Wild plants can do without liquid manure and fertilizers – but for plants that need to grow vigorously, investing in soil fertility can be very worthwhile. Vegetables, bedding plants, some fruits, lawns and roses respond well to soil amendment. Feeding other plants is of lesser value.
Manure (and compost) contain modest amounts of nutrients, often as little as 0.5 percent. However, large amounts are used, typically 5-10 kg per square meter, so the overall nutrient content is significantly increased.
Most of the manure also adds carbon to the soil, some of which is used by soil organisms to produce materials that bind the soil into water and nutrient sparing “clumps” surrounded by interstices through which water drains, air seep away and roots can migrate.
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Another benefit of manure is that the benefits last for several years, with the nutrient content slowly being released as the material rots.
Fertilizers are highly concentrated (5-40% nutrient content) so relatively little is needed, typically 30-200g per square meter.
The main nutrients are nitrogen, phosphorus and potassium, and to a lesser extent magnesium and sulphur. Trace elements – boron, copper, iron, manganese, molybdenum and zinc – are only required in small amounts.
Gardeners typically base their plant feeding on manure, compost, and organic-based fertilizers. These contain enough trace elements that deficiencies in the garden rarely occur, although they are common in agriculture.
Compost Bin (Photo: Neil Hepworth/RHS)
The nutrient content of manure and compost is very variable. An analysis is possible, but only useful on an agricultural scale. For gardens, adding the recommended amounts is sufficient without overdosing.
Fertilizers tend to be overused. By law, the analysis of fertilizers should be printed on the packaging, although some are sold without. These are best avoided because while they likely contain useful amounts of nutrients, the levels are likely to be variable and you cannot be sure what to get or how much to use.
Nutrients in soil can be unavailable (“locked up”) for plants if the soil is too acidic or too alkaline. A pH test kit, available relatively inexpensively from garden centers, is sufficient for measuring pH. Nutrients are best available at pH6.5 or slightly acidic. Adding lime (ground limestone) to very acidic soils raises the pH, but realistically, using sulfur dust to lower the pH of alkaline soils is rarely practical.
Smart gardeners only lime parts of their plots where very fertile soil is needed, since most ornamental plants thrive well enough in acidic soil and some – heather and rhododendrons for example – need it. These “Ericaceae,” or acid-loving plants, are a great choice for gardeners lucky enough to have acidic soil.
Adding chicken manure pellets to a new bed can help provide nutrients to the plants (Picture: Tim Sandall/RHS)
Testing the soil every four years is a good investment when growing hungry crops like vegetables and adding fertilizer to correct any deficiencies in phosphorus, potassium or magnesium before planting.
Where nutrient levels are good but growth is disappointing, consider nitrogen fertilizers. Plant growth is directly related to nitrogen (which cannot be accurately tested), but overuse of nitrogen can lead to weak, diseased, or insect-infested growth.
Nitrogen fertilizer is expensive due to the current disruption in gas supply and can pollute both air and water. In a garden situation, lack of water is probably just as important as lack of nitrogen.
Where plants have adequate water, adding moderate amounts of organic, high-nitrogen fertilizer such as poultry manure pellets or hoof and horn manure is effective.
Poultry manure pellets could become scarce next year due to the impact of bird flu on poultry farms, but a good alternative is alfalfa pellets, sold as pet or pony feed.
Adding powdered lime (calcium carbonate) to the soil in RHS garden Wisley, Surrey (Photo: Tim Sandall/RHS)
Plants in containers cut off from soil reserves will need regular fertilizing throughout the summer, but that’s something to consider later. Now is the time to bring in manure or compost as few gardens can produce enough home-made compost for all their needs.
It can be incorporated by digging or other cultivation, but is arguably best when applied as a mulch to perennial crops such as soft fruit or asparagus, or to raised beds where there is none unless trampled all traffic on the paths has to dig . Worms and other soil organisms absorb the fertilizer naturally.
With shortages and delivery delays following the global fertilizer supply crisis, it’s best to buy enough fertilizer now to get through the growing season. However, it is even better to use blocking fertilizer and compost. This will save you money this year and build up reserves of soil fertility if the current situation persists.