An Urban Tree Care Conference
January 30th & 31st, 2020

There are different types of soil. Those differences can have a major impact on tree establishment and growth. Soil types are based on the size of the mineral particles in the soil. Clay soils have very, very small particles, while sand soils have relatively large particles. Silt soils are in between. All soils are a combination of those three size classes (clay, silt and sand), plus some amount of organic matter.

Clay soils are “heavy”. They absorb water slowly, but hold a lot of it; they don’t like to give it up, though. Clay soils easily become waterlogged (too much water, no available oxygen) which can be very harmful to tree roots. Clay soils can have a good nutrient load, and they can hold applied nutrients well. Trees in clay soils need to be irrigated less often, but with more water, compared to other soil types.

Sand soils are “light”. They are easily worked, and harder to compact than other soils (though it is quite possible to do so!). Sand soils absorb water quickly, but they neither hold much, nor for long – it just keeps sinking lower into the soil. Sand soils have very low nutrient levels, but are more easily amended and worked with. Trees in sand soils need to be watered more often, but with less water, compared to other soil types.

Silt soils are in between the clay and sand in size and in water holding/nutrient holding capacities.

Many soils are complex combinations of sand, silt and clay, and may vary substantially over short distances.

Water moves in soil based on two forces. One is gravity, which always pulls water downward. The other is capillary pull. This force moves water into the spaces between soil particles, and it is caused by the electrical charges on the surface of the soil particles interacting with the electrical charges on water molecules. The smaller the space between particles, the stronger capillary pull is. In a sandy soil, gravity overwhelms capillary pull and water moves largely (and quickly) straight down. In a clay soil, capillary pull is very strong, and water moves horizontally about as fast as it moves downward; water will spread out farther in a clay soil. Clay soils hold a lot of water, but they take it in slowly and they don’t give it all up; sand soils don’t hold much water, but the water they do hold is very plant available.