Today compost can be hard to define, we use the term for the decomposed organic matter we use as a soil conditioner in our garden. We use it to describe a variety of growing media that we use when growing from seed or growing plants in pots and containers.

So how do you choose the right compost?

On the name of the product? The picture on the bag? The name of the manufacturer? The price? Unfortunately the latter is often the deciding factor. In today’s retail world we are conditioned into choosing the multi-buy deal and as a result compost has become a commodity. Costs of production, transport and packaging have increased. So how can the price be maintained – maybe by reducing the content of the bag both in terms of price and quality?

So what’s the big deal about peat?

Peat is a natural resource and supplies are limited. Peat does regenerate but it takes time and regeneration is slower than consumption so peat is a finite resource. Alternatives have been trialled for many years and considerable progress has been made. Horticulture is not the only use for peat and one could argue that is one of the more valuable uses but that is not within the scope of this article.

Why is peat so good?

Peat is a wonderful growing medium that is naturally sterile so needs no heat treatment to destroy weed seeds and harmful organisms before use. Peat is very efficient at holding water and dissolved nutrients and maintains lots of air space in a growing medium. A peatbased compost therefore maintains its water-holding qualities through the depth of the compost. It is clean to handle, light and easy to grow in.

What’s wrong with the alternatives?

In many cases the best alternatives: recycled green waste, wood fibres, paper waste and coir all work but they are trickier in terms of water management and do not always give such consistent results as peat based composts. They are mostly satisfactory for mature plants but can be unpredictable when raising plants from cuttings and seeds.

Is there a compromise?

Yes, reduced peat composts. These use a proportion of peat alongside peat alternatives with the addition of slow release nutrients. The type and quality of the peat alternatives affects the performance and management of the compost but considerable progress has been made due to the need to reduce peat consumption.

What about loam-based composts?

Loam-based composts: John Innes Composts are the ones we are familiar with, consist of fine particles of mineral soil. These are very good at hanging on to water and nutrients. For some they can be too good at this, so care needs to be taken not to overwater after repotting or when growing from seed.

So what is the ideal compost?

It does depend what you are growing. For general potting of mature plants a mixture of John Innes no 3 with Multi-Purpose compost with added John Innes is ideal. One bag of J.I. 3 mixed with one large bag of Multi-Purpose with added J.I. Proportions are not crucial as both are complete composts. This compost does contain a proportion of peat, but only a fraction of a peat-based compost. It is not suitable for lime-hating plants and is no good for seed sowing because it contains too high a level of nutrients.

So what about feeding plants in pots?

Most multi-purpose composts contain enough nutrients for the first six weeks from the time of potting. After this you need to feed with a liquid fertiliser or you need to add a controlled release fertiliser such as Osmacote. For ericaceous plants use Vitax Conifer & Shrub Fertiliser. Cheaper composts use cheaper base fertilisers therefore the nutrients may be exhausted more quickly. They often also use cheaper peat and cheaper peat alternatives which can give inferior results.

What about seed sowing?

Multi-purpose composts may not give good results for sowing seeds, especially if they have a high proportion of green waste or wood fibre. Both these ingredients are usually still decomposing so they take some nitrogen from the compost. They can also introduce fungal mycelia which compete with delicate young seedlings. Compost that is very low in nutrients is required for best germination; too many nutrients damage young roots and can inhibit germination. So always use specific seed compost for seed sowing. If you are going to use an all-peat compost this is when to use it.

So which specific composts should I buy?

For seed sowing choose Westland Surestart. This is a beautiful, finely milled, good quality compost available in small quantities. It is easy to use and gives great results.

For young plants, in other words starter plants, plug plants, rooted cuttings and pricking out seedlings use John Innes No. 1 compost. This has a lower nutrient level than John Innes No. 3 and is good at retaining moisture. Alternatively use Westland Multi-Purpose compost. These young plants will not be in it for long, so the nutrients are adequate.

For permanent planting in pots choose J.Arthur Bowers Multi-Purpose with added John Innes and add more John Innes No.3. Mix a bag of each together in a wheelbarrow. This is also an ideal mixture for summer bedding plants, and the same compost can be used afterwards for the autumn planting of spring-flowering bulbs.

Can I add one of those water retaining gels?

The granules of water retaining gel are ideal in containers planted with summer bedding plants if you want to reduce the frequency of watering. They should not be used in permanently planted containers or in pots planted for winter interest as water holding in winter can increase the chances of winter damage to plants and containers.

What about ericaceous plants?

For rhododendrons, azaleas, camellias etc. Use John Innes Ericaceous compost. These plants are sensitive to lack of water and this loam-based formula will give the best results. Peat-based ericaceous compost is dependent on a wetting agent that helps dry peat compost to absorb water. After a few years the wetting agent breaks down and it becomes difficult to get water into a dried out pot of compost and the plant suffers.

So what is peat based ericaceous compost used for?

The Westland peat-based ericaceous compost is excellent as a mulch around newly planted and established ericaceous plants in the open ground. It is very beneficial as a top dressing around heathers when they have been trimmed after flowering. It is also ideal for winter containers planted with heathers, leucothoe and gaultheria.

What about compost for use when planting new trees and shrubs?

We produced our own Hillier Shrub and Tree planting compost specifically for this purpose. This is a reduced peat compost with slow release fertiliser, high in phosphate for root development, but balanced for good growth. It carries full planting instructions on the bag to make sure that you get the best results from your newly planted trees, shrubs, roses, climbers and perennials.

What is the best compost when preparing a new bed or border, or the vegetable plot?

In this case we are talking soil conditioning, in other words adding organic matter rather than nutrients. The latter will be added separately by adding a base fertiliser: we recommend Vitax Q4. As a conditioner Harrington and Jessop’s composted stable manure is ideal. This is very good at increasing the humus content of soil – this is the sticky water holding part of the soil between the soil particles. This manure adds some nitrogen and other nutrients but you will still need to add a slowrelease base fertiliser.

What should I use when growing fruit and vegetables in pots and containers?

If you plant a fruit tree or a soft-fruit bush in a pot use John Innes No.3 unless you are growing blueberries in which case use John Innes ericaceous compost. For vegetables try Westland’s Organic Vegetable Compost. This is ideal for short-term crops such as salad crops. If growing in pots or planters add Fish, Blood and Bone fertiliser after four weeks.