Trees are an important part of our urban landscape and most people can name a couple of ways trees benefit us and our urban environment. Soil, however, is a much less appreciated resource that is vital to our health and that of our urban forests. Soil is such an important factor in our lives and the environment that the United Nations General Assembly has made 2015 the International Year of Soils.

In the first part of our “Life of an Urban Tree” series of blog posts, we will discuss soils and how they affect the trees we have in our community.

What is the ideal soil for a tree?

The ideal soil for trees has good structure, is moist but well drained, and is relatively high in organic matter.

What exactly is “good soil structure”?

Soil is made up of lots of little particles. These particles, whether they are sand, silt or clay have pore spaces between them. The size of the particles themselves is referred to as the soils “texture”. The structure of the soil refers to how these particles come together and the pore spaces between them.

Sand has bigger pore spaces than silt and clay has the smallest pores of all three. Soils with good structure have enough pore spaces to allow for oxygen and water to be accessed by the roots of a tree. When you have poor structure, such as in a heavy clay or a heavily-compacted site, the roots of the tree are limited in their ability to absorb nutrients, water, and oxygen from the soil.

Water in the Soil

Drainage and water availability are fairly obvious factors in the health of a tree. Everyone knows that trees need water to live. The question is how much?

If a tree has too little water during a period of drought then the tree can become stressed. It may wilt for a little while and then bounce back or it could permanently wilt. When this happens the tree has to use more energy to recover or it may die back completely.

Watering newly planted trees is especially important as they do not have as big a root zone as an established tree. Trees that get too much water and have “wet feet” for long periods can also be severely damaged. The tree will not be able to access oxygen from the soil and the roots may begin to rot. Some species are more tolerant of wet conditions than others so planting the right tree for the right site is important. We will discuss that more in our planting blog post.

The structure and texture of the soil combined with the amount of organic matter present will determine how quickly the water drains through the soil. Sand drains water away quickly, while clay takes much longer for the water work its way through the small pore spaces. In each case, working some organic matter into the soil will help improve the outcome.

What is Organic Matter?

Organic matter is all the things nature leaves behind and lets rot.  Its full of tiny organisms that decompose, improve soil structure and make nutrients available for the roots of a tree.

There are all kinds of bacteria, fungi, insects, and worms living in the top layer of a healthy soil. Some fungi (micorrhizae) live in a kind of symbiotic relationship with the roots of the tree allowing the roots to access more of the nutrients in the soil. Adding more organic matter to any type of soil whether sand, silt, or clay will greatly improve its structure, nutrient levels, and water holding capacity.

Most urban soils are lacking sufficient amounts of organic matter because we want to have “clean” yards. We rake away the things that break down over time,  stuff them into a yard waste bag and take them somewhere else. Then we buy it back as compost to add to our gardens.  This breaks the nutrient cycle in our yard. We encourage people to compost in their own yards to save money on waste disposal and soil amendments.

Soil Conditions that Undermine Root/Tree Health

  • Heavy compaction at base of tree
    Heavy compaction at base of tree

    Compacted soils make it difficult for roots to grow and access oxygen, water, and nutrients. Heavy foot traffic, parking on the root zone of a tree and heavy equipment can all lead to compacted soils.

  • Hard Surfaces such as driveways, sidewalks, and patios interrupt the ability of a tree’s roots to spread. They are also devoid of any organic matter and water runs off hard surfaces into the storm drains instead of filtering through the soil.
  • Changes in soil level can greatly reduce a trees survivability. Burying a trees roots and root collar make it difficult if not impossible (depending on how deep the extra soil is) to find oxygen. A tree that has had its roots buried will show signs of decline over time and may eventually die.
  • Construction or trenching in the root zone of a tree can disturb and damage the roots causing problems in the rest of the tree. A tree that has had trenching nearby may not show signs of stress for a few years leading people to wonder what happened when it finally does start to die back. It can sometimes take five or so years to really notice an issue but by then the damage has been done. Installing barriers around the root zone can help protect a tree when construction is happening around it. Hand digging and manually cutting the roots with a sharp exact cut can also reduce some of the stress of trenching.
  • Competition from surrounding plants such as grasses or other garden plants can be hard on a tree. Grass is especially difficult and we suggest mulching trees to limit competition and protect the tree from lawn mower damage.
  • Fertilizer can be harmful to tree roots over time. We recommend to avoid chemicals altogether and instead work to improve soil quality over all making fertilizer unnecessary.
  • Road salt can be very destructive to trees. It can lower nutrient availability/uptake and can cause many problems in the crown such as witches broom and leaf scorch. Some trees are more salt tolerant than others and this should be a consideration when planting near roads.
Mulched Pear

How can I improve soil quality in my yard?

A layer of compost and a good amount of mulch around the roots of a tree can do wonders! It helps add biological activity and organic matter back to the soil under the tree. It also helps prevent competition from grass and other weeds.

Mulch breaks down over time adding more organic matter back to the soil. It is important to not mulch high up on the trunk of the tree as this can lead to moisture problems and rot. Add the mulch around it like a doughnut making sure that it does not bury the root collar.  Do not mulch higher than 6 inches above the soil. This can also limit the availability of oxygen. It is better to mulch often than too much all at once.

Other Steps to Take:

  • Have your soil tested for pH and macro nutrients.
  • Avoid heavy compaction – do not park on tree roots!
  • Use Myke when planting to help introduce helpful micorrhizae to the soil
  • Use compost around trees/in the garden and rake a thin layer over the lawn once every year or so.
  • Avoid the use of chemical fertilizers, herbicides and pesticides.
  • Limit the use of salt in the winter or try to find a eco-friendly alternative.

Whether you have newly planted or mature trees the soil they are in will be the limiting factor to their success. If you want more information on how the soil in your yard may be impacting your tree contact us for a consultation.