Chapter 2 The Soil

Perhaps the greatest evidence of knowing little about the soil is to label it dirt. Soil is the underground environment of plants.

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

A cross-sectional slice made down into the earth’s surface to show the different layers of soil formations is called a soil profile. Over time, distinctive layers develop in undisturbed soils. The subsoil is between the parent layer and the top soil. The subsoil is finely weathered like the topsoil, but it lacks organic matter in the quantity found in the topsoil layer.

Organic matter comes from the decomposition of plant and animal tissue. Humus- organic compounds that do not decompose quickly, eventually succumb to enzymatic action, forming a complex mixture. Humus

Plowed under green plants are known as green manure. Organic material increases both water and mineral holding capacity of the soil.

Water and air exist around and between the soil particles. As much as 50% of the topsoil may be air and water in liquid or vapor form. The ratio of air to water depends on the texture of the soil and how wet it is.

Even though the original parent stone may be the same or similar, differences in the subsoil and topsoil may result from variations in five things. Weathering elements Soil movement Topography Climate Amount of organic matter

Transported soils have been moved by forces of nature. Soils that weather from bedrock and remain in place are termed sedentary. Transported soils have been moved by forces of nature. Colluvial soils have moved in response to gravity. Alluvial soils are carried in water such as rivers. Aeolian soils are transported and deposited by winds Glacial till is soil deposited by glaciers

The best agricultural soils are usually alluvial and glacial till. Weathering parent material forms particles of different sizes, called soil separates. In decreasing order of size, the separates are: Gravel, sand, silt, and clay

The relative proportion of soil separates of different sizes in any one soil create the soil texture. Most soils in nature contain sand, silt, and clay in some proportions. Loam soil proportion- 40% sand 40% silt 20% fine clay.

Sand Particles have assorted shapes and sizes, depending on how they were weathered Spaces between particles are large Water passes through quickly because of the large pore space Air is present in the greatest quantity Low in mineral nutrients

Silt Particles are irregularly shaped and much smaller than most sand particles Has greater surface area than sand Holds water in the soil better than sand, but it does not provide as much space for air Low nutrient level

Clay Very small, plate-like particles Possesses the greatest surface area of all the separates Water is held tightly to the particles and passes very slowly through the soil Has an adhesive quality when moistened and squeezed This is what sometimes creates a sticky, hard-to-plow field

At least 17 separate chemical elements has been proven repeatedly through tests that demonstrate growth abnormalities when any one of these essential elements is lacking. An element is essential if the plant cannot grow and develop normally without it. Essential elements are broken down into 2 categories, depending on how much of the element is needed Macronutrients Micronutrients

Essential Elements Macronutrients Micronutrients Calcium (Ca) Carbon (C) Hydrogen (H) Magnesium (Mg) Nitrogen (N) Oxygen (O) Phosphorus (P) Potassium (K) Sulfur (S) Micronutrients Boron (B) Chlorine (Cl) Copper (Cu) Iron (Fe) Manganese (Mg) Molybdenum (Mo) Nickel (Ni) Zinc (Zn)

Plants only obtain carbon, hydrogen, and oxygen from sources other than the soil All other nutrients must be obtained as minerals from the soil around the plant’s roots Beneficial elements have been found to promote plant growth in many species, but have not been proven to be absolutely necessary for completion of the plants’ life cycle.

Plants may exhibit symptoms of nutrient deficiency for several reasons The element may be lacking totally or not be present in sufficient quantity The element may be bound in a chemical form unavailable or too slowly available to the plant There may be an overall imbalance of nutrients in the soil Nitrogen is the most common element lacking sufficient quantities in the soil

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

Soil particles adhere together to form larger particles called aggregates. The soils’ water, held between the particles and granules of the soil contains dissolved mineral salts. This liquidation is known as the soil solution.

When soil contains more hydrogen ions than hydroxyl ions, this makes them acidic. When soil contains more hydroxyl ions than hydrogen ions, this makes them alkaline. When a soil contains equal concentrations of hydrogen and hydroxyl ions, its termed neutral.

pH=7 neutral pH<7 acidic pH>7 alkaline

Cation exchange refers to the capacity of colloidal particles to attract positively charged ions (cations) and to exchange one ion for another. Without cation exchange, nutrients would be readily leached from the soil. To make a soil more acidic (lower pH), sulfur is usually added to the soil To make a soil more alkaline (raise pH) calcium or calcium-magnesium compounds are used

The uptake of water and the uptake of minerals are independent processes. Minerals enter root cells through a permeable membrane when the concentration f of the mineral salts in the soil solution is greater than in the root cell

Fertilizers are nutrient additives applied to the soil periodically to maintain optimum crop productivity. The need for fertilization may result form a deficiency of one or more mineral elements in the soil, their presence in a form unavailable to the plant, or the leaching of elements into the soil to a depth below the root zone.

Soil elements used in greatest quantity by the green plant: nitrogen, phosphorus, potassium. A fertilizer which provides all three elements is termed a complete fertilizer. Examples of organic fertilizers: dried blood, cocoa meal, animal manures, dried sewage sludge, bone meal.

Ammonification is the conversion of nitrogen in organic compounds to ammonia. Nitrification is the conversion of ammonia to nitrate. Phosphorus and nitrogen are present in the soil in very small amounts. Potassium is present in the soil in much larger quantities than either nitrogen or phosphorus.