Earth’s Resources: Soil

What is soil? The outermost layer of the earth’s crust The outermost layer of the earth’s crust Made up of both organic (from living things) and inorganic materials. Made up of both organic (from living things) and inorganic materials. It’s divided into layers according to the ratio of organic to inorganic material at various depths It’s divided into layers according to the ratio of organic to inorganic material at various depths It’s classified according to what kind of material you can find in a sample It’s classified according to what kind of material you can find in a sample

Layers of the soil The layers vary as to content The layers vary as to content The closer to the top, the more organic material you’ll find The closer to the top, the more organic material you’ll find Leaves Sticks Dead animals (insects as well as larger critters) Finally, at the bottom, you will hit BEDROCK!

Layers = HORIZONS The topmost layer, or O horizon, includes “litter” or detritus from formerly living things. The topmost layer, or O horizon, includes “litter” or detritus from formerly living things. The A horizon is the most valuable – It is the topsoil where plants get most of their nutrients. The decaying organic matter in this layer is called humus. The A horizon is the most valuable – It is the topsoil where plants get most of their nutrients. The decaying organic matter in this layer is called humus. (litter)

Humus As living things (both dead plants and dead animals) decompose, their molecular components become available for new plants to take up as nutrients. As living things (both dead plants and dead animals) decompose, their molecular components become available for new plants to take up as nutrients. This decayed material is called humus. This decayed material is called humus. Humus is mostly insoluble, so it remains in these upper layers of soil. Humus is mostly insoluble, so it remains in these upper layers of soil. It is very dark, nearly black in color. It is very dark, nearly black in color. Human created humus is called compost! Human created humus is called compost!

TOPSOIL Five tons of topsoil spread over an acre would only be as thick as a dime! Five tons of topsoil spread over an acre would only be as thick as a dime! It takes over 500 years for organic material to collect and decompose naturally to form only ONE INCH of topsoil! It takes over 500 years for organic material to collect and decompose naturally to form only ONE INCH of topsoil! 5 to 10 tons of animal life can exist in an acre of topsoil! 5 to 10 tons of animal life can exist in an acre of topsoil!

The E horizon is where the nutrients from the organic material (humus) begin to dissolve. The E horizon is where the nutrients from the organic material (humus) begin to dissolve. The deepest of plants’ roots sometimes extend this far. The deepest of plants’ roots sometimes extend this far. Layers = HORIZONS (litter)

Leaching After rain or other sorts of precipitation, water moves into the soil. This is called infiltration. After rain or other sorts of precipitation, water moves into the soil. This is called infiltration. As it moves from the topsoil layer into the subsoils and deeper, it dissolves inorganic minerals from the upper layers. This is called leaching. As it moves from the topsoil layer into the subsoils and deeper, it dissolves inorganic minerals from the upper layers. This is called leaching. Minerals and other substances may be carried into groundwater supplies and thus into drinking water in this manner. Minerals and other substances may be carried into groundwater supplies and thus into drinking water in this manner.

The B horizon The B horizon This layer contains a smaller amount of organic material. This layer contains a smaller amount of organic material. It has a heavy concetration of the minerals leached from the above layers. It has a heavy concetration of the minerals leached from the above layers. Layers = HORIZONS (litter)

The C and R horizons are the underlying rock. The C and R horizons are the underlying rock. C horizon is weathered and broken rock or regolith. Leached minerals may be found here. C horizon is weathered and broken rock or regolith. Leached minerals may be found here. The R horizon is bedrock (almost solid crustal material.) The R horizon is bedrock (almost solid crustal material.) Layers = HORIZONS

Layers = Horizons Soil in different locations may or may not have all of these horizons Soil in different locations may or may not have all of these horizons For instance, in a construction zone, all of the topsoil may have been removed. For instance, in a construction zone, all of the topsoil may have been removed. Topsoil may have eroded through wind or water effects. Topsoil may have eroded through wind or water effects. Heavy earthquake or storm damage may have disrupted the existing layers Heavy earthquake or storm damage may have disrupted the existing layers

Examples of Soil Profiles Limestone soil in the USVI (organic limestone) Loamy sand from Kansas

Other components of soil Water – after rains or floods, water infiltrates the soil, carrying the nutrients to plant roots and into lower horizons Water – after rains or floods, water infiltrates the soil, carrying the nutrients to plant roots and into lower horizons Air – soil is composed of approximately 25% air, aided by the movement of worms and insects in the topsoil layers Air – soil is composed of approximately 25% air, aided by the movement of worms and insects in the topsoil layers 5% living things – plant roots, worms, microorganisms make up part of the soil. 5% living things – plant roots, worms, microorganisms make up part of the soil.

Types of Soils Soil is classified according to the type of mineral base Soil is classified according to the type of mineral base Sand Sand Silt Silt Clay Clay Loam Loam

Sandy Soil Sandy soil has large particles which don’t fit together well Sandy soil has large particles which don’t fit together well Water (and dissolved nutrients) seep right through and are not usable by plants’ roots Water (and dissolved nutrients) seep right through and are not usable by plants’ roots This type of soil dries out quickly This type of soil dries out quickly Particles feel grainy Particles feel grainy

Silty Soils Particles are smaller than sand Particles are smaller than sand Particles are very light, they erode easily with both wind and water Particles are very light, they erode easily with both wind and water With water, this is what we think of as mud. With water, this is what we think of as mud. Particles feel like flour Particles feel like flour Dusty texture when dry Dusty texture when dry

Clay-type soils Clayey soil has very small particles, and gums together. It holds water and nutrients well, but plants find it hard to root in clay soil. Clayey soil has very small particles, and gums together. It holds water and nutrients well, but plants find it hard to root in clay soil. While it holds water well, it takes a long time for water to permeate a clay soil. While it holds water well, it takes a long time for water to permeate a clay soil. Brief, heavy rains may not reach plant roots. Brief, heavy rains may not reach plant roots. Particles feel slippery Particles feel slippery

Loamy Soil The perfect mix of particle size and texture to hold water and nutrients! The perfect mix of particle size and texture to hold water and nutrients! About 40% sand About 40% sand 40% silt 40% silt 20% clay 20% clay

Types of soils The ideal type of soil for plant (crop) growth is partway between the three types. The ideal type of soil for plant (crop) growth is partway between the three types. Loamy soil has neither too small nor too large particles (It’s just right!) Loamy soil has neither too small nor too large particles (It’s just right!) Able to drain, but also hold nutrients Able to drain, but also hold nutrients Loamy soil has a good percentage of decayed plant material or humus which provide the nutrients that plants need. Loamy soil has a good percentage of decayed plant material or humus which provide the nutrients that plants need.

Further Resources National Resources Conservation Service website. Try out the Web Soil Survey tool to see what’s under your feet National Resources Conservation Service website. Try out the Web Soil Survey tool to see what’s under your feet National Resources Conservation Service National Resources Conservation Service US Environmental Protection Agency US Environmental Protection Agency US Environmental Protection Agency US Environmental Protection Agency Digging to China? Digging to China? Digging to China? Digging to China? Introduction to Soils Introduction to Soils Introduction to Soils Introduction to Soils