Soil

For May, the arboretum looks down upon the soil our plants and trees rely upon so heavily. In this short article, we’ll provide a better understanding of soil, its importance, and how we can take care of it.

Pedology is the study of soil’s physical and chemical properties, its formation and classification, and how it is used in agriculture, environmental conservation, and land management. Those who study soil are known as pedologists and they can specialize in various fields like soil chemist, who focuses on the chemical composition of soil, including nutrient levels and mineral content; soil physicist, who studies the physical properties of soil, such as structure, texture, and how water moves through it; soil microbiologist, who researches microorganisms that live in soil and their role in nutrient cycling and decomposition; environmental soil scientist, who works with developers to ensure land is suitable for projects and may focus on soil contamination and restoration; and agronomist, who studies soil and crop production, often focusing on improving crop yields and soil fertility.

Soil is made up of three basic particles categorized by size: sand, silt, and clay. Sand is the largest in size, the least weathered, and most recognized particle. These particles increase a soil’s drainage and reduce bulk density. Clay is the smallest particle, often known for its less-than-ideal property of being hard to dig in dry conditions. It holds onto water due to its high surface area. When wet, this particle feels sticky and can be extruded and worked into ribbons or pottery. Silt is the anti-celebrity of soil in that it is not easy to feel like sand or easy to apply use like clay. Silt is the goldilocks of soil particles —- it’s not too big or too small but alone as a soil it has the best and worst traits, leaving weather patterns and precipitation to dictate soil extremes. Soils high in silt can be too wet, too dry, and just right depending on the time of year. That said, they have good nutrient holding and workability traits for planting and gardening. It’s very rare to find pure silt soil.

These three particles, present in different percentages, define 12 distinct soil texture classes. From those primary 12 soil texture orders, soils are then broken down into other series based on parent material, weathering, moisture, and chemical properties. There are over 20,000 different soil series in the US alone. Want to know what types of soils are in your area or your backyard? Visit the USDA Soil Maps. You can also find the print versions of these maps by county at your local library or university extension offices.

Soil should be one of several factors taken into consideration when selecting new trees to plant, as well as a variable to consider when diagnosing tree health issues. Many species of trees have adapted and evolved to be tolerant of and suited for either a broad range or very specific soil conditions. Trees continue to be adaptable to what humans have labeled as less than ideal soil conditions for growing crops. The most common soil issues that negatively impact trees are compaction, change in hydrology, or the removal or addition of soil to an area where an existing tree is. Most of these issues are caused by human actions.