Gardeners are raised on the idea that a thick mulch will benefit the garden, but what if this is wrong? Research from Chicago Botanical Gardens has found it is better for plants and soil microbes to live in leaner conditions, only applying compost at 2.5cm maximum while using the odd soil amendment to aid soil life. Or an even more extreme thought, what if we cut out soil completely?
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Using sand and biochar in place of soil
Growing without soil is an approach used in urban planting to reduce maintenance costs, tackle more extreme weather and combat limited resources. It sounds radical and it challenges everything we know about our plants. A pioneer of this technique is Swedish gardener, nurseryman and designer Peter Korn, who with his partner Julia Andersson has created a horticultural Eden at Klinta Trädgård in southern Sweden growing purely in sand.
By using sand as both a permanent mulch in the garden and a growing medium Korn raises plants that are stronger and more resilient to adverse conditions. The idea is to grow them slowly and in a harsher environment that often results in a smaller, more delicate form, requiring no staking and less ongoing care. The astonishing moisture retention of the sand and the strong fungal and mycorrhizal communities it encourages further aid water and nutrient uptake, making regular watering unnecessary. This may be because sand contains fewer microbial competitors than rich soil. Peter Korn’s planting survived through two record-breaking Swedish droughts without irrigating once. While it won’t satisfy all horticultural needs – vegetables and the traditional beefy, hungry English herbaceous border still love the rich soil – sand planting has a huge amount of benefits.
Alternatively, consider a sustainable and beneficial soil amendment, biochar. Biohar is a by-product of intensive, low-oxygen burning, a process called pyrolysis. This isn’t anything new, it is thought ancient Amazonians used to burn all their farm waste in a similar process. When used with the soil the biochar acts as a sponge, absorbing and storing water and nutrients. Biochar improves soil structure, reduces soil compaction, increases soil fertility and creates habitat space for soil microbes. All that is needed is a thin scattering mixed into your compost and the benefits of biochar will last well over a thousand years.
Biochar retains more than half of its carbon, reducing the amount released into our atmosphere. Compost only traps around 10-20 per cent, which results in huge carbon loss – a key contributor to global warming.
Growing in sand beds
• If your garden soil is already rich in organic matter from years of compost, then a sand mulch cannot be applied as it will remove the sand’s ability to retain moisture by drawing it out.
• The sand used must be a sharp sand measuring 0.05-0.08mm in size and must be pre-washed to remove any salt or lime.
• Lay the sand as a mulch at around 10-15cm to prevent weed growth or simply place into a raised bed. • Always plant bare-rooted into the sand. Before planting make sure
you wash the plant’s roots to remove all compost and soil.
• Once planted give the plant a good watering to close air pockets around the roots and stop when the sand becomes smooth again.
• Although most plants benefit from this system, some won’t. Avoid extremely hungry plants and varieties, such as exotics and vegetables.
• You can make your own biochar stove but it will require some research. However, you can buy ready-made biochar.
• Always inoculate homemade or fresh biochar, otherwise it will act as a nutrient sponge for a few years before offering any benefit to the soil. The simplest way to inoculate your biochar is by adding it to a compost heap where it will become full of microbes and nutrients. This will have the additional benefit of speeding up the decomposition time of the heap.
• Apply the biochar-activated compost as a thin mulch on top of the soil’s surface. • Inoculating biochar rapidly increases soil microbial growth and especially the development of mycorrhiza, a key fungus for healthy plant development.
Discover more about soil
FIND OUT MORE Garden: Giving Plants What They Want by Peter Korn (Peter Korn, 2013).