c-ray
02-03-2007, 10:09 PM
from http://www.organicproducermag.com/index.cfm?fuseaction=feature.display&feature_id=77&CFID=19132&CFTOKEN=90686492
by Dr. Gareth Evans
12/26/06 Organic Producer
Humans have been busily cultivating soil for millennia and though it is easy to suppose that the stuff beneath our feet holds few surprises after centuries of tillage, even now new findings continue to emerge. Not even the distinctive smell of freshly ploughed soil is immune the role of the common soil bacteria Streptomyces in producing geosmin, the chemical responsible, has been known for some time, yet it was only within the last few years that the gene for making it was found.
Soil is a considerably more complex substance than many often suppose, principally consisting of a collection of fine mineral particles, with varying amounts of air, water and organic matter. The interaction between these physical, chemical and biological aspects is what essentially defines the character of any given soil and as the primary growing medium its ability to establish a healthy root environment which is, of course, fundamental to all plant growth.
Although it may account for only around 5% in average dirt, the presence of organic matter typically comprising plant roots, soil organisms and humus is effectively its distinguishing feature. While pure sand, for example, can obviously contain both water and air within its inter-particulate spaces, it would not be considered a true soil.
Texture, Composition and Character
Texture is one of the most important soil properties particularly in respect of water movement and a number of aspects of the soils mechanical properties which have a profound influence on cultivation and landscape engineering, are heavily dependent on texture and composition.
Despite the importance of the organic component, unless it is present in significantly higher amounts than normal such as might be found in peat-rich soils it is the mineral material which typically defines soil character. Since this fraction accounts for the bulk of the solid constituents, the size and nature of the mineral particles principally determine soil texture, though the fundamental character of any given soil is effectively fixed during the process of its original formation.
Texture is defined by the relative proportions of sand, silt and clay sometimes referred to as the fine earth fraction, and defined as particles of less than 2mm diameter. Particles larger than this typically gravel and stones are collectively termed rock fragments and play no part in the textural analysis, though they would be separately catalogued on the pedon description a comprehensive and complete record of all the inclusions present.
The fine earth fractions are defined in terms of their particle size, clay being the smallest with a diameter of less than 0.002mm, followed by silt (0.002 0.05 mm) and finally sand as the largest (up to 2mm) often further subdivided into fine, medium, coarse and very coarse. This is a tremendous range, the diameter of very coarse sand grains being a thousand times bigger than even the largest clay particle.
Soils can be categorized by the type and relative proportions of these three basic components. Such a classification can be very precise and a highly specific class determined. Often however, it is more useful to speak in general terms describing broad groups or families defined by the properties and characteristics of the soil.
Establishing the texture can be done using a number of methods. Laboratory techniques typically employ sieving and sedimentation to produce a series of standard grades, ultimately yielding a percentage by dry weight and enabling a soil to be classified very precisely.
However, for many applications, this level of accuracy and expense may be unnecessary and a discussion of broader groups or classes of soil prove more useful. Such an approach, while not as comprehensive, is often much easier to work with in practice, though it remains an indicative rather than a definitive measure.
Simply rolling a sample of the soil between finger and thumb to assess its constituents by its feel is one commonly used field method. This approach depends on the skill and knowledge of the individual, but with practice, most people can learn to make the sort of textural determination that is satisfactory for most purposes. The main advantages of this approach are that it is quick and easy to perform, requires no particular equipment, and costs nothing to do unsurprisingly it is probably the most widely used of all the methods available.
Soil Horizons
In determining the texture of a soil, it is important to remember that the soil on the surface may feel very different between the fingers than the soil deeper underground. A trench dug through almost any soil reveals a succession of layers each lying roughly parallel to the surface, each with its own distinct character, and often clearly defined by distinct variations in color. These layers are termed horizons and form the basis for the international classification system of soils.
Seven major soil horizons are recognized, though often it is possible to differentiate further sub-layers to describe the complete profile in very accurate detail. Not all of the possible horizons are present in every soil and transitional strata, with intermediate characteristics, may occur. In addition, the layers themselves vary considerably in thickness, permitting a very precise vertical map to be drawn.
The horizons are labeled by the letters H-O-A-E-B-C-R.
H - saturated conditions (not necessarily permanent) which accumulate organic matter.
O - the organic horizon, but with no long term saturation of water
A - high in fine organic matter and humus associated with clay
E - high in sand or silt; aluminium and iron compounds have been leached out
B - where materials washed from upper layers have accumulated
C - parent mineral material only slightly affected by the processes of soil formation
R - underlying bedrock
In the majority of cases the O horizon composed of degrading plant material and completely decomposed humus forms the uppermost layer.
The underlying A horizon primarily consists of minerals, often mixed with some organic material. It is a translocation zone from which soluble substances and finer particles have been removed by eluvation the process by which material moves in solution from one layer to another. Commonly, the A horizon shows gradation into a dark colored upper margin due to organic accumulation, and a lighter lower boundary layer where eluvation has caused mineral loss.
By contrast, the B horizon shows the influence of illuvation the deposition of mineral substances such as iron, aluminium and calcium-containing compounds, washed from the upper A horizon. In addition, clay particles are often concentrated here, which increases the overall bulk density of the horizon.
The C horizon is parent material which has not yet been significantly affected by the processes of soil formation, mineral translocation or organic modification, while un-weathered underlying rock comprises the R horizon.
From a practical standpoint, most soils can be considered as having three functional zones top soil (O, A), sub-soil (B) and parent material (C).
Soil Structure
Soil structure the way in which the primary particles are arranged has a major influence on many of its functional characteristics such as drainage ability, water-holding capacity and nutrient availability. There are two component parts the organization of the primary particles into crumbs, and the way these crumbs then bind to each other (aggregation).
The proportion of each of the primary particles and their geometric shapes have a major influence on the way any given soil fits together. This in turn dictates aggregate formation and gross physical structure. Aggregation helps build and sustain soil porosity, the percentage of the total volume of soil that consists of open spaces, and ideally, there should be a range of primary crumb sizes, optimally lying between 15mm. Much larger than this reduces cohesion while much smaller, the natural drainage and aeration channels can become blocked.
While some soils such as the massive accretions in soil pans may truly be described as structure-less and others rich in coarse sand have a primary structure of single-grain particles and thus an ill-defined secondary structure, most fall into one of five crumb categories.
Blocky Large blocks of soil
Columns Tall, round topped soil blocks, formed in vertical columns
Granular Small blocks of soil
Platy Long, shallow soil blocks running in the horizontal direction like plates
Prismatic Columns with non-rounded tops
Soil formation
The size and nature of the mineral particles in a soil are themselves the result of the interaction of five factors which govern the geological processes of soil formation, namely the nature of the parent rock, the climate, geographical relief of the area, biological activity, and time.
These factors are inter-related. The geographical relief can affect the local climate, which in turn dictates the likely organisms present. When considering soil formation, it is also important to understand that the time factor refers not solely to the weathering and erosion period of the underlying parent material, but also represents the historical influences of previous climates, organisms and land topology throughout this process. In addition, although all of these are important, in different regions the relative influence of any one factor can change, which accounts for the variability of soils between otherwise apparently similar locations.
The root environment is central to all plant growth and, with the exception of soil-less techniques, such as hydroponics or nutrient film applications, it is defined by the growing medium. While an extensive knowledge of the geology and chemistry of soil is, clearly, not essential for the grower, an insight into its fundamental nature can only be of benefit.
by Dr. Gareth Evans
12/26/06 Organic Producer
Humans have been busily cultivating soil for millennia and though it is easy to suppose that the stuff beneath our feet holds few surprises after centuries of tillage, even now new findings continue to emerge. Not even the distinctive smell of freshly ploughed soil is immune the role of the common soil bacteria Streptomyces in producing geosmin, the chemical responsible, has been known for some time, yet it was only within the last few years that the gene for making it was found.
Soil is a considerably more complex substance than many often suppose, principally consisting of a collection of fine mineral particles, with varying amounts of air, water and organic matter. The interaction between these physical, chemical and biological aspects is what essentially defines the character of any given soil and as the primary growing medium its ability to establish a healthy root environment which is, of course, fundamental to all plant growth.
Although it may account for only around 5% in average dirt, the presence of organic matter typically comprising plant roots, soil organisms and humus is effectively its distinguishing feature. While pure sand, for example, can obviously contain both water and air within its inter-particulate spaces, it would not be considered a true soil.
Texture, Composition and Character
Texture is one of the most important soil properties particularly in respect of water movement and a number of aspects of the soils mechanical properties which have a profound influence on cultivation and landscape engineering, are heavily dependent on texture and composition.
Despite the importance of the organic component, unless it is present in significantly higher amounts than normal such as might be found in peat-rich soils it is the mineral material which typically defines soil character. Since this fraction accounts for the bulk of the solid constituents, the size and nature of the mineral particles principally determine soil texture, though the fundamental character of any given soil is effectively fixed during the process of its original formation.
Texture is defined by the relative proportions of sand, silt and clay sometimes referred to as the fine earth fraction, and defined as particles of less than 2mm diameter. Particles larger than this typically gravel and stones are collectively termed rock fragments and play no part in the textural analysis, though they would be separately catalogued on the pedon description a comprehensive and complete record of all the inclusions present.
The fine earth fractions are defined in terms of their particle size, clay being the smallest with a diameter of less than 0.002mm, followed by silt (0.002 0.05 mm) and finally sand as the largest (up to 2mm) often further subdivided into fine, medium, coarse and very coarse. This is a tremendous range, the diameter of very coarse sand grains being a thousand times bigger than even the largest clay particle.
Soils can be categorized by the type and relative proportions of these three basic components. Such a classification can be very precise and a highly specific class determined. Often however, it is more useful to speak in general terms describing broad groups or families defined by the properties and characteristics of the soil.
Establishing the texture can be done using a number of methods. Laboratory techniques typically employ sieving and sedimentation to produce a series of standard grades, ultimately yielding a percentage by dry weight and enabling a soil to be classified very precisely.
However, for many applications, this level of accuracy and expense may be unnecessary and a discussion of broader groups or classes of soil prove more useful. Such an approach, while not as comprehensive, is often much easier to work with in practice, though it remains an indicative rather than a definitive measure.
Simply rolling a sample of the soil between finger and thumb to assess its constituents by its feel is one commonly used field method. This approach depends on the skill and knowledge of the individual, but with practice, most people can learn to make the sort of textural determination that is satisfactory for most purposes. The main advantages of this approach are that it is quick and easy to perform, requires no particular equipment, and costs nothing to do unsurprisingly it is probably the most widely used of all the methods available.
Soil Horizons
In determining the texture of a soil, it is important to remember that the soil on the surface may feel very different between the fingers than the soil deeper underground. A trench dug through almost any soil reveals a succession of layers each lying roughly parallel to the surface, each with its own distinct character, and often clearly defined by distinct variations in color. These layers are termed horizons and form the basis for the international classification system of soils.
Seven major soil horizons are recognized, though often it is possible to differentiate further sub-layers to describe the complete profile in very accurate detail. Not all of the possible horizons are present in every soil and transitional strata, with intermediate characteristics, may occur. In addition, the layers themselves vary considerably in thickness, permitting a very precise vertical map to be drawn.
The horizons are labeled by the letters H-O-A-E-B-C-R.
H - saturated conditions (not necessarily permanent) which accumulate organic matter.
O - the organic horizon, but with no long term saturation of water
A - high in fine organic matter and humus associated with clay
E - high in sand or silt; aluminium and iron compounds have been leached out
B - where materials washed from upper layers have accumulated
C - parent mineral material only slightly affected by the processes of soil formation
R - underlying bedrock
In the majority of cases the O horizon composed of degrading plant material and completely decomposed humus forms the uppermost layer.
The underlying A horizon primarily consists of minerals, often mixed with some organic material. It is a translocation zone from which soluble substances and finer particles have been removed by eluvation the process by which material moves in solution from one layer to another. Commonly, the A horizon shows gradation into a dark colored upper margin due to organic accumulation, and a lighter lower boundary layer where eluvation has caused mineral loss.
By contrast, the B horizon shows the influence of illuvation the deposition of mineral substances such as iron, aluminium and calcium-containing compounds, washed from the upper A horizon. In addition, clay particles are often concentrated here, which increases the overall bulk density of the horizon.
The C horizon is parent material which has not yet been significantly affected by the processes of soil formation, mineral translocation or organic modification, while un-weathered underlying rock comprises the R horizon.
From a practical standpoint, most soils can be considered as having three functional zones top soil (O, A), sub-soil (B) and parent material (C).
Soil Structure
Soil structure the way in which the primary particles are arranged has a major influence on many of its functional characteristics such as drainage ability, water-holding capacity and nutrient availability. There are two component parts the organization of the primary particles into crumbs, and the way these crumbs then bind to each other (aggregation).
The proportion of each of the primary particles and their geometric shapes have a major influence on the way any given soil fits together. This in turn dictates aggregate formation and gross physical structure. Aggregation helps build and sustain soil porosity, the percentage of the total volume of soil that consists of open spaces, and ideally, there should be a range of primary crumb sizes, optimally lying between 15mm. Much larger than this reduces cohesion while much smaller, the natural drainage and aeration channels can become blocked.
While some soils such as the massive accretions in soil pans may truly be described as structure-less and others rich in coarse sand have a primary structure of single-grain particles and thus an ill-defined secondary structure, most fall into one of five crumb categories.
Blocky Large blocks of soil
Columns Tall, round topped soil blocks, formed in vertical columns
Granular Small blocks of soil
Platy Long, shallow soil blocks running in the horizontal direction like plates
Prismatic Columns with non-rounded tops
Soil formation
The size and nature of the mineral particles in a soil are themselves the result of the interaction of five factors which govern the geological processes of soil formation, namely the nature of the parent rock, the climate, geographical relief of the area, biological activity, and time.
These factors are inter-related. The geographical relief can affect the local climate, which in turn dictates the likely organisms present. When considering soil formation, it is also important to understand that the time factor refers not solely to the weathering and erosion period of the underlying parent material, but also represents the historical influences of previous climates, organisms and land topology throughout this process. In addition, although all of these are important, in different regions the relative influence of any one factor can change, which accounts for the variability of soils between otherwise apparently similar locations.
The root environment is central to all plant growth and, with the exception of soil-less techniques, such as hydroponics or nutrient film applications, it is defined by the growing medium. While an extensive knowledge of the geology and chemistry of soil is, clearly, not essential for the grower, an insight into its fundamental nature can only be of benefit.