The Dirt on Dirt: Soil Composition and Importance

Dirt is a complex, finite natural resource. Dirt, or soil, provides plants with nutrients and water, and supports root structure and development. It is a growing medium, but also a habitat and ecosystem. What if I told you it was also a living entity? Soil has both biotic (living) and abiotic (non-living) components. These components combine to give it the important features and properties that make soil vital to life on earth. 

Soil science is an entire field of its own. What grows best in what soil? How can we modify soil compositions to improve yield or resilience of crops or vegetation? Is there an ideal soil composition? Soil science has 2 main branches: pedology, which focuses on formation, chemistry, morphology and classification of soils, and edaphology, which studies how soil affects living organisms, including plants and trees. Applications include agricultural management, environmental conservation and urban planning. Knowing what is making up the ground beneath our feet is very important to everyone and everything!

Formation of Soil:

Soil is formed by the weathering of rocks. Physically, rock is mechanically weathered by wind, ice or temperature changes which produces smaller rock particles. Chemically, water and atmospheric gases alter rock to break it into minerals. Finally, biologically, living organisms break down rock and minerals into organic acids that aid in mineral dissolution. Accumulation of organic matter (dead plant and animal matter) builds soil fertility and enhances the structure of soil. Organic matter and weathered rock are combined via leaching (aka water soluble substances are washed through the soil layers). There are 5 horizons or levels to the profile of soil: 

1. O- Organic humus- Surface litter, partially decomposed organic matter

2. A- Topsoil- contains humus (dark organic matter), living organisms and inorganic material.

3. B-Subsoil- Iron, aluminum and humic compounds accumulate and leach downwards from Horizon A(Topsoil).

4. C-Parent material- weathered parent material or inorganic material partially broken down.

5. R- Bedrock- Unweathered, compacted rock. 

Soil Components:

The components are always the same, but the composition is different depending on the region. The components include minerals, organic matter, water, air and organisms. The most ideal or average composition is 45% minerals, 5% organic matter, 25% water, and 25% air plus the organisms. 

• Mineral Particles - weathered rock broken down to particle size.

  • Sand - 0.05mm to 2mm sized particles, coarse/gritty texture, high drainage capabilities and low nutrient retention;

  • Silt - 0.002mm to 0.05mm sized particles, smooth texture, retains moisture and nutrients, compactable;

  • Clay- less than 0.002mm sized flat particles, dense, sticky texture, retains water and nutrients very well, poor drainage. 

• Organic matter- decomposed animal and plant matter.

  • Supports:

    • Nutrient supply- releases nutrients in a usable form;

    • Soil structure- binds mineral particles;

    • Moisture retention- increase organic matter increases water retention;

    • Microbial activity- cycles nutrients and improves soil health.

• Water- soil moisture. 

  • Acts as: 

    • Solvent for nutrients- dissolving nutrients and minerals into a solution so that they are available for plant uptake; 

    • Organism support- plants and animals, including microbes, rely on water for basic function;

    • Influence for physical and chemical properties. 

• Air- air in the soil is not the same as air in the atmosphere due to the biological activity in the soil. Gas exchange between the soil and atmosphere enables biochemical processes such as nutrient cycling and decomposition. Air is held between the pores of mineral particles. High moisture levels or compaction can lead to reduced pore size, and thus reduced air in the soil. In these cases, aeration is often required to remediate the soil.

  • Oxygen (O2)- Level in soil less than level in atmosphere;

  • Carbon dioxide (CO2)- Level in soil more than level in atmosphere.

• Organisms- bacteria, fungi, protozoa, nematodes, earthworms, insects and small mammals all inhabit the soil and contribute to the most complex ecosystem on earth. 

  • Important for:

    • Decomposition- break down nutrients and recycle organic matter;

    • Soil structure- burrowing insects/mammals and earthworm activity creates channels that improve aeration and drainage;

    • Nutrient cycling- microbial activity converts organics and inorganics into plant accessible forms;

    • Disease suppression- diverse soil microbe populations suppress soil-borne diseases.

Types of Soil:

• Sand- mostly made up of sand, with small percentage of silt/clay

  • Coarse

  • Well draining

  • Nutrient poor

• Silt-  mostly made up of silt, with small percentage of sand/clay

  • Smooth

  • Retains water, but well draining if not compacted

  • Retains nutrients well

• Clay- mostly made up of clay, with small percentage of sand/silt

  • Dense

  • Retains water, poor drainage

  • Nutrient rich

• Loam- 40% sand, 40% silt, 20% clay

  • Soft and crumbly

  • Good water retention and drainage

  • Good nutrient retention

• Peat- High in organic matter

  • Spongey and acidic

  • Retain water

  • Nutrient rich

• Chalky- High in calcium carbonates 

  • Gritty and alkaline

  • Well draining

  • Poor nutrient retention

Physical Properties:

Color

Soil color changes based on organic matter, minerals and moisture levels. Soil color is often associated with the quality of soil, but the color of the soil does not provide enough information to determine “quality”. It can however give clues about its condition or make up. For example, dark soil can suggest wet soils, while light colored soil can indicate a high sand component. 

Texture 

Texture is based on the individual particle type and its composition within the soil. A particle's ability to react to its environment is directly related to its surface area. This is a permanent feature. You cannot change the texture of the soil without changing its composition or surface area entirely, which is a much bigger feat than it sounds. Let's consider a 6in deep 1 acre lot. If the water retention of the soil is too high, one may suggest adding sand to change the texture and increase drainage potential. That 6in deep 1 acre lot has a total mass of 2 million lbs. To change the sand composition even 1%, 20 000lbs of sand would need to be added and mixed into the lot to make even a small difference in overall texture. 

Structure 

Structure refers to how the components act together. We are no longer considering the individual particles, but a group of particles into larger pieces known as aggregates. Aggregates do not have texture but granular structure. Good granular structure allows movement of air and water through the soil, while poor granular structure can decrease movement and limit root growth. Topsoil should have a good granular structure that is similar to cookie crumbs. 

Water-holding Capacity 

Water retention is based on the pore size between particles. Water enters the soil via precipitation or irrigation and it exits via evaporation and transpiration through leaves. Ideally, soils will have equal amounts of water and air filling the pores for optimal water-holding capacity, or "field capacity”. This capacity changes when forces of gravitational pull over power the force of cohesion between soil components and pores. If pore sizes are too big or too small, gravity will cause water molecules to leach past topsoil to subsoils or bedrock where it is inaccessible to plants. 

Chemical Properties:

pH

pH stands for “Potential Hydrogen” and dictates the acidity or alkalinity of a solution. This is important to soil science because it determines the solubility of nutrients and thus nutrient availability to the plant or tree. Lower pH (0-6) suggests acidic conditions and high pH (8-14) suggests alkaline conditions, with neutral conditions being 7. Ideal growing conditions for general plant life is 5-7. A change in pH of soil is caused by differing moisture levels, high temperatures, and the presence of some minerals, including Manganese, Zinc, and Aluminum. pH is a constantly changing condition and there is no one-size-fits all for growing conditions. Each plant or tree species will require different amounts of micro and macro nutrients, all of which require a certain pH to be soluble and available for uptake by surrounding roots. So if your tree or plant is declining due to lacking a certain mineral, it could just be that the pH of your soil is just not allowing that mineral to be soluble or available. 

Cation Exchange Capacity

CEC or Cation Exchange Capacity determines the retention of nutrients in soil. Nutrients are positively charged and will only be retained if the soil has an appropriate negative charge (i.e. positive and negative charges attract). As water flows through soil with high CEC, the high negative charge of the soil will hold the positively charged nutrients rather than let them flow along with the water. Soil condition and composition both contribute to CEC. For example, sandy soils typically have lower CEC due to particle size and texture which does not hold negative charge or water well. Contrarily, organic matter and clay can increase CEC due to its water holding capacity and high content of positive nutrients. pH also affects the CEC of soil. When the pH drops (becomes more acidic), the concentration of positive Hydrogen ions are reduced, increasing the negative charge as a result. An increase in pH works in the opposite way- an increase in Hydrogen concentration reduces the negative charge of the soil, leading to a decrease in CEC. Ideally, soil will maintain a high CEC to support plant life. Rapid changes in the charge of soil, or CEC, can lead to a shock to the plants, jeopardizing the stability of the soil as a whole ecosystem.

Importance:

Now, that is a lot of information on dirt! So how is this information helpful or important? On a global scale, healthy soils support ecosystems, water regulation and climate stability. It is the pillar for agriculture, forestry and food production, making it essential to food security and human livelihood. Soil science provides hope for pollution mitigation, with research into its natural filtering and binding capacity at an all-time high. In a world that is struggling economically, ecologically and sociologically, it is hard to believe that dirt can be the answer to all our problems. But it very well could be. Soil provides raw materials  for construction, manufacturing and horticulture, all of which are intimate parts of building a community. Soil health is an accumulation of all of its properties and functions, and directly relates to the health of all life in that region. If everyone had a better understanding of how soils work, how to assess the health of local soils and how to optimize its potential, we could build healthier communities underpinned by the vitality of DIRT.