-rotted manure or garden compost, which should help balance the alkaline nature of the mushroom compost. It’s also not suitable for fruit crops, which need a more neutral or acid soil.
Animal manure is a fantastic soil conditioner, but it must be well rotted before adding to the soil, or the concentration of nitrogen will scorch young plants.
If you are offered fresh manure, you’ll need to create a space to rot it down for at least six months before spreading it across the soil in spring, a few weeks before planting. Break up any lumps by raking it and mix in some topsoil.
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