Buying fertiliser can be an expensive business for both the pocket and the planet. RHS Chief Horticulturist Guy Barter looks at how you can use compost and manure in its place to feed the soil.
Following high natural gas prices and increased world demand after manufacturers suspended production due to Covid problems, fertiliser doubled in wholesale price in 2021. It’s all the more reason to make the best use of manures and composts, which can replace much fertiliser and in many cases meet all plant nutrient needs, such as with tree fruit, red and white currants and ornamental plants.
As well as meeting nutrient needs, bulky organic manures and composts feed the soil, boosting soil health and productivity in ways that fertiliser cannot. Soils rich in organic matter support healthier plants that need less fertiliser.
But manures and composts are highly variable depending on what animals are involved and how they have been fed and bedded. Compost nutrients depend on the materials used in their production. The table at the bottom of this page gives very approximate figures for comparing the nutrients in each type.
Use manure at about 10kg(22lb) per square metre for hungry vegetables such as beetroot, brassicas, potatoes and runner beans. Half that is enough for other crops.
In practical terms, adding manure one year out of three will maintain soil organic matter, but more frequent applications will raise organic matter in the soil. Adding a thin surface layer every year as no-dig gardeners do will greatly enhance soil fertility.
Ideally allow fresh manure to rot down for a year before use, especially for crops that are eaten raw. This process gets rid of most disease organisms such as E. coli.
Poultry manures are more concentrated than stable or farmyard manures. They add plenty of nitrogen and phosphorus but little organic matter. Fresh poultry manure is best added to the compost bin. Dried pellet poultry manure is a good fertiliser for vegetables as well as blackcurrants and plums.
Some councils and waste management companies sell composted municipal wastes, which can be good value as long as they are free of plastic and other contaminants.
Garden compost contains less nutrients than manures but adds good quality organic matter. Composting household and garden wastes is a particularly sustainable activity, but aim to turn your compost heap regularly to speed up rotting and prevent the anaerobic conditions that can lead to methane production, which is a greenhouse gas.
Bear in mind that making enough garden compost to fully fertilise the soil is a challenge in most gardens and at least some fertiliser will be needed in addition to the compost for good results.
Nutrients in fertiliser, manure and compost
||Dry matter %
|Municipal compost (made from waste collections)
|Dried poultry manure pellets
Pick of the crop
Look for the RHS Award of Garden Merit (AGM) when buying vegetable seed or small plants. You can also download the RHS lists of recommended cultivars.