Natural Notes: Soil Science
This post consists of my notes from a lecture on soil science as part of a training to become a certified arborist by the International Society of Arborists. The speaker was Brenda Ochhiuzzo, a natural resources specialist with the Forest Preserves of Cook County and her lecture took place on January 23, 2015 at the Salt Creek Resource Management facility in Willow Springs, Illinois.
Introduction
Trees and soil are interdependent. Soil has the greatest health factor on a tree. When planting and managing trees in urban environments, soil texture, structure, pH, and water-holding capacity are often overlooked during planning.
Biological Properties of Soil
Soil it its own ecosystem. There are billions of organisms living within the soil. Worms accelerate decay and aeration. Nematodes are parasites on roots and can carry disease. Bacteria and fungi can help decompose. Native soil is soil that is the result of thousands or even millions of years of weathering from bedrock (parent) material.
Soil layers
There are five major soil horizons (layers):
Nutrient Cycling
Trees grow and absorb essential minerals from soil solution (hence one of the reasons they need water). This allows them to produce new leaves and woody material. During a tree's seasonal cycle, leaf drop will occur, hence starting again nutrient cycling.
Soil Texture
Soil texture refers to the fine or course material such as sand, silt, or clay (figure 2). Soil texture affects the ability for it to hold water and oxygen for tree roots.
Ideal soil (loam soil) is a mix of three different particle sizes (textures). Ideal soil consists of 50% solids and 50% pore space for water and air.
Soil aggregates form soil structure and are clumped together. The size and shape of soil aggregates are important factors to water and air uptake by tree roots.
Compaction reduces pore spaces and can inhibit percolation and increase runoff (figure 3).
Half (50%) of soil is space or pores between particles. There are two types of pores. Macropores are larger spaces mainly between aggregates and are too large to hold water. Microspores are found between soil particles and retain water.
Bulk density refers to the weight of dried soil. It measures soil airspace. The lower the bulk density, the lower the weight and the more pore space. The higher the bulk density, the higher the weight and less pore space. Soil compaction (see figure 4) reduces total pore space and alters the proportion of macropores to micropores.
Soil pH
pH is the measure of alkalinity or acidity (see figure 5). It ranges from 0-14 and is logarithmic. A soil pH of 5.5 or lower results in the unavailability of phosphorous while other elements may be toxic. Certain pH compounds form insoluble in water and may be toxic to trees.
Buffering capacity is the ability to resist change of pH in soil. Soil amendments can be used as a buffer, but are temporary and change the pH only in a small area for a short amount of time.
Soil minerals
Soil minerals are required for tree growth. Minerals dissolve in water which allows roots to absorb them. Elements in solutions consist of negatively and positively charged ions, or anions and cations, respectively. Cation exchange capacity (CEC) is the soil's ability to attract, retain, and exchange positively charged ions. CEC is used by arborists to gauge soil fertility. Organic matter and clay have negative density - they attract and hold cations which helps to retain water, soil, minerals, and essential elements. This attraction also minimizes leaching.
Cations and anions are essential elements in soil. However, ions can accumulate to harmful levels.
Soils with excess soluble salts are saline soils. Soils with the cation sodium (Na+) in high percentage are called sodic soils. Sodic soils can form crusts, can have a high pH, and are toxic to some plants. A correction to this problem is possible using low sodium irrigation water or gypsum.
The Rhizosphere
The rhizosphere is the area of intense biological activity near elongating roots (see figure 6).
Actinomycetes are bacteria that decompose organic material to form hummus.
Mycorrhizae are also found within the rhizosphere. Some tree species form mycorrhizae networks where some trees are interconnected by sharing mycorrhizae root fungus. New trees arrive within mycorrhizal network (e.g. Douglas Fir forests).
Water Holding Capacity
Water holding capacity is the amount of water soil can hold. Clay has higher water holding capacity because it has more micropores. Sand has less water holding capacity because it has more macropores.
Gravitational water drains from larger macropores. Field capacity is the remaining water that is held by the soil particles after gravitational water has drained away (capillary water is retained in micropores). See figure 7.
The infiltration rate of water is dependent on soil structure (e.g. course sand or fine clay).
Urban Soils
Urban soils are often altered, and are generally unfavorable for tree growth due to compaction (which is often the biggest problem!). Structured soils are "designed" to be compacted to meet engineering requirements and allows for tree root growth and development. Also, suspended sidewalks provide adequate root space under an urban environment. The use of structural cells to distribute vehicle and pedestrian weight are also effective amendments for trees in urban soils. Improvements to the soil itself may also be performed to correct issues, such as addressing drainage problems or introducing organic matter.
Introduction
Trees and soil are interdependent. Soil has the greatest health factor on a tree. When planting and managing trees in urban environments, soil texture, structure, pH, and water-holding capacity are often overlooked during planning.
Biological Properties of Soil
Soil it its own ecosystem. There are billions of organisms living within the soil. Worms accelerate decay and aeration. Nematodes are parasites on roots and can carry disease. Bacteria and fungi can help decompose. Native soil is soil that is the result of thousands or even millions of years of weathering from bedrock (parent) material.
Soil layers
There are five major soil horizons (layers):
 |
| Figure 1: The soil profile and soil horizons |
Nutrient Cycling
Trees grow and absorb essential minerals from soil solution (hence one of the reasons they need water). This allows them to produce new leaves and woody material. During a tree's seasonal cycle, leaf drop will occur, hence starting again nutrient cycling.
Soil Texture
Soil texture refers to the fine or course material such as sand, silt, or clay (figure 2). Soil texture affects the ability for it to hold water and oxygen for tree roots.
 |
| Figure 2: Soil textures |
Soil aggregates form soil structure and are clumped together. The size and shape of soil aggregates are important factors to water and air uptake by tree roots.
Compaction reduces pore spaces and can inhibit percolation and increase runoff (figure 3).
 |
| Figure 3: The effects of soil compaction. |
Half (50%) of soil is space or pores between particles. There are two types of pores. Macropores are larger spaces mainly between aggregates and are too large to hold water. Microspores are found between soil particles and retain water.
Bulk density refers to the weight of dried soil. It measures soil airspace. The lower the bulk density, the lower the weight and the more pore space. The higher the bulk density, the higher the weight and less pore space. Soil compaction (see figure 4) reduces total pore space and alters the proportion of macropores to micropores.
 |
| Figure 4: Soil compaction |
pH is the measure of alkalinity or acidity (see figure 5). It ranges from 0-14 and is logarithmic. A soil pH of 5.5 or lower results in the unavailability of phosphorous while other elements may be toxic. Certain pH compounds form insoluble in water and may be toxic to trees.
 |
| Figure 5: Simplified pH scale. |
Soil minerals
Soil minerals are required for tree growth. Minerals dissolve in water which allows roots to absorb them. Elements in solutions consist of negatively and positively charged ions, or anions and cations, respectively. Cation exchange capacity (CEC) is the soil's ability to attract, retain, and exchange positively charged ions. CEC is used by arborists to gauge soil fertility. Organic matter and clay have negative density - they attract and hold cations which helps to retain water, soil, minerals, and essential elements. This attraction also minimizes leaching.
Cations and anions are essential elements in soil. However, ions can accumulate to harmful levels.
Soils with excess soluble salts are saline soils. Soils with the cation sodium (Na+) in high percentage are called sodic soils. Sodic soils can form crusts, can have a high pH, and are toxic to some plants. A correction to this problem is possible using low sodium irrigation water or gypsum.
The Rhizosphere
The rhizosphere is the area of intense biological activity near elongating roots (see figure 6).
 |
| Figure 6: Simple diagram of the rhizosphere |
Mycorrhizae are also found within the rhizosphere. Some tree species form mycorrhizae networks where some trees are interconnected by sharing mycorrhizae root fungus. New trees arrive within mycorrhizal network (e.g. Douglas Fir forests).
Water Holding Capacity
Water holding capacity is the amount of water soil can hold. Clay has higher water holding capacity because it has more micropores. Sand has less water holding capacity because it has more macropores.
Gravitational water drains from larger macropores. Field capacity is the remaining water that is held by the soil particles after gravitational water has drained away (capillary water is retained in micropores). See figure 7.
 |
| Figure 7: Field capacity, saturation, and wilting point |
Urban Soils
Urban soils are often altered, and are generally unfavorable for tree growth due to compaction (which is often the biggest problem!). Structured soils are "designed" to be compacted to meet engineering requirements and allows for tree root growth and development. Also, suspended sidewalks provide adequate root space under an urban environment. The use of structural cells to distribute vehicle and pedestrian weight are also effective amendments for trees in urban soils. Improvements to the soil itself may also be performed to correct issues, such as addressing drainage problems or introducing organic matter.
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