Soil Composition
Soil is formed of four distinct parts: minerals (45%), organic matter (5%), water (25%) and air (25%). Soil occurs in layers, each of which has distinct composition and properties. The mineral portion comes from weathered rock. It provides essential nutrients for plants as well as pore spaces for water and air. Different rocks are composed of different minerals, so soils that come from different parent material will vary in mineral composition and chemical properties. Rocks rich in aluminum form acidic soils whereas those with magnesium and iron silicates form soils that may be deficient in calcium, nitrogen, and phosphorus.
The age of soil also affects its mineral composition. Older soils are more weathered and lower in certain essential nutrients. Large portions of Australia, South America and India have old, infertile soils whereas in the Northern Hemisphere, relatively new soils were formed when glaciers pulverized bedrock as they moved across the land in the last ice age. (10,000 years ago) Fertile soils are also found in areas of volcanic activity where fresh layers of lava create new rock to be weathered in to soil.
Animal waste, leaf litter and the decomposing remains of plants and animals make up the organic portion of soil. This portion is full of microorganisms such as bacteria and fungi that decompose the organic material. During decomposition, essential nutrients are released into the soil. Organic matter also increases the soil's water-holding capacity by acting much like a sponge. These increases in nutrients and water holding capabilites are why many gardeners put organic matter into their gardens. The dark brown or black organic matter that remains after decomposition is called humus. (not to be confused with the chickpea spread you buy at the grocery!) On average, humus persists in soil for about 20 years. Earthworms, ants and termites continue to break it down further.
Soil has many pore spaces around the soil particles. This takes up about 50% of the volume of soil and is filled with air or water. Both are needed to produce a moist and aerated soil that sustains plants and other organisms that live within the soil. Water is generally held in the smaller pores (less than .05 mm in diameter). Air would typically be found in larger pores.
Precipitation drains downward into soil and some is absorbed by the roots of plants and some "sticks" to the soil particles. The remaining water percolates or leaches through the layers of soil carrying dissolved nutrients with it. The deposition of leached minerals into a lower level of soil is called illuviation.
Air in the soil is similar in composition to that in the atmosphere. However, organisms in the soil create carbon dioxide as a result of respiration. As a result, there is typically more carbon dioxide and less oxygen in soil than in atmospheric air. Nitrogen is another important gas found in soils. It is used by nitrogen-fixing bacteria that take nitrogen and make it accessible for uptake by plants.
It is important to note that the carbon dioxide in the soil combines with water to form carbonic acid. This is a weak acid that accelerates the weathering process of rocks during soil formation.
The age of soil also affects its mineral composition. Older soils are more weathered and lower in certain essential nutrients. Large portions of Australia, South America and India have old, infertile soils whereas in the Northern Hemisphere, relatively new soils were formed when glaciers pulverized bedrock as they moved across the land in the last ice age. (10,000 years ago) Fertile soils are also found in areas of volcanic activity where fresh layers of lava create new rock to be weathered in to soil.
Animal waste, leaf litter and the decomposing remains of plants and animals make up the organic portion of soil. This portion is full of microorganisms such as bacteria and fungi that decompose the organic material. During decomposition, essential nutrients are released into the soil. Organic matter also increases the soil's water-holding capacity by acting much like a sponge. These increases in nutrients and water holding capabilites are why many gardeners put organic matter into their gardens. The dark brown or black organic matter that remains after decomposition is called humus. (not to be confused with the chickpea spread you buy at the grocery!) On average, humus persists in soil for about 20 years. Earthworms, ants and termites continue to break it down further.
Soil has many pore spaces around the soil particles. This takes up about 50% of the volume of soil and is filled with air or water. Both are needed to produce a moist and aerated soil that sustains plants and other organisms that live within the soil. Water is generally held in the smaller pores (less than .05 mm in diameter). Air would typically be found in larger pores.
Precipitation drains downward into soil and some is absorbed by the roots of plants and some "sticks" to the soil particles. The remaining water percolates or leaches through the layers of soil carrying dissolved nutrients with it. The deposition of leached minerals into a lower level of soil is called illuviation.
Air in the soil is similar in composition to that in the atmosphere. However, organisms in the soil create carbon dioxide as a result of respiration. As a result, there is typically more carbon dioxide and less oxygen in soil than in atmospheric air. Nitrogen is another important gas found in soils. It is used by nitrogen-fixing bacteria that take nitrogen and make it accessible for uptake by plants.
It is important to note that the carbon dioxide in the soil combines with water to form carbonic acid. This is a weak acid that accelerates the weathering process of rocks during soil formation.