Soils

What is soil?

• Soil is a complex mixture of eroded rock, minerals, nutrients, decaying organic matter, water air, billions of organisms, microscopic decomposersorganisms, microscopic decomposers

• Soil is potential renewable resource but it takes a long time to make it– Created by weathering of rocks, deposits of minerals, decomposing dead organisms of minerals, decomposing dead organisms and organic materials

What is so important about soil?

• It influences the productivity of ecosystems

• Provides plants important nutrients

5 Soil Forming Factors

1. Parent material– Determines the starting of mineral composition and particle size

– Factors that may move or influence; wind, water, gravity, glacial

2. Climatewater, gravity, glacial

2. Climate– Determines the speed of weathering, leaching, etc..

3. Biotic Factors– Influences the distribution and amount of organic matter

5 Soil Forming Factors

4. Topography– Determines the speed of weathering, leaching, etc…

– Influences are slopes, depression, well drained depression, well drained vs. poorly drained

5. Time– The period over which the processes have acted. Determines the maturity and depth of soils

• True soils are influenced, changed, and supplemented by living organisms– Fungus and bacteria decompose dead plants and animals to make humus

– Termites, earthworms, and beetles will move organic material aroundmove organic material around

Humus

• Humus is the dark colored semi-soluble organic substance formed from decomposition of soil organic matter.

• Humus provides soil with a number of very important benefits:– It enhances a soil's ability to hold and store moisture.

– It reduces the eluviation of soluble nutrients from the soil profile. nutrients from the soil profile.

– It is the primary source of carbon and nitrogen required by plants for their nutrition.

– It improves soil structure which is necessary for plant growth.

Soil Horizons

• Mature soil are arranged in a series of zones called soil horizonshorizons

Soil Horizons are caused by

1. Weatheringa. Physical, chemical or biological breakdown of rocks and minerals into smaller sized particles

2. Eluviationa. Movement of humus, chemical substances, and mineral

particles from the upper layers of a soil to lower layers by a. Movement of humus, chemical substances, and mineral

particles from the upper layers of a soil to lower layers by the downward movement of water through the soil profile.

3. Illuviationa. Deposition of humus, chemical substances, and fine mineral

particles in the lower layers of a soil from upper layers because of the downward movement of water through the soil profile

4. Organic decomposition

• Each horizon has a distinct texture and composition – These will vary with different types of soil

• Soil scientists will examine the cross-examine the cross-sectional view soil horizon called soil profile– Most mature soils have at least 3 soil horizons

• Soil can have up to 5 horizons– Types of horizons: O, A, B, C, R

• O horizon– Uppermost layer– Also known as the – Also known as the surface-litter layer or organic layer

– Made up of plant material, fungi, animal wastes, twigs

– Normally black or brown in color

• A horizon– Just below the O horizon

– Also know as the – Also know as the topsoil layer

– Mainly composed of minerals and humus

– Dark in color, light in texture, and porous

• Both the O and A soil layers contain most of the roots and stems of plants

• Well-developed soils that contain bacteria, fungi, earthworms, etc…bacteria, fungi, earthworms, etc…

• Humus is well developed in A horizon– Produces high crop yields

– Helps hold water and nutrients

• Color of topsoil can tell a lot about how useful it will be to use for growing crops– Dark brown or black topsoil is nitrogen rich and high in organic and high in organic matter

– Gray, bright yellow, or red topsoil are low in organic matter and need nitrogen to support crops

• B horizon– Below the A horizon

– Also called subsoil

– Influenced by illuviation and receives eluviated material from the A horizon

– More dense – More dense

– May be reddish in color due to the iron oxides from the layers above

• C horizon– Composed of the parent material

– Particle size varies from small particles to boulder sizeboulder size

• R horizon– Unweathered bedrock

• Soils develop and mature slowly– Takes about 200-1000 years to develop an inch of topsoil

• The more soil horizons visible means the • The more soil horizons visible means the ecosystem is older and well-established

• Leaching– Process in which water removes and transports soil humus and inorganic nutrients in solution

E Horizon

Soil Texture

• Texture refers to the size distribution of the mineral particles found in a representative sample of soil. – Particles are normally grouped into three main classes: sand, silt, and clay. Particles are normally grouped into three main classes: sand, silt, and clay.

– Soils with equal mixtures of sand, silt, and clay are called loams

Type of Mineral Particle Size Range

Sand 2.0 - 0.06 millimeters

Silt 0.06 - 0.002 millimeters

Clay less than 0.002 millimeters

• Clay is probably the most important type of mineral particle found in a soil. – Particles have a very large surface area relative to their volume.

• Highly reactive• Highly reactive• Has the ability to attract and hold positively charged nutrient which then are available to plant roots for nutrition.

– Particles somewhat flexible and plastic because of their lattice-like design.

• Allows clay particles to absorb water and other substances into their structure.

• When feeling soil to describe its texture, you take a small amount of moist soil and rub between your fingers and thumb– If it feels gritty, means there is a lot of sand

– If it feels sticky, it means there is a lot of clayclay

• Should be able to roll in clump

– If it feels smooth like flour, then there is a lot of silt

• Soil texture helps determine the porosity of soil– Soil porosity measures the volume of pores or spaces per volume of soil and the or spaces per volume of soil and the average distance between those pores

• More pores, more water and air can be held in soil

• The number of pores within soil helps determine soil permeability• Soil permeability = the rate at which water and air move from upper to lower soil layers

• Soil permeability, porosity, and texture determine the soil’s water-holding capacity, oxygen content, and workabilityoxygen content, and workability• Important for crops

• Loams are the best soils for growing most crops • Hold lots of water but do not tightly constrict plant roots

• Sandy soils are easy to work with but water runs quickly through them

• Clay soils can hold a lot of water but • Clay soils can hold a lot of water but tend to be clumpy and dense

pH of Soil

• One of the most important chemical properties influencing reactions in a soil is pH.

• Influences the uptake of nutrients by • Influences the uptake of nutrients by plants

• Affects soil fertility– Best is between a pH of 6.0-7.2

pH of Soil

• Primarily controlled by the concentration of free hydrogen ions in the soil matrix.

• Soils with a relatively large concentration of hydrogen ions tend to

• Soils with a relatively large concentration of hydrogen ions tend to be acidic.

• Alkaline soils have a relatively low concentration of hydrogen ions (more basic).

• If soil is too acidic, an alkaline substance (e.g. lime) is added.– Lime speeds up the decomposition of organic matter in soils

• Manure and other organic fertilizer are added to soils to maintain soil fertilityadded to soils to maintain soil fertility

• If soil is too basic (too much Ca+), sulfur is added– Bacteria will convert it to sulfuric acid to decrease pHdecrease pH

Human Influence on Soil

• Over tilling (turning over of soil)– Disturbs the organisms that live in the topsoil

– Dries out the soilIncreases erosion (both wind and water)– Increases erosion (both wind and water)

• Burning of fossil fuels– Releases sulfur dioxides and nitrogen oxides which leads to acid deposition

• Leads to loss of soil fertility, reduce crop and trees growth, makes plants vulnerable to drought, disease, and pests