Thursday 30 July 2009

Soil Gas

Land Fill Without Trees

Soil gas is often the forgotten member of the the soil composition trilogy. While the solid and liquid proportions get all the attention the soil atmosphere is often left literally up in the air.

Soil consists of 3 primary components solids liquids and gases. The solid components take up approximately half of the total soil volume and consist mostly of minerals and a small percentage of roots, plants, and animals referred to as the organic component. The other half of soil consists of even amounts of soil water and soil atmosphere.
While the solids and liquids perform obvious functions like support and nutrient transport, soil gases are more nebulous in their critical relationship to tree growth.
This need for air should be second nature to us all as we share the need for oxygen with our green friends. Without oxygen there is no life and gas exchange is critical to all plants. While
green photosynthetic tissues give off oxygen they also give off carbon dioxide when they burn energy to create chemicals and perform life processes. This process is called respiration and we animals do this part ourselves. Roots and stems do not normally photosynthesize and respire almost exclusively. They need and ample supply of oxygen and good gas exchange to keep everything in order.
Presence of oxygen is the reason that 80% of all tree roots are concentrated in the upper ten inches of the soil.
The arch enemy of soil gas exchange is soil compaction. Soil compaction reduces and can even eliminate small pore spaces in soils. Without soil pore spaces fresh air can not enter the soil and sour gases from decomposition or plant waste processes become toxic to tree roots. Tree roots will often be seen growing at or close to the surface of the soil. More than a futile attempt to anger the lawn mower, this is an indication of compacted soils. Soils that are saturated with water also force tree roots to grow closer to the soils surface. Excess soil moisture can exclude soil gases and eliminate root activity. This is what happens in the classic case of killing with kindness by over watering potted plants. The same thing can happen to trees and shrubs in our landscapes.
The composition of soil atmosphere can have a beneficial or detrimental effect on root activity. Loose porous soils exchange gases like carbon dioxide, created in root respiration and allow for fresh oxygen laden air to reach actively growing roots. Benificial mychorizal fungi benefit from the presence of oxygen and depend on gas exchange to thrive.
When soil moisture levels are high, oxygen levels become low and tree roots die and decay. Roots decaying with out oxygen give off toxic gases like methane that further injure living roots and soil organisms.
In some instances buried or decaying vegetation or construction debris in the sub soil can give off toxic gases that kill roots. Yards that are built on fill or with soil that has been deposited on existing vegetation when grades were drastically altered can slowly decompose. Decomposition gasses are the primary reason that trees are seldom seen growing on old landfills. If you have more questions about soils and soil composition contact your Certified Arborist.