Getting ready for autumn’s arrival

With new Covid restrictions imminent, our gardens are more important to us than ever, whatever the weather may be

With our erratic weather consisting of an exotic medley of droughts and deluges, it seems that the best way to help your plants and garden is to get the soil in good shape. Talking to Tim O’Hare (below), a soil scientist who works all over the world, advising on soils from Wisley to Oman via the Olympic Parks, it becomes clear that current research on soil management, in gardening terms, turns a lot of perceived wisdom on its head.

I cannot remember the number of times I have heard gardeners being recommended to dig a massive deep trench when planting a hedge and fill with good soil and well-rotted manure. And when planting big trees, to take out metre-deep pits, again to be filled with topsoil and manure. But putting topsoil into a depth greater than 30-40cm when planting anything is detrimental. Soil needs air, and it gets starved of oxygen, becomes anaerobic, below this depth: the aerobic bacteria die off and anaerobic bacteria develop. These anaerobic bacteria produce methane and ammonia, which give rise to toxic conditions. The tree roots become short of oxygen, they cannot take up food and water and so they suffocate.

Instead, O’Hare recommends just using topsoil for the top 30-40cm when planting. Only dig as deep as you need to accommodate the plant’s roots, or the tree’s rootball. The more you dig soil, the more you ruin the structure, and it is far easier to dig the minimal amount. Digging disrupts the all-important top 70-100mm, which contains the valuable microorganisms, mainly bacteria and fungi, and digging reduces the precious population of earthworms. When O’Hare was supervising the planting of huge trees at the Olympic

Park, trees with trunks 60-100cm in circumference, they filled the lower portion of the pits with compactionresistant subsoils or washed sands and the topsoil was limited to the top 30-40cm.

For soil improvers, O’Hare is a big fan of green compost (formerly called green waste, recycled from garden waste). I have been using this for well over 10 years at home and on other gardens and find it a brilliant soil improver. Like most compost, it is naturally quite alkaline, but slowly – after three or four years – the soil it is applied to will settle back to its natural pH. Green compost has a high potassium content, generally thought to promote flowering and fruiting, and it is a slow-release source of nitrogen, phosphorous, secondary nutrients and trace elements plus masses of beneficial fungi and bacteria. When forming “manufactured topsoil”, green compost is often blended with quarry overburdens (the unwanted sand deposits from quarries) to form a clean, organic rich medium for generating good growth.

It is often perceived that adding green material or unbroken-down materials such as mown grass clippings to soils as a surface mulch robs the soil of nitrogen as it is broken down. O’Hare believes this does not occur as long as you do not dig it in. It will slowly break down, through the activity of earthworms and the microorganisms in the soil. Green shreddings from tree surgeons, with leaves and twigs all mulched up, are a great, inexpensive (or often free if the tree surgeon is working nearby) mulch, ideal for boosting organic matter, nutrients, keeping the soil free of weeds and holding in the moisture. So perhaps if we leant on our spades a bit more, rather than dug with them, we would do our gardens a big favour.

Current research on soil management turns perceived wisdom on its head