The rule that grounds care professionals have always known - that soil compaction is the enemy - is little understood in the landscape field with sometimes disastrous results, according to the soil scientist who has advised projects from Gardens by the Bay in Singapore to the Olympic Park in east London.
The best soil for landscaping is completely different from that favoured by agriculture, something not enough people appreciate, Tim O'Hare Associates founder Tim O'Hare told delegates at the second annual SoilsCon conference organised by his firm and held in Henley-on-Thames, Oxfordshire, on 5 October.
He said any kind of landscape construction immediately has implications for natural soil, which is not designed to be moved - and that is without worrying about the heavy plant, other vehicles and people moving around on top of it.
Landscape crews usually start their job with soil that has been deliberately made worse, from a horticultural point of view at least, said O'Hare. "Civil engineers like to compact soil. They don't want the soil to move." In fact, the profession defines compaction as a "soil improvement technique".
For soils to function properly they need good aeration, drainage, water storage, plant nutrients and soil fauna, both micro and macro. "Half of good soil is water and air," said O'Hare, the elements civil engineers are so keen to squeeze out. But without these, plants die.
Compacted soils "can cause flooding from a lack of filtration. In anaerobic soil plants can't take up the water because there is no oxygen there. Plant roots have no space to grow. Soil pathogens come into play and attack the plants. There are detrimental bacteria. Soil plasticity is a major issue because it has too much density and moisture."
The best type of soil for landscaping is sandy soil, then sandy clay. Any kind of silty soils are the worst, O'Hare pointed out, even though they are recognised as good soils for agriculture. "That's the problem in landscaping and land construction," he said, adding that ideally it would be undertaken during summer, when soils are friable (crumbly) and not plastic, but that is not often possible. If not, landscapers should pay proper attention to soil storage.
"If the soil goes into a stockpile dry, even if you leave it there two or three years, when you come to take it out it will be dry. Even if you have to handle a dry soil in January it's easy, it's not plastic. If the soil is wet when it goes in the stockpile, it doesn't matter how long you leave it there, the bulk will be as wet as it was when it went in and it will be highly anaerobic."
O'Hare recently completed the Defra Construction Code of Practice for the Sustainable Use of Soils on Construction Sites, which shows good methods to keep dry soil dry, and dry out wet soil, during periods of storage.
Another tip is using compaction-resistant soils, O'Hare advised, particularly where you are dealing with roof gardens or podiums, as landscape professionals do increasingly. "Here sand is by far the best material to use because you can handle it all year round and trample it as much as you like," he added. "Sharp" sand is most useful for big trees.
"At the Olympic Park they had to travel all over the subsoil with diggers. We had to make sure that the soil was compaction-resistant," he said. Sand is by far the best material to use on podiums, he explained, "because you can handle it all year round and trample it as much as you want".
He said all contractors should have a ripper claw attachment for excavators, adding "there are no excuses" not to have this essential bit of kit. But equally there is nothing wrong with a garden fork. "How many people who lay turf go back and spike the lawn? That's the most important thing as all the soil is compacted." The roots of the grass will travel down the holes. Contractors should also do this when planting. "By driving the fork in, you're shattering the soil, allowing the water to get in, the air to go in and the plants will romp away."
Treework Environmental Practice managing director Luke Fay wanted to dispel the many myths about tree roots, saying they grow out - willow tree roots can spread as far as 40m - not down and do not smash through buildings. To thrive, trees need a symbiotic relationship with around two billion bacteria in the soil in which they grow.
"Trees can't draw in nutrients on their own," he said. "They are absolutely dependent on the microorganisms in the soil. What you have is a rhizoplane around the root, there will be fungi, there will bacteria, there will be aerobic bacteria, there will be protozoa and nematodes. These eat the fungi and bacteria and excrete the nutrients in a form the tree can take up. The tree is not just passing the carbohydrates and sugars into itself but is also excreting chemicals.
"Even two trees planted next to each other at the same time can have radically different health if, for example, traffic compacts the soil over the roots." Fay showed some sad examples to delegates. "There is no reason we should not get this right."
RHS Chelsea Flower Show 2016 best in show winner Andy Sturgeon also showed how bad soil can disrupt even the best designs, bravely displaying pictures of some soil-related failures. At one site good imported topsoil was ruined by being trampled over during construction, while at a rush-job build compacted soil led to a lot of plants dying. "Control of your soil on site during construction is important," he said, and starting with the wrong soil could have "heartbreaking" results.
The other speakers at the conference were British Sugar Topsoil general manager Andy Spetch, on manufactured topsoil; Leap Environmental director Richard Brinkworth, on contaminated land; Tim O'Hare Associates senior associate Tim White, on wild flower grasslands; and regional operations director at The Landscape Group - now part of idverde - Alistair Bayford.
Questions - How landscapers can help to avoid problems with soil
Landscape professionals visit nurseries, said British Sugar Topsoil’s Andy Spetch, and they should visit soil suppliers too. He suggested asking:
? What is the source of the topsoil?
? Greenfield, brownfield, skip, manufactured?
? Is there an auditable process?
? Are quality ingredients being used?
? How is the topsoil stored?
? How much is available?
? What is the period of availability?
? Is there enough for your project from one batch?
? What is their sampling protocol?
? What is their frequency of testing?
? What was the date of the last analysis?
? Is there a suite of analysis?
Compaction - Common causes and how to limit occurrences
Obvious Soil excavation and stockpiling, vehicle trafficking, foot trampling, site compounds, storage of materials, piling mats.
Less obvious Self-compaction, surface mulches, excessive topsoil depths, tree root balls.
How to minimise compaction Restrict soil handling, use good stock, choose compaction-resistant soils if necessary, consider effects of tree pits, use tracked machinery rather than wheeled.