AP Notes - Soils
SOIL
Soil is a thin layer on top of earth’s land surface.
Soil is a natural resource and deeply affects every other part of the ecosystem. Soil holds nutrient and water for plants and animals; water is filtered and cleansed as it flow through soils; and soils affect the chemistry of water and the amount of water that returns to the atmosphere to form rain.
Soil is composed of 3 main ingredients: minerals of different size, organic materials from the remains of dead plants and animals, and open spaces (pores) that can be filled with air or water.
A good soil for growing most plants should have about 45% minerals (with a mixture of and, silt and clay), 5% organic matter, 25% air, and 25% water.
Soils Develop in response to several factors:
1. Parent Material: The rock or minerals from which the soil derives. The nature of the parent rock can either be native to the area or transported to the area by wind, water, or glacier, has a direct effect on the ultimate soil profile.
2. Climate: This is measured by precipitation and temperature. It results in partial weathering of parent material, which forms the substrate for soil.
3. Living Organisms: These include nitrogen-fixing bacteria, Rhizobium, fungi, insects, worms, snails etc. that help decompose litter and recycle nutrients.
4. Topography: This refers to the physical characteristics of the location when the soil is formed. Topographic factors that affect a soil’s profile include drainage, lope direction, elevation, and wind exposure.
With sufficient time, a mature soil profile reaches a state of equilibrium. Feedback mechanism involving both abiotic and biotic factor work to preserve the mature soil profile. The relative abundance of sand, silt and clay are called the soil texture according to the USDA (US Department of Agriculture). The category with the smallest particles is clay (<0.002 mm). The next largest is silt 0.002-0.05mm in diameter. And sand is the coarsest soil with particles 0.05-2.0 mm in diameter. Sand particles are too large to sick together, and sandy soils have larger pores; which means they can hold more water. Clays easily adhere to each other and there is little room between particles for water; clay soils are extremely compact.
Another very important characteristic of soil types is soil acidity or alkalinity. pH ranges from 0-14 and is a measurement of the concentration of hydrogen ions. Most soils fall into the pH range of 4-8, meaning that soils range from being neutral to slightly acidic. Soil pH is important because it affects the solubility of nutrients; and thus in turn determines the extent to which these nutrients are available for absorption by plant roots. If the soil is too basic or too acidic, certain soil nutrients will not be able to be used by regional plants.
WHERE DOES SOIL COME FROM?
Soil is a combination of organic materials and rock that has been broken down by chemical and biological weathering. The types of minerals found in the soil in a particular region will depend on the type of base rock in that region.
Water, wind, and living organisms are agents of weathering. All weathering is placed into three broad categories:
● Physical weathering (mechanical weathering) – any process that breaks rocks down into smaller pieces without changing the chemistry of the rock. The forces responsible for it include wind and water.
● Chemical Weathering – occurs as a result of chemical reactions between water and other atmospheric gases, and the bedrock in a region.
● Biological Weathering: Weathering that takes place as a result of the activities of living organisms.
Soil is made of distinct layers with very different characteristics. These are shown in the picture and table below.
SOIL LAYERS/HORIZION
Horizon Name Description
O Surface Litter Upper most layer. Leaves, waste, live organisms and partially decomposed organic debris including decomposing organisms. The dark, crumbly material resulting from decomposition of organic material is called humus. Maybe very thick in deciduous forests and very thin in tundra or desert.
A Topsoil Made up of weathered rock and some organic matter that has traveled down from the O layer. Some living organisms and inorganic materials. Topsoil is typically very thick in grasslands. Important role in plant growth. E Zone of Leaching Dissolved and suspended materials move downward.
B Subsoil
Zone of Illuviation Tends to be yellowish in color due to the accumulation of iron, aluminum, humic compounds, and clay leached down from the A and E horizons. It can be rich in nutrients in areas where rainwater leeched (washed) nutrients from the topsoil.
C Weathered Parent Material Partially broken down organic minerals. Large pieces that have not undergone much weathering.
R Bedrock Bedrock
SOIL COMPONENTS
COMPONENT DESCRIPTION
Clay Very fine particles. Compacts easily. Forms large, dense clumps when wet. Low permeability to water; therefore, upper layers become water logged.
Gravel Coarse particles. Consists of rock fragments.
Loam About equal mixtures of clay, sand, silt and humus. Rich in nutrients. Holds water but does not become waterlogged.
Sand Sedimentary material coarser than silt. Water flows through too quickly for most crops. Good for crops and plants requiring low amounts of water.
Silt Sedimentary material consisting of very fine particles between the size of sand and clay. Easily transported by water.
SOIL PROBLEMS FOR (AND CAUSED BY) HUMANS
In order to be able to grow the food we humans consume, we must have enough arable (suitable for plant growth) soil to meet our agricultural needs. Soil fertility refers to the soil ability to provide essential nutrient like nitrogen (N), potassium (K), and phosphorus (P) to plants. Humus is also an important component because it is rich in organic matter.
Soil composed of roughly the same amount of all three textures (sand, silt, and clay) are described as being loamy, and these are the best for plant growth. Another important characteristic of soil for agricultural purposes is the extent to which it aggregates, or clumps. The most fertile soils for agricultural purposes are aggregates of soils of different textures bound together with organic materials.
Monoculture
Agricultural activities can change the texture of the soil. For example repeated plowing tends to break down soil aggregates, leaving “plow pan” or “hard pan”, which is hard, unfertile soil.
Traditionally communities planted many different types of crops in a field; however, in modern agriculture the monoculture, or the planting of one type of crop, predominates. Over the history of agriculture, a significant decrease in genetic diversity of crop species has taken place. The creates problems such as: A lack of genetic diversity makes crops more susceptible to pests and diseases.
The consistent planting of one crop in an area eventually leaches the soil in that area of the specific nutrients that the plant needs to grow. Can be prevented by crop rotation (in which different crops are plant in each area in each growing season).
Other problems with modern agriculture include its reliance on large machinery (which can damage soil), and the fact that as an industry, agriculture is a huge consumer of energy. Energy consumed both in the production of pesticides and fertilizers, and the use of fossil fuels to run farm machinery.
The past 50 years have seen a huge increase in worldwide productivity, and this is largely due to the mechanization of farming resulted from the Industrial revolution. The boom in agriculture productivity is know as the Green Revolution, and unfortunately it has had many detrimental effects. For examples, the use of chemical pesticides results in an emergence of new species of insects
Another drawback to the Green Revolution resulted from the dramatic that were pesticide-resistance. Recently, the introduction of genetically modified plants has enabled researchers to take steps in solving problems of pesticide-resistant insect species.increase in irrigation worldwide; over-irrigated soils undergo salinization. In salinization, the soil becomes water-logged and when it dries out, salt forms a layer on its surface; this eventually leads to land degradation. In order to combat this problem, researches have developed drip irrigation, which allots an area only as much water as in necessary, and delivers the water directly to the roots.
Erosion
Soil erosion is the movement of weathered rock or soil components from one place to another.
Erosion is caused by flowing water, wind and human activity (cultivating inappropriate land, burning of native vegetation, deforestation, and construction).
Soil erosion destroys the soil profile, decreases water-holding capacity of the soil, and increases soil compaction.
Because water cannot percolate through the soil, it runoff the land, taking more soil with it (positive feedback loop).
Because the soil cannot hold water crops grown in areas of soil erosion frequently suffer from water shortages.
In areas of low precipitation, erosion leads to significant droughts.
Poor agriculture techniques that lead to soil erosion include monoculture, row cropping, overgrazing, improper plowing the soil and removing crop wastes instead of plowing the organic matter back into the soil.
Three types of soil erosion
1. Sheet erosion – soil moves off as a horizontal layer.
2. Rill erosion – fat-flowing water cuts small channels in the soil.
3. Gully erosion – extreme case of rill erosion, where over time, channels increase in size and depth.
Soil erosion causes damage to agriculture, waterways (canals), and infrastructures (dams). It interferes with wetland ecosystems, reproductive cycles (as in salmon), oxygen capacity, and pH of the water.
SOIL CONSERVATION
To conserve soil resources several best management practices have been developed. These practices return organic material to the soil, slow down the effect of wind, and reduce the amount of damage done to the soil by tillage (plowing). Some common methods include:
Use animal wastes (manure) and the residue of plants to increase the amount of organic matter in the soil.
Modify tillage practices to reduce the breakup of soil and to reduce the amount of erosion. These include contour plowing and strip planting.
Use trees and other wind barriers to reduce the force of winds.
SOIL LAWS
Two laws relate to preserving soil.
Soil and Water Conservation Act (1977) – Soil and water conservation programs to aid landowners and users; also sets up conditions to continue evaluating the condition of US soil, water, and related resources.
Food Security Act (1985) – Nicknamed swamp buster, this act discouraged the conversion of wetlands to non-wetlands. 1990 federal legislation denied federal farm supplements to those who converted wetlands to agriculture, and provided a restoration of benefits to those who unknowingly converts lands to wetlands.
Facts
One inch of topsoil can take up to 1,00 years to form.
Due to extensive soil erosion in the US, more soil is lost each year than during the 1930 Dust Bowl.
In 1850, soil in the prairie states averages 14 inches of topsoil. Today it is 7 inches.
More than half of the fertilizer used today is required to replace organic nutrients lost through soil erosion
Dust Bowl: Occurred in the 1930s in Oklahoma, Texas, and Kansas. It was cause by plowing the prairies and resulted in the loss of natural grasses that rooted the soil. Drought and winds that occurred blew most of the topsoil away, causing people to leave the Area.
1935 Soil Erosion Act: Established Soil Conservation Service. Mandates the protection of the nation’s soil reserves. Deals with soil erosion problems, carries out soil surveys, and does research on soil salinity. Provides computer databases for research.