In this guide, Dan Ori MHort explains how to make compost in a community garden or at home, and how to choose commercially produced compost. This content was presented as part of a workshop at Seaford Community Garden, and supported by the Ouse Valley Climate Action Fund (OVCA).

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What is compost?

Types of commercially produced compost

What to consider when buying compost

Making your own compost

The 3 bay compost method

Compost tumblers

Compost ‘Darleks’

What is compost?

In the UK, compost refers to garden compost (made in a compost heap or bin) or commercially produced compost (the type we buy from the garden centre or nursery). Commercially produced compost is manufactured from controlled blends of organic materials. Usually, commercially produced compost is produced at much higher temperatures, which means it’s sterilised to remove weeds and pests – but this does kill off some of the beneficial microbes. Home compost is usually produced at lower temperatures, which means it might contain weed seeds and roots, but it’s usually much richer in nutrients.

Commercially produced compost

If you visit your local garden centre, you’ll probably find dozens of different types of compost, at different prices. However, there are really only 2 types of compost you need:
Seed and cutting compost: this is a finely textured compost, and has a low nutrient level. High nutrient levels in compost can inhibit germination of some seeds. It can be difficult to find peat-free versions, so Dan suggests sieving old compost, or even using the sweepings from the potting shed floor. 
Multipurpose compost: this is a general purpose compost and can be used for almost everything else.

What to consider when buying compost

Peat: the extraction of peat for gardening causes huge environmental and climate damage. Despite, this, you can still find peat compost for sale. When buying compost, always choose peat-free, and check the packet which should say that your compost is ‘peat-free’. 

Source: buy your compost from a reputable supplier. 

The minerals plants need

Before we talk about fertilizers (see below), it’s useful to understand why and when we might need them. And this is all about the minerals, or nutrients plants need to produce leaves, roots, fruits and flowers. (It’s a bit like vitamins for humans!)

If your soil, or compost, is healthy, and you’re growing the right type of plants, you probably won’t need to worry about exact nutrient levels. However, if your plants start to look unhealthy, it may be that you have a mineral deficiency.

If you’re growing in pots or containers, or growing ‘hungry’ plants which are working hard to produce lots of fruit, vegetables or flowers, then it can be a good idea to add some fertilizer to replenish the nutrients or minerals in your soil or compost.

Find out more about the minerals plants need.

Fertilizers

Fertilizers are simply mixes of minerals, or nutrients, needed by plants to grow. Fertilizers can be home-made (eg nettle tea, comfrey tea) or commercially produced.

You’ll find dozens of different fertilizers at your local garden centre, at hugely varying prices. Understanding how fertilizers are formulated can help us make the best – and most economical – choice. The good news is that most of the time you only need two: a multipurpose fertilizer and a high potassium fertilizer.

NPK Ratio

This refers to the ratio of the 3 most important plant nutrients:

• N: Nitrogen – for healthy growth of stems and leaves
• P: Phosphorus – for healthy growth of roots
• K: Potassium – for healthy growth of fruits and flowers

A general, or multipurpose fertilizer will have a balanced ratio of Nitrogen, Phosphorus and Potassium. Most of the time, this is the only type of fertilizer you’ll need.

A high Potassium fertilizer (usually sold as a tomato or rose fertilizer) has a higher level of Potassium, which means it’s good for healthy fruits, vegetables and flowers. This isn’t just for tomatoes or roses: you can use this on all of your vegetables, fruits as well as your flowers and roses.

Controlled release, slow release or liquid fertilizers

This refers to the rate at which fertilizers release their nutrients. Liquid feeds release nutrients quickly, but need to be reapplied weekly. Slow release fertilizers are usually natural materials (such as bonemeal) which release nutrients more slowly over a few weeks. Controlled release fertilizers are formulated to release nutrients over several months.

It doesn’t really matter which one you choose – it depends on the cost and how much time you have, ie controlled release fertilizers are generally more expensive, but take less time because they only need to be applied once or twice during the growing season.

Home made fertilizers

Home made liquid fertilizers (ie compost tea) can be a great way to produce your own fertilizer. You can use nettles, comfrey or really any plant you have to hand. Remember it’s illegal to collect seaweed without a licence though! Making compost tea can be a smelly and time consuming process – so if it’s not for you, don’t feel bad about getting the shop-bought feed!

When using home made compost teas, do be careful to dilute it well. It can be very strong and can ‘burn’ plants if not used carefully.

Soil additives: how to improve your soil

Whatever your soil type, there’s a simple answer to improving it, which is to add organic matter. The following options are cheap or free.

• Garden Compost
• Well-Rotted Manure
• Leaf Mould
• Woodchip

There are also a range of other additives you can use to improve your soil, or the compost in your containers or raised beds. These have a cost, and environmental impact, but can be worth it in terms of reduced water needs and healthier and more productive plants.

Vermiculite

A mineral that expands when heated, resembling small golden-brown flakes. It can absorb up to four times its weight in water and also helps with temperature regulation.

Perlite

A lightweight, volcanic glass that, when heated, expands and becomes porous. It isn’t as water absorbing as vermiculite but is good for increasing drainage, for example with mediterranean plants. It’s also good for growing cuttings, which can be grown in pure perlite.

Horticultural Grit

Small, sharp stones added to soil to enhance drainage and prevent compaction. Dan recommends buying horticultural grit rather than general builder’s grit or sand, as the latter may contain other substances, and hasn’t been cleaned.

Water Retentive Gels

Granules that absorb and hold water, releasing it slowly to plant roots. Not particularly recommended as these can contain a variety of chemicals and vermiculite works just as well.

Coir

This is a by-product – a natural fibre extracted from the husk of coconuts. It improves soil structure, aeration, and moisture retention. It has no nutritional value and should be neutral pH. Always buy from a reputable supplier to ensure it’s clean and any salinity has been removed.

Mycorrhizal Fungi

Marketed as beneficial fungi that form symbiotic relationships with plant roots, aiding in nutrient and water uptake, and improving plant resilience. However, more recent research has indicated that there is little benefit to adding mycorrhizal powder.