Soil
Soil is not dirt but it is a complex system that provides many services to the environment. Soil is an ecosystem with living and non-living components that provide many functions
Soil Formation
Soil formation is influenced by five factors. These factors include: climate as it relates to precipitation and temperature; biological activities (plants, animals, micro organisms); relief (aspect and slope); parent material (original material exposed at the surface and subjected to breakdown); and time. The characteristics of the soil formed are dependent on the interaction of these factors.
Climate influences soil pH, microbial activity, vegetation type, and soil depth. Just as plant materials change from west to east and north to south, soils are vastly different across the country. Western climates are drier and warmer than those found in the eastern United States. Dry, arid climates promote water evaporation and lime accumulation in the soil. Increased rainfall promotes increased plant and microbial growth. As a result of this increased growth, more soil organic matter is created and soil depth increases in eastern soils. The greatest influence on soils when moving north to south is the amount of organic matter accumulation. Increased soil temperature increases soil microbial activity and breakdown of organic matter. More organic matter accumulation occurs in cooler northern areas because the rate of decomposition or breakdown is slow. Climate influences soil formation as rainfall rate affects how well things accumulate, dissolve, and/or leach. Climate also affects the degree of vegetation (productivity and decomposition) and erosion level. Living organisms include plants, animals, and microorganisms.
These organisms influence the amount of organic matter accumulation in the soil. Organisms such as earthworms and microbes decompose organic material produced by plants and animals, such as leaves, roots, and animal waste. This organic material serves as food sources for decomposers and allows them to increase their population. An organic product called humus is the final, stable product of decomposition. The type of plant influences the rate of decomposition and amount of humus found in the soil. Topography influences soil organic matter accumulation, colour, drainage, and depth. Soils formed on top of hills are usually deep.
Soils formed on slopes are usually shallow due to erosion. Soils formed in low areas are usually deeper due to the deposition of soil from erosion. Darker soil colours may come from increased water accumulation in low areas. Soils with poor drainage or high water tables usually have higher organic matter content than those which are well-drained because of slow rate of decomposition.
Parent material is the substance from which soil is developed and influences the mineral and nutrient composition of the soil. Parent material is classified based on how it was deposited. Glacial till parent material was transported by ice. This method transported all sizes of particles the same distance, so soil created from this parent material has many different particles sizes. Alluvium is parent material carried by water. Water is a good sorter of particle sizes, and as a result, larger soil particles are dropped closer to the water sources, while small particles settle further away from the water source. Colluvium parent material is moved by gravity. Like ice, gravity does not sort particle size effectively. As a result, soil particle sizes are mixed throughout the site. Parent material deposited by wind has several names based on where it originated. Loess is the most common windblown parent material. Like water, wind is a good sorter of soil particle sizes. Lacustrine and marine parent materials are deposited by lakes and oceans. Since water is the moving force, there is a high degree of soil particle separation. Residual or residuum parent material is weathered where it was found. There is no soil particle separation because it has not moved. There are several parent materials for Nebraska, but windblown sand and loess make up the majority of the parent materials.
Soil formation is a complex process that takes many years. The rate of weathering varies depending on the climate, living organisms, topography, and parent material. When new parent material is exposed or deposited, the process starts anew.
Landscape Soils
Landscape soils are different from agricultural and native or undisturbed soils. They are usually highly disturbed. During construction, topsoil (i.e., ‘A’ horizon or surface soil) is often removed and not replaced to its original depth after construction is completed. This results in limited organic matter and microbial populations, which are critical for healthy plant growth. Landscape soils are commonly compacted from construction vehicles. This limits air in the soil, water infiltration, and proper plant rooting.
Construction debris (concrete, sand, lumber, trash) is often left behind, sometimes buried just below the soil surface. This creates problems with increased soil pH, dry spots, and limited root growth. Disturbed landscape soils increase soil variability and site problems, which makes managing the landscape challenging.
Physical Properties of Soil
Physical properties directly affect the soil’s ability to support seed germination and plant growth, cycle essential nutrients, absorb and hold water, facilitate gas and heat exchanges, reduce erosion, filter pollutants, and support roads and structures. Properties such as soil texture and depth do not normally change with management, but properties such as soil aggregate stability, infiltration rate, bulk density, and water and nutrient holding capacity can change over time due to landscape management practices.
Soil texture refers to the relative proportion of soil separates that make up the mineral or inorganic portion of the soil. The soil separates are sand, silt, and clay and each have different sizes, shapes, and feel. There are 12 major classes of soil texture. In each textural class, a range of sand, silt, and clay amounts is present. Soil texture affects soil aggregation, water infiltration, water holding capacity, plant available water, temperature fluctuations, nutrient storage, and ease of tillage (tilth). Coarse-textured soils (sandy soils) have high infiltration rates, low water and nutrient holding capacity, low aggregate stability, high temperature fluctuations, and are easy to till, while fine-textured soils (clayey soils) have the opposite characteristics.
Soil structure describes the arrangement of soil particles into groups called aggregates. Aggregates have different forms and shape, which define the soil structural characteristics of a given soil. Some processes that affect aggregation include soil organic matter decay, wetting and drying, freezing and thawing, microbial activity, and cation adsorption. These processes bind soil particles and form aggregates. Soils with good structure or stable aggregates allow favourable movement of air and water, unlike those wilting point levels. Plant avail with weak aggregates. Since plant roots move through the same channels in the soil as air and water, well-structured soils allow extensive root development. Organic matter is one of the main properties that promote soil aggregation and soil structural development. Tilling a wet soil should be avoided as it degrades soil structure.
Soil water comes from natural precipitation and irrigation. Water enters the soil through cracks, earthworm holes, and openings between the soil aggregates. The vertical movement of water into the soil is called infiltration. Saturation, field capacity, and permanent wilting point describe the amount of water held by a soil.
Saturation is the soil water content at which all pores are filled. Field capacity is the soil water after saturated soil has drained for 24-48 hours, while wilting point is the soil water content at which plants can extract no more water. Plant available water refers to soil water between the field capacity and able soil water and water holding capacity varies with soil texture
Soil drainage is defined as the rate and extent of water movement across the soil surface and downward in the soil. Slope or lack of slope is important to external (horizontal) soil drainage. Other factors that affect external and internal (vertical) drainage include texture, structure, and physical condition of surface and subsoil layers. Excess soil moisture negatively influences plant growth due to lack of oxygen required for root respiration. Not enough water negatively influences plant growth through slowed photosynthesis. The most desirable soil moisture condition is one in which approximately one-half of the pore space of the soil is occupied by water.
Soil colour is another measure of soil conditions. Colour is determined by organic matter content, drainage, and the degree of oxidation or extent of weathering. Organic matter, manganese, and iron are the primary colouring agents in soil. Light or pale surface soil colours indicate low organic matter content and are typically associated with relatively coarse textured and highly leached soils. Dark colours indicate high organic matter content and may result from high water table condition (poor drainage). Dark colours may also result from the parent material. Dark soils warm up faster than light coloured soils. Red, yellow, and brown subsoil colours indicate a well-drained and well-aerated soil. Gray-coloured sub soils indicate wetness and lack of oxygen. Drainage is usually poor in gray-coloured soils.
Bulk density is the mass of dry soil per unit volume. Soil texture, organic matter, and the degree of compaction influence soil bulk density. Soils with large pore spaces often have low bulk densities. Clay soils often have lower bulk densities than sandy soils with low organic matter. Water movement, rooting depth, and soil aeration are restricted as bulk density increases. Poor landscape management practices such as untimely tillage (wet soil) increase bulk density and reduce the pore spaces.
Soil depth is the vertical distance into the soil from the surface to a layer that essentially reduces the downward growth of plant roots. Rock, sand, gravel, dense clay, or a compacted layer may reduce root growth. Soils that are deep, well drained, and have desirable texture and structure are suitable for the production of most landscape plants. Deep soils hold more plant nutrients and water, and are more productive, than shallow soils with similar textures. Trees growing in shallow soils have shallow root systems and are more frequently blown over by wind than those growing in deep soils.
Summary
Soil is an important factor in healthy plant growth. Its formation is dependent on climate, living organisms, topography, parent material, and time. The amount of organic matter, mineral matter, and pores space a soil has will influence its physical and chemical properties. Ultimately, the physical and chemical properties of a soil will impact plant selection and management practices.
Soil Formation
Soil formation is influenced by five factors. These factors include: climate as it relates to precipitation and temperature; biological activities (plants, animals, micro organisms); relief (aspect and slope); parent material (original material exposed at the surface and subjected to breakdown); and time. The characteristics of the soil formed are dependent on the interaction of these factors.
Climate influences soil pH, microbial activity, vegetation type, and soil depth. Just as plant materials change from west to east and north to south, soils are vastly different across the country. Western climates are drier and warmer than those found in the eastern United States. Dry, arid climates promote water evaporation and lime accumulation in the soil. Increased rainfall promotes increased plant and microbial growth. As a result of this increased growth, more soil organic matter is created and soil depth increases in eastern soils. The greatest influence on soils when moving north to south is the amount of organic matter accumulation. Increased soil temperature increases soil microbial activity and breakdown of organic matter. More organic matter accumulation occurs in cooler northern areas because the rate of decomposition or breakdown is slow. Climate influences soil formation as rainfall rate affects how well things accumulate, dissolve, and/or leach. Climate also affects the degree of vegetation (productivity and decomposition) and erosion level. Living organisms include plants, animals, and microorganisms.
These organisms influence the amount of organic matter accumulation in the soil. Organisms such as earthworms and microbes decompose organic material produced by plants and animals, such as leaves, roots, and animal waste. This organic material serves as food sources for decomposers and allows them to increase their population. An organic product called humus is the final, stable product of decomposition. The type of plant influences the rate of decomposition and amount of humus found in the soil. Topography influences soil organic matter accumulation, colour, drainage, and depth. Soils formed on top of hills are usually deep.
Soils formed on slopes are usually shallow due to erosion. Soils formed in low areas are usually deeper due to the deposition of soil from erosion. Darker soil colours may come from increased water accumulation in low areas. Soils with poor drainage or high water tables usually have higher organic matter content than those which are well-drained because of slow rate of decomposition.
Parent material is the substance from which soil is developed and influences the mineral and nutrient composition of the soil. Parent material is classified based on how it was deposited. Glacial till parent material was transported by ice. This method transported all sizes of particles the same distance, so soil created from this parent material has many different particles sizes. Alluvium is parent material carried by water. Water is a good sorter of particle sizes, and as a result, larger soil particles are dropped closer to the water sources, while small particles settle further away from the water source. Colluvium parent material is moved by gravity. Like ice, gravity does not sort particle size effectively. As a result, soil particle sizes are mixed throughout the site. Parent material deposited by wind has several names based on where it originated. Loess is the most common windblown parent material. Like water, wind is a good sorter of soil particle sizes. Lacustrine and marine parent materials are deposited by lakes and oceans. Since water is the moving force, there is a high degree of soil particle separation. Residual or residuum parent material is weathered where it was found. There is no soil particle separation because it has not moved. There are several parent materials for Nebraska, but windblown sand and loess make up the majority of the parent materials.
Soil formation is a complex process that takes many years. The rate of weathering varies depending on the climate, living organisms, topography, and parent material. When new parent material is exposed or deposited, the process starts anew.
Landscape Soils
Landscape soils are different from agricultural and native or undisturbed soils. They are usually highly disturbed. During construction, topsoil (i.e., ‘A’ horizon or surface soil) is often removed and not replaced to its original depth after construction is completed. This results in limited organic matter and microbial populations, which are critical for healthy plant growth. Landscape soils are commonly compacted from construction vehicles. This limits air in the soil, water infiltration, and proper plant rooting.
Construction debris (concrete, sand, lumber, trash) is often left behind, sometimes buried just below the soil surface. This creates problems with increased soil pH, dry spots, and limited root growth. Disturbed landscape soils increase soil variability and site problems, which makes managing the landscape challenging.
Physical Properties of Soil
Physical properties directly affect the soil’s ability to support seed germination and plant growth, cycle essential nutrients, absorb and hold water, facilitate gas and heat exchanges, reduce erosion, filter pollutants, and support roads and structures. Properties such as soil texture and depth do not normally change with management, but properties such as soil aggregate stability, infiltration rate, bulk density, and water and nutrient holding capacity can change over time due to landscape management practices.
Soil texture refers to the relative proportion of soil separates that make up the mineral or inorganic portion of the soil. The soil separates are sand, silt, and clay and each have different sizes, shapes, and feel. There are 12 major classes of soil texture. In each textural class, a range of sand, silt, and clay amounts is present. Soil texture affects soil aggregation, water infiltration, water holding capacity, plant available water, temperature fluctuations, nutrient storage, and ease of tillage (tilth). Coarse-textured soils (sandy soils) have high infiltration rates, low water and nutrient holding capacity, low aggregate stability, high temperature fluctuations, and are easy to till, while fine-textured soils (clayey soils) have the opposite characteristics.
Soil structure describes the arrangement of soil particles into groups called aggregates. Aggregates have different forms and shape, which define the soil structural characteristics of a given soil. Some processes that affect aggregation include soil organic matter decay, wetting and drying, freezing and thawing, microbial activity, and cation adsorption. These processes bind soil particles and form aggregates. Soils with good structure or stable aggregates allow favourable movement of air and water, unlike those wilting point levels. Plant avail with weak aggregates. Since plant roots move through the same channels in the soil as air and water, well-structured soils allow extensive root development. Organic matter is one of the main properties that promote soil aggregation and soil structural development. Tilling a wet soil should be avoided as it degrades soil structure.
Soil water comes from natural precipitation and irrigation. Water enters the soil through cracks, earthworm holes, and openings between the soil aggregates. The vertical movement of water into the soil is called infiltration. Saturation, field capacity, and permanent wilting point describe the amount of water held by a soil.
Saturation is the soil water content at which all pores are filled. Field capacity is the soil water after saturated soil has drained for 24-48 hours, while wilting point is the soil water content at which plants can extract no more water. Plant available water refers to soil water between the field capacity and able soil water and water holding capacity varies with soil texture
Soil drainage is defined as the rate and extent of water movement across the soil surface and downward in the soil. Slope or lack of slope is important to external (horizontal) soil drainage. Other factors that affect external and internal (vertical) drainage include texture, structure, and physical condition of surface and subsoil layers. Excess soil moisture negatively influences plant growth due to lack of oxygen required for root respiration. Not enough water negatively influences plant growth through slowed photosynthesis. The most desirable soil moisture condition is one in which approximately one-half of the pore space of the soil is occupied by water.
Soil colour is another measure of soil conditions. Colour is determined by organic matter content, drainage, and the degree of oxidation or extent of weathering. Organic matter, manganese, and iron are the primary colouring agents in soil. Light or pale surface soil colours indicate low organic matter content and are typically associated with relatively coarse textured and highly leached soils. Dark colours indicate high organic matter content and may result from high water table condition (poor drainage). Dark colours may also result from the parent material. Dark soils warm up faster than light coloured soils. Red, yellow, and brown subsoil colours indicate a well-drained and well-aerated soil. Gray-coloured sub soils indicate wetness and lack of oxygen. Drainage is usually poor in gray-coloured soils.
Bulk density is the mass of dry soil per unit volume. Soil texture, organic matter, and the degree of compaction influence soil bulk density. Soils with large pore spaces often have low bulk densities. Clay soils often have lower bulk densities than sandy soils with low organic matter. Water movement, rooting depth, and soil aeration are restricted as bulk density increases. Poor landscape management practices such as untimely tillage (wet soil) increase bulk density and reduce the pore spaces.
Soil depth is the vertical distance into the soil from the surface to a layer that essentially reduces the downward growth of plant roots. Rock, sand, gravel, dense clay, or a compacted layer may reduce root growth. Soils that are deep, well drained, and have desirable texture and structure are suitable for the production of most landscape plants. Deep soils hold more plant nutrients and water, and are more productive, than shallow soils with similar textures. Trees growing in shallow soils have shallow root systems and are more frequently blown over by wind than those growing in deep soils.
Summary
Soil is an important factor in healthy plant growth. Its formation is dependent on climate, living organisms, topography, parent material, and time. The amount of organic matter, mineral matter, and pores space a soil has will influence its physical and chemical properties. Ultimately, the physical and chemical properties of a soil will impact plant selection and management practices.