It is important to choose the right type of compost, as not all composts are the same. Some composts are built for seeds and cuttings, others designed for trees and shrubs or just multi-purpose use. Here we have a breakdown explaining common ingredients found in compost, a guide to choosing the right one, which are right for different plants and even how to improve store bought compost.
Ingredients in compost
Compost is often made from different base ingredients, including:
- Peat: A naturally occurring substance that can range in colour from black to dark brown to pale brown and is made from decaying vegetation (sphagnum moss or sedge). Although peat in its raw form typically lacks nutrients, it provides a warm, aerated environment for plant roots and retains moisture and additional nutrients without becoming saturated with water.
- Peat-free: There are a lot of composts that don’t contain peat, and many of them work well. Given that peat-based products have a high environmental cost, they are an obvious choice. How to select and utilise peat-free composts, as well as the advantages of making your own, are discussed in this section.
- Sterilised loam: A loam that has undergone heat treatment to ensure that all weed seeds and diseases have been eradicated.
- Sand: Washing the sand ensures that any salt traces have been removed.
- Grit: Incorporated to maintain the compost’s open structure, enhancing drainage and aeration around the roots. Grit particles can range in size from very fine to almost pea-sized.
- Coir: A naturally occurring by-product made from coconut husks that retains water well and does not easily become waterlogged.
- Bark: Can be utilised as an alternative to peat. Before being used in compost, bark must be thoroughly composted or it can hinder plant growth.
- Composted wood waste: This is also an alternative to peat and is typically constructed from composted chipboard.
- Composted green waste: Local councils typically produce this, which is made from garden and kitchen waste.
- Composed stable manure: Pretty straightforward, in addition to the horse manure, this typically includes some composted straw and/or other bedding materials.
Fertilisers used in compost
Many compost manufacturers will add additional fertilisers to their compost to aid plants in their growth. Plants need major nutrients (Nitrogen, Phosphorous and Potassium) along with secondary nutrients and trace elemebts. The two main fertilisers used are:
- Soluble fertiliser: Water slowly dissolves these. In most cases, soluble fertilisers only supply the plants with enough nutrients for the first six weeks. However, a lot of the composts use a special mix that contains both the major nutrients and the trace elements that are just as important for healthy plant growth.
- Granular fertiliser: Granular fertilisers come in two varieties: those with controlled release and those with a slow release. Fertilisers with a “slow release” release their nutrients whenever the compost is damp because they need moisture in the soil to function. The plants won’t be able to take in the nutrients if the compost gets too wet or dries out. Fertilisers called “controlled release” are made to slowly release all major nutrients and trace elements over up to six months. In contrast to slow-release, the nutrients are released based on soil temperature and reach the plant when it needs them for growth.
Other common ingredients for composts are:
- Lime: The natural pH of peat is between 3.5 and 4. Compost mixes typically contain lime, which is added to raise the pH to 6 or 6.5. Except for ericaceous plants, which require specialised ericaceous compost, this is the ideal pH for most plants.
- Wetting agent: It can be challenging to re-wet compost that has been allowed to dry out because the water will likely run off and bead off. The surface tension of the compost can be reduced by treating it with a wetting agent. Because of this, it is much simpler for the water to seep into the compost and not escape.
A guide to choosing the right compost
“Finding the right compost for the job can be extremely difficult. Each compost has a different job, and for anyone starting out in gardening, this can be a very daunting job. I recommend using Compost King – Multi-purpose compost if you aren’t sure which soil as it is an overall good compost with a slow release fertiliser. But for roses, topiary, shrubs, trees, fruit trees in particular, I recommend Compost Kings Shrub, tree and rose compost as it has all the needed nutrients for them to develop.”Chris Bonnett, owner of Gardening Express & gardening expert for The Express
The right compost for different plants
- Sowing seeds
- Potting on plug plants
- Rooting cuttings (also add a little vermiculite or prelite)
- Plants in pots for less than a year (including hanging baskets, summer bedding and veg)
- Plants in pots for more than a year (equal parts multi-purpose compost and good top soil)
- Large plants in large pots being kept outside, including shrubs and fruit, we recommend either a multi-purpose or shrub, tree & rose compost.
- Mulching beds, use a well-rotted garden compost or decorative/chipped bark
How to improve store bought compost
Mix with homemade compost
You can incorporate store-bought compost if you have some homemade compost but not enough to cover the entire area. Because the decomposition of homemade compost can take up to seven months, you’ll need to prepare it well in advance. Use store-bought and homemade compost in the garden in the same way as always.
Because it contains all of the essential nutrients that plants require, well-rotted manure is an excellent method for enhancing the quality of store-bought compost. Mix homemade compost with store-bought compost to a ratio of 20 to 50 per cent and use as usual. To ensure that your plot is free of antibiotics, chemicals, or pesticides, it is essential to obtain manure from a reputable source.
Want some more information about composts and composting? View our other related articles here