Chapter 2Chapter 2

The SoilThe Soil

Perhaps the greatest evidence of Perhaps the greatest evidence of knowing little about the soil is to label it knowing little about the soil is to label it dirt.dirt.

Soil is the underground environment of Soil is the underground environment of plants.plants.

Soil is the thin outer layer of the Soil is the thin outer layer of the earth’s crust, made up of weathered earth’s crust, made up of weathered minerals, living and nonliving minerals, living and nonliving organisms, water, and air. organisms, water, and air.

A cross-sectional slice made A cross-sectional slice made down into the earth’s surface down into the earth’s surface to show the different layers to show the different layers of soil formations is called a of soil formations is called a soil profile.soil profile.

Over time, distinctive layers Over time, distinctive layers develop in undisturbed soils.develop in undisturbed soils.

The subsoil is between the The subsoil is between the parent layer and the top soil. parent layer and the top soil.

The subsoil is finely The subsoil is finely weathered like the topsoil, weathered like the topsoil, but it lacks organic matter in but it lacks organic matter in the quantity found in the the quantity found in the topsoil layer.topsoil layer.

Organic matter comes from the Organic matter comes from the decomposition of plant and animal tissue. decomposition of plant and animal tissue.

Humus- organic compounds that do not Humus- organic compounds that do not decompose quickly, eventually succumb decompose quickly, eventually succumb to enzymatic action, forming a complex to enzymatic action, forming a complex mixture.mixture.

Humus

Plowed under green plants are known Plowed under green plants are known as green manure.as green manure.

Organic material increases both Organic material increases both water and mineral holding capacity water and mineral holding capacity of the soil. of the soil.

Water and air exist around and Water and air exist around and between the soil particles.between the soil particles.

As much as 50% of the topsoil may be As much as 50% of the topsoil may be air and water in liquid or vapor form.air and water in liquid or vapor form.

The ratio of air to water depends on the The ratio of air to water depends on the texture of the soil and how wet it is.texture of the soil and how wet it is.

Even though the original parent Even though the original parent stone may be the same or similar, stone may be the same or similar, differences in the subsoil and topsoil differences in the subsoil and topsoil may result from variations in five may result from variations in five things.things.

– Weathering elementsWeathering elements– Soil movementSoil movement– Topography Topography – ClimateClimate– Amount of organic matterAmount of organic matter

Soils that weather from bedrock and Soils that weather from bedrock and remain in place are termed sedentary.remain in place are termed sedentary.

Transported soils have been moved by Transported soils have been moved by forces of nature. forces of nature.

– Colluvial soils have moved in response to Colluvial soils have moved in response to gravity.gravity.

– Alluvial soils are carried in water such as Alluvial soils are carried in water such as rivers.rivers.

– Aeolian soils are transported and deposited Aeolian soils are transported and deposited by windsby winds

– Glacial till is soil deposited by glaciersGlacial till is soil deposited by glaciers

The best agricultural soils are usually The best agricultural soils are usually alluvial and glacial till. alluvial and glacial till.

Weathering parent material forms particles Weathering parent material forms particles of different sizes, called soil separates.of different sizes, called soil separates.

In decreasing order of size, the separates In decreasing order of size, the separates are: are: – Gravel, sand, silt, and clayGravel, sand, silt, and clay

The relative The relative proportion of soil proportion of soil separates of separates of different sizes in different sizes in any one soil create any one soil create the soil texture. the soil texture.

Most soils in nature Most soils in nature contain sand, silt, contain sand, silt, and clay in some and clay in some proportions.proportions.

Loam soil Loam soil proportion- 40% proportion- 40% sand 40% silt 20% sand 40% silt 20% fine clay.fine clay.

SandSand

Particles have assorted shapes and Particles have assorted shapes and sizes, depending on how they were sizes, depending on how they were weatheredweathered

Spaces between particles are largeSpaces between particles are large Water passes through quickly Water passes through quickly

because of the large pore spacebecause of the large pore space Air is present in the greatest quantityAir is present in the greatest quantity Low in mineral nutrientsLow in mineral nutrients

SiltSilt

Particles are irregularly shaped and Particles are irregularly shaped and much smaller than most sand much smaller than most sand particlesparticles

Has greater surface area than sandHas greater surface area than sand Holds water in the soil better than Holds water in the soil better than

sand, but it does not provide as sand, but it does not provide as much space for airmuch space for air

Low nutrient levelLow nutrient level

ClayClay

Very small, plate-like particlesVery small, plate-like particles Possesses the greatest surface area of Possesses the greatest surface area of

all the separatesall the separates Water is held tightly to the particles Water is held tightly to the particles

and passes very slowly through the soiland passes very slowly through the soil Has an adhesive quality when Has an adhesive quality when

moistened and squeezedmoistened and squeezed– This is what sometimes creates a sticky, This is what sometimes creates a sticky,

hard-to-plow fieldhard-to-plow field

At least 17 separate chemical elements At least 17 separate chemical elements has been proven repeatedly through has been proven repeatedly through tests that demonstrate growth tests that demonstrate growth abnormalities when any one of these abnormalities when any one of these essential elements is lacking.essential elements is lacking.

An element is essential if the plant An element is essential if the plant cannot grow and develop normally cannot grow and develop normally without it.without it.

Essential elements are broken down Essential elements are broken down into 2 categories, depending on how into 2 categories, depending on how much of the element is neededmuch of the element is needed– MacronutrientsMacronutrients– MicronutrientsMicronutrients

Essential ElementsEssential Elements

MacronutrientsMacronutrients– Calcium (Ca)Calcium (Ca)– Carbon (C)Carbon (C)– Hydrogen (H)Hydrogen (H)– Magnesium (Mg)Magnesium (Mg)– Nitrogen (N)Nitrogen (N)– Oxygen (O)Oxygen (O)– Phosphorus (P)Phosphorus (P)– Potassium (K)Potassium (K)– Sulfur (S)Sulfur (S)

MicronutrientsMicronutrients– Boron (B)Boron (B)– Chlorine (Cl)Chlorine (Cl)– Copper (Cu)Copper (Cu)– Iron (Fe)Iron (Fe)– Manganese (Mg)Manganese (Mg)– Molybdenum (Mo)Molybdenum (Mo)– Nickel (Ni)Nickel (Ni)– Zinc (Zn)Zinc (Zn)

Plants only obtain carbon, hydrogen, Plants only obtain carbon, hydrogen, and oxygen from sources other than and oxygen from sources other than the soilthe soil

All other nutrients must be obtained All other nutrients must be obtained as minerals from the soil around the as minerals from the soil around the plant’s rootsplant’s roots

Beneficial elements have been found Beneficial elements have been found to promote plant growth in many to promote plant growth in many species, but have not been proven to species, but have not been proven to be absolutely necessary for be absolutely necessary for completion of the plants’ life cycle. completion of the plants’ life cycle.

Plants may exhibit symptoms of Plants may exhibit symptoms of nutrient deficiency for several nutrient deficiency for several reasonsreasons– The element may be lacking totally or The element may be lacking totally or

not be present in sufficient quantitynot be present in sufficient quantity– The element may be bound in a The element may be bound in a

chemical form unavailable or too slowly chemical form unavailable or too slowly available to the plantavailable to the plant

– There may be an overall imbalance of There may be an overall imbalance of nutrients in the soilnutrients in the soil

Nitrogen is the most common Nitrogen is the most common element lacking sufficient quantities element lacking sufficient quantities in the soilin the soil

Leaching is when nitrogen, in the Leaching is when nitrogen, in the nitrate form, is not absorbed by the nitrate form, is not absorbed by the colloidal particles of the soil, it passes colloidal particles of the soil, it passes quickly through the root region of the quickly through the root region of the soil. soil.

Soil particles adhere together to form Soil particles adhere together to form larger particles called aggregates.larger particles called aggregates.

The soils’ water, held between the The soils’ water, held between the particles and granules of the soil particles and granules of the soil contains dissolved mineral salts. This contains dissolved mineral salts. This liquidation is known as the soil liquidation is known as the soil solution. solution.

When soil contains more hydrogen When soil contains more hydrogen ions than hydroxyl ions, this makes ions than hydroxyl ions, this makes them acidic.them acidic.

When soil contains more hydroxyl When soil contains more hydroxyl ions than hydrogen ions, this makes ions than hydrogen ions, this makes them alkaline.them alkaline.

When a soil contains equal When a soil contains equal concentrations of hydrogen and concentrations of hydrogen and hydroxyl ions, its termed neutral. hydroxyl ions, its termed neutral.

pH=7 neutralpH=7 neutral pH<7 acidicpH<7 acidic pH>7 alkalinepH>7 alkaline

Cation exchange refers to the capacity Cation exchange refers to the capacity of colloidal particles to attract positively of colloidal particles to attract positively charged ions (cations) and to exchange charged ions (cations) and to exchange one ion for another. one ion for another.

Without cation exchange, nutrients Without cation exchange, nutrients would be readily leached from the soil.would be readily leached from the soil.

To make a soil more acidic (lower pH), To make a soil more acidic (lower pH), sulfur is usually added to the soilsulfur is usually added to the soil

To make a soil more alkaline (raise pH) To make a soil more alkaline (raise pH) calcium or calcium-magnesium calcium or calcium-magnesium compounds are usedcompounds are used

The uptake of water and the uptake The uptake of water and the uptake of minerals are independent of minerals are independent processes. processes.

Minerals enter root cells through a Minerals enter root cells through a permeable membrane when the permeable membrane when the concentration f of the mineral salts in concentration f of the mineral salts in the soil solution is greater than in the the soil solution is greater than in the root cellroot cell

Fertilizers are nutrient additives Fertilizers are nutrient additives applied to the soil periodically to applied to the soil periodically to maintain optimum crop productivity. maintain optimum crop productivity.

The need for fertilization may result The need for fertilization may result form a deficiency of one or more form a deficiency of one or more mineral elements in the soil, their mineral elements in the soil, their presence in a form unavailable to the presence in a form unavailable to the plant, or the leaching of elements into plant, or the leaching of elements into the soil to a depth below the root the soil to a depth below the root zone.zone.

Soil elements used in greatest quantity Soil elements used in greatest quantity by the green plant: nitrogen, by the green plant: nitrogen, phosphorus, potassium.phosphorus, potassium.

A fertilizer which provides all three A fertilizer which provides all three elements is termed a complete elements is termed a complete fertilizer. fertilizer.

Examples of organic fertilizers: dried Examples of organic fertilizers: dried blood, cocoa meal, animal manures, blood, cocoa meal, animal manures, dried sewage sludge, bone meal.dried sewage sludge, bone meal.

Ammonification is the conversion of Ammonification is the conversion of nitrogen in organic compounds to ammonia. nitrogen in organic compounds to ammonia.

Nitrification is the conversion of ammonia to Nitrification is the conversion of ammonia to nitrate.nitrate.

Phosphorus and nitrogen are present in the Phosphorus and nitrogen are present in the soil in very small amounts. soil in very small amounts.

Potassium is present in the soil in much Potassium is present in the soil in much larger quantities than either nitrogen or larger quantities than either nitrogen or phosphorus.phosphorus.