Excellent 15 minute video about soil composition


  1. Soil Testing and Analysis: Start by conducting a soil test to determine the pH and nutrient levels of your soil. This information will guide your soil improvement efforts. You can often obtain soil testing kits from local agricultural extension offices or gardening centers. Adjust the soil’s pH and nutrient levels based on the test results to create an ideal growing environment for plants.                           
  2. Add Organic Matter: Incorporate organic matter, such as compost, well-rotted manure, or leaf mulch, into your soil. Organic matter improves soil structure, water-holding capacity, and nutrient content. Spread a layer of compost over your garden beds and work it into the top few inches of soil.                   
  3. Mulching: Apply a layer of organic mulch, such as wood chips or straw, to the soil surface. Mulch helps retain soil moisture, suppress weed growth, regulate soil temperature, and prevent erosion. As the mulch breaks down, it adds valuable organic matter to the soil.                                                     
  4. Practice No-Till Gardening: Minimize or avoid tilling your soil, as tilling can disrupt soil structure, lead to compaction, and expose beneficial soil organisms to the surface where they may die. Instead, use techniques like lasagna gardening or raised beds to create healthy, undisturbed soil layers.                                                           
  5. Select Native and Well-Adapted Plants: Choose plants that are well-suited to your local climate, soil type, and environmental conditions. Native plants are often adapted to the soil and climate of your region and require less maintenance. They also support local ecosystems and wildlife.

Soil formation is a complex process that is influenced by factors such as climate, parent material, topography, organisms, and time. Weathering and erosion of rocks by physical, chemical, and biological processes release minerals into the soil, while plants, animals, and microorganisms contribute to organic matter accumulation and decomposition. Over time, these processes develop distinct soil profiles composed of stratified layers called horizons.

The topmost layer, the O horizon, contains living and decomposed organic matter like bugs, leaves, and twigs, enriching the soil with nutrients. Below that lies the A horizon, or topsoil, where the roots of plants grow and where much of the biological activity occurs. The B horizon, or subsoil, contains accumulated minerals and leached materials from the layers above, while the C horizon consists of partially weathered parent material. Lastly, the R horizon, or bedrock, is the solid rock that underlies all the other soil layers.

Soil composition determines its suitability for supporting plant life and other ecological functions through various physical, chemical, and biological properties. Physical properties include color, texture (proportions of sand, silt, and clay), structure (arrangement of soil particles), porosity (air and water-holding capacity), and permeability (the ability of water to move through the soil). Chemical properties encompass pH level and cation exchange capacity, which influence nutrient availability. Biological properties relate to the abundance and diversity of soil organisms, such as bacteria, fungi, and earthworms, which play critical roles in nutrient cycling and soil health.

Looking at the composition and formation of soil from an urban lens, it directly impacts the health of street trees. Urban soils can be severely degraded due to compaction and disruption during construction. Compacted soil restricts the infiltration of water and air, making it difficult for tree roots to access essential nutrients and oxygen. Tree roots struggle to penetrate the ground, leading to shallow root systems that lack stability and resilience. In contrast, loose, well-aerated soil allows roots to spread wide, enabling trees to anchor securely and access a more extensive network of nutrients and water. That’s why it’s important to choose tree species with adaptable root systems that can thrive in urban soil conditions, promote healthier growth, and reduce the risk of uprooting during severe weather events.

Following this, ample and consistent soil moisture is critical for the survival of urban trees, particularly during drought conditions. Urban environments often face challenges with water retention due to impermeable surfaces, such as asphalt and concrete, which prevent water from infiltrating the soil. This is why you see watering bags (gator bags) on young street trees throughout the city. For more information on watering urban trees, read our section on watering here.