THE DIRT

“Growing plants is the goal,” said James Urban, FASLA, Urban Trees + Soils, at the 2013 ASLA Annual Meeting in Boston. To grow healthy plants, one needs healthy soils, and landscape architects who understand soils and know how to call a soil scientist. In a wide-ranging talk, Urban and his co-presenter, soil scientist Norm Hummel, discussed how landscape architects can design with new soils the right way, particularly in challenging, damaged urban landscapes.

Whether natural or man-mixed, soils have physical, environmental, and chemical properties. These are all important to the health of a growing medium. Physical properties include organic matter, water, drainage, and aeration. Environmental characteristics include light and temperature. Chemical elements include the pH balance, and the presence (or not) of phosphorous, nitrogen, and potassium, which are all critical elements for plants.

To determine what kind of soil is needed for a project, Urban said goals and requirements are needed early on in the design process. Questions that need to be asked: “What type of trees and plants are you trying to grow? How big do you want these plants to get?” As an example, depending on the requirements, an oak can grow to 25 feet and last 50 years, or grow to its full extent and live hundreds of years. Landscape architects have think through these things in terms of soil early on.

It’s also important to know how a site is being used. A landscape may have lawn, but is that walked on a few times a year or thousands of times? Urban said the National Mall’s turf gets a quarter of a million visitors per day. That space gets 3,000 events a year. Use will determine what kind of irrigation and soils are needed.

Urban said there are eight critical properties of soils, which soil biologists can test to determine if soils meet specifications. They include structure, texture, density, nutrients, pH, organic matter, and density, which are all “inter-connected.”

More often than not, Urban said trees and plants don’t do well because of the physical properties of soils rather than the chemical. If something goes wrong — a tree is stressed, shows early fall color, or even dies — landscape architects may be planting the wrong trees and plants for the soil types.

Some details on soil’s physical properties: The structure of soils has to do with how well-glued together the soil particles are. Particles are attracted to other particles — and organic matter glues them together. Clay soil has a strong structure due to the stickiness of the soil. Silt soil has a weaker structure, while sand has no structure at all. Sandy soils are useful in areas that need to drain.

Urban added that man-made mixed soils are very different from natural soils. Mixed soils include soils that have been broken apart and put back together.

Soils are also made up of spaces or voids where water can flow. Ideal forest soils have a void space of about 50 percent, while urban compacted soils are around 20-30 percent. With the Sustainable Sites Initiative™ (SITES®), Urban said more landscape architects will need to measure soil structure.

Soil texture is also important to examine. Clay, silt, and sand all have different surface areas given the unique sizes of the particles. Fine sand is .24mm, while silt is 2.4mm, and clay, nearly 24mm. Just within the family of sand, there are huge differences as well, with fine sand having properties distinct from coarse grains.

Hummel, who said he has examined over 100,000 soil samples in his career, said organic matter is a major contributor to soil health. Organic matter can be amended with either peats or composts.

He said many peats are actually not sustainable and shouldn’t be used to augment the organic matter in damaged soils. Peat farming can strip an area of nutrients, creating environmental damage. However, he made an exception for sphagum peat, which is more expensive, but a renewable resource. For Hummel, sphagum peat is “superior to compost, which breaks down rapidly.”

But compost is most often added to soils to boost the amount of organic matter. Compost is often used with disturbed urban soils that have suffered from erosion and compaction. Compost types include yard waste (grass, wood chips), bio-solids (treated municipal sewage), animal manure, and mixed waste. Some regional compost specialties include pine bark and rice hulls. Hummel added that soils have a “disease suppressive capacity.” Still, he cautioned against the practice of using 90 percent compost and 10 percent soil, saying that a “tree planted in that will simply fall over or die.”

Hummel also delved into the chemical properties of soils – and whether it’s possible to chemically amend damaged soils. He concluded that altering the PH balance of existing site soils is “unrealistic.” What’s better is to focus on the availability of nitrogen, potassium, and phosphorous in the soil.

While sending soil samples to a lab will yield data on all these properties, these properties can also be requested in soil specifications. Hummel said landscape architects can even specify things like permeability in soils.

Urban concluded that it’s best to reuse dirt where possible, but sometimes grading and compaction have “killed the soils.” To understand the problems and solve them, landscape architects can use web soil surveys, study soil maps, take their own samples, examine them, and send them to the lab. “Landscape architects need to learn how to do this.”

To learn how to go on to the next step and fix soils, check out Urban’s book, Up by Roots.

Image credit: Sugar Beach, Toronto, by Claude Cormier / Deeproot