User:Paleorthid/1938 USDA soil taxonomy
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[edit] 1
Taking the country as a whole, Marbut (1922, 1935) divided the soils into two parts on the basis of characteristics of the solum (A and B horizons taken together) calling them pedocals (ped = soil; cal = calcium) were the soils of the low rainfall regions with a lime accumulation horizon. The pedalfers (ped = soil; al = alumina; fer = iron) were the leached soils, without lime, exposing conspicuously the alumina and iron. The chemical reaction (pH) of the pedocals was neutral to alkaline, that of the pedalfers was acid. The line dividing these great solum groups followed approximately the eastern rim of the watershed of the Red River of the North from the Canadian boundary southward through western Minnesota, turning southwestward at the Iowa line into Nebraska, then southward passing somewhat west of Lincoln, Nebraska, and Wichita, Kansas, then slightly west of south past El Reno, Oklahoma, to south central Texas, slightly west of the 99 meridian, where it made a sweeping eastward curve to the Gulf coast near Corpus Christi. Although it must be shown on a printed map as a line, it was more accurate to think of it as a transitional zone. Except at the northern end, it was not far from the 98 meridian, the line so much discussed in the literature of the plains region as the line dividing the prairie from the low plains. The Great Soil Groups or Provinces (Marbut, 1935) were subdivisions of the solum groups. In the pedalfer area, except the Prairie Peninsula, from north to south they were tundra soils in Canada; podzols near the borders of the two nations associated largely with the northern evergreen forests; gray-brown podzolic soils of the oak-hickory deciduous forests, and red soils and yellow soils of the southern hardwood and evergreen forests regions. All soils formed under forests were of limited or even inferior fertility, a fact which runs counter to popular traditions. The prairie soils constituted a group by themselves in the region of the Prairie Peninsula and southward through the prairie-forest transition country to the Gulf. Intermixed, however, were gray-brown podzolic soils which occupied most of the eroded valleys. In the pedocal area, just west of the pedocal-pedalfer division line, lay the chernozem belt forming a north-south belt from near the Rio Grande into Canada.
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Farther west were the dark-brown soils of the plains and the brown soils of the plains-desert transition, and the gray soils of the deserts and the Great Basin. The Pacific coast region had a wide range of soils; from gray-brown podzolic to gray-desert.
The prairie soils were the subject of much discussion among pedologist. It was much like the problem among botanists, of grass in a high rainfall climate that, by all the known rules, should have produced forests. The prairie soils are chernozem-like soils formed under a grass cover in a mid-rainfall region, but they lie in a high rainfall region, and are surrounded by a network of valley soils of the gray-brown podzolic group which theoretically seem to be proper soils for the entire Prairie Peninsula. Marbut (1934, 1935) insisted that they are normal soils, while Joffe 1936), more directly in the Russian tradition, called them degraded chernozems. At any rate, it is agreed that the prairie soils are unique, that no other substantial area of similar soils exist elsewhere in the world. They possessed the remarkable fertility of a grassland soil, were only slightly acid, and received ample rainfall to make them highly productive. As the prairie belt ran north and south from near the Canadian line to the Gulf, the temperature range, produced differences, the southern line of Kansas marking roughly an irregular division line between southern prairie soils and northern prairie soils.
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In the classification of soils of the United States into Great Soil Groups, Marbut assigned such designations only to soils which he held were mature. By his standard of maturity, nearly half of the soils were designated as immature. In his maps (1935) for the Atlas of American Agriculture, one was designated for the Great Soil Groups, and on it the immature soils were not shown. A second map indicated the immature soils. The two must be studied together. Usually only the first of Marbut's maps was reproduced in books using his soil maps. The use of both maps is particularly important to the mixed grass prairie-plains where a large part of the soils were designated as immature. Furthermore, some of these were among the most productive of the central grassland. Most pedologist took the view that only in early stages of soil formation did parent materials largely influence its characteristics. In more developed soils, the mark of the parent materially lessened until in mature soils that influence was practically non-existent, climate and vegetation fully determining the soil properties. In the Yearbook of the United States Department of Agriculture, 1938, Soils and Men, a substantial revision of the Marbut classification was presented, introducing in part new terminology as well as reclassification. The southern prairie soils became reddish prairie, but the area was broken up into several types of soils. The chernozems were retained, the northern dark browns were renamed chestnut, but the southern chernozems and southern dark browns were substantially rearranged and the name reddish-chestnut given to the major portion. The brown and desert gray soils underwent the most extensive revision and one that is traceable only by reference to the detailed maps. In dealing with the low-rainfall regions, Shaw (1937) still held to classifications of soils which emphasized parent materials, and Graham (1941, 1943) concluded that in Missouri the parent materials continued to influence plant growth even in mature soils and that soil classification should recognize this fact. Robinson (1936) noted a swing back to greater recognition of parent materials. The whole subject of soil science was too conspicuously in its formative stage that it was important to recall Robinson's (1936) admonition, that all soil classifications were provisional. The problem of minor elements placed a new emphasis on parent materials of soils as well as upon occurrence of mineral nutritional diseases of plants and animals. The importance of the subject was such that the editor of Soil Science sponsored a symposium of twenty-one papers dealing with sixteen minor elements which were published in the July and August issues, 1945, including a map of minor element distribution in the United States which affected plants and animals.
http://www.kancoll.org/books/malin/mgchap06.htm
[edit] INTRODUCTION TO SOIL GEOGRAPHY
http://www.siue.edu/~fodemer/geog210.htm
A. DEFINITION OF SOIL B. COMPONENTS OF SOIL:
Solid Materials:
- mineral matter
- organic matter
Pore Spaces:
- soil air
- soil water
C. PROPERTIES OF SOILS:
- Soil Texture
- Soil Structure and Consistence
- Soil Horizon
- Soil Chemistry
D. SOIL FORMATION:
- Factors of Soil Formation
- Soil Forming Processes:
processes of material accumulation
processes of horizon differentiation:
--laterization
--podsolization
--calcification
--salinization
--gleization
E. SOIL CLASSIFICATION
DEFINITION OF SOIL:
* top loose materials of the earth surface which support plant growth * However, soil means different things to different people * geologists see soil as all the loose materials produced by weathering and composed of inorganic and inert materials * engineers see soil as all loose used for civil engineering construction * soil chemists see soil as a chemical lab where various chemical processes and reactions take place to produce nutrients for plant benefit * agricultural scientists see soil as the few top layers of weathered materials that plants root and grow * pedologists (e.g. V.V. Dokuchaev and Nikolai Sibirtzev) see soil as the superficial layer of loose materials subjected to genetic and environmental factors that is capable of supporting life * According to V.V. Dokucheav (1870-1890),soil is seen as an organized natural body distinguishable on the basis of their intrinsic properties, processes of formation and distribution over space * V.V. Dokucheav's work led to the development of the concept of a living soil * the idea that soil is simply the product of geologic processes gave way to the fact that biological and biochemical processes are equally important * V.V. Dokuchaev and Nikolai Sibirtzev systematized the study of soil into a scientific discipline * American soil scientist - C. F. Mabut in the 1930s translated the work of V.V. Dokuchaev into English * he exposed the Russian ideas to the international scientific community * in general, soil geography is concerned with soil-forming processes, its distribution, its management and conservation
FUNCTIONS OF SOILS
* medium for plant growth * support and anchorage for growing plants * medium for water to reach plant roots * supply of nutrients to plant life * serves as habitat for organisms whose biological activity is responsible for the recycling of mineral nutrients derived from organic matter.
B. SOIL COMPONENTS
* Composed of two major components:
solid materials:
- mineral matter
- organic matter
Pore Spaces:
- Macro-pores housing soil air
- micro-pores housing soil water
hence, soil components are:
- mineral matter
- organic matter
- soil air
- soil water
* the foregoing components combine to produce the physical and chemical properties of soils
* such properties are important for:
- soil classification and description
- soil utility and biological activity
Mineral Matter
* includes all inorganic materials derived:
- from parent rocks through weathering
- or from organic sources through mineralization
* main forms of mineral elements in soil:
- silicate minerals (most abundant in soil)
- macro-nutrients
- micro-nutrients
* soil mineral content determines:
- soil productivity
- soil fertility status
* most depleted minerals in soils are:
- nitrogen
- phosphorous
- potassium
* often replaced by addition of mineral or organic fertilizers and manures
* nitrogen is replaced by the use of legumes (nitrogen fixing plants) in crop rotation
* clay minerals hold water and nutrients
* quartz helps with aeration and water percolation
Organic Matter:
* includes all living and dead organisms
* dead organics decomposed to form humus
* organic matter:
- improves soil tilth (workability)
- is a store and supplier of plant nutrients like N2, P, S, K
- provides a home to living organisms
* Humus:
- contains colloidal substances which facilitates the transfer of
nutrients to plants
- holds and exchanges nutrient cations
- binds soil particles together
- reduces cohesion in clayey soils and encourages granulation
* soils with high humus content are chemically active and more fertile
* living organisms in soils are:
macro-organisms:
- earthworms, ants, bettles, grubs
- burrowing animals
- help in the mixing of soil materials
- burrows provide avenues for movement of air and water through the soil
micro-organisms:
- bacteria and protozoa, etc
- responsible for the decay and disintegration of organic materials
- contribute to the break up of mineral particles
* soils with abundant organisms are often more productive
Soil Air
* soil macro-pores:
- form soil atmosphere
- source of oxygen for plants and animals
- place for the disposal of CO2 and other noxious gases
* oxygen diffuses into the root cells and aid respiration
* aerobic micro-organisms, bacteria, actinomycetes and fungi convert nutrients in organic matter into soluble forms for plants re-use
Soil Water
* soil water are in the forms of:
gravitational water (free water):
* water percolating downward in response to the pull of gravity
* most abundant soon after rainfall
* facilitates the process of leaching and eluviation
* important in soil horizon differentiation
* first water to be depleted during drought
* not useful to plants
* capillary water (water of cohesion) water held by surface tension on and within micro-pores * water moves downward, upward or laterally according to climatic conditions * useful to plants * disappears from soil after a prolonged drought * most effective in loamy soil
hygroscopic water (water of adhesion)
* held as microscopical thin layer around each individual soil particles
* it is of no use to plants
* last water to be lost from the soil
C. PROPERTIES OF SOILS:
- soil texture
- soil color
- soil structure
- soil consistence
- soil horizon
- soil chemistry
Soil Texture
* refers to the size, shape and arrangement of individual soil particles
* names of the individual particles are determined by size, hence:
- clay <0.002 mm (in diameter)
- silt 0.002 - 0.05 mm
- sand 0.05 - 2.00 mm
- gravel >2.00 mm
* most soils contain a mixture of particles of different sizes
* hence, USDA uses the soil texture triangle to determine soil textural classes based on the proportions of sand, silt and clay it contains
* For example:
- a soil containing 20% sand, 60% silt and 20% clay is classified as a
silt loam
* soil texture:
- influences soil properties
- influences, nutrient supply, soil tilth, absorption, retention and transport
of air and water
Soil Structure
* refers to ways individual soil particles are clumped together to form aggregates or soil peds
* main types are:
- granular structure and platy structure
- lens-like structure and blocky structure
- prismatic structure and columnar
- crumbs
* strength of soil peds can be described as:
- weak (indistinct peds)
- moderate (well-formed distinct peds but not in undisturbed soil)
- strong (distinct in undisturbed soil)
* ped formation aided by:
- plant roots which move particles closer
- removal of water by roots causing soil shrinkage and cracking
- animal activity, wetting and drying, freezing and thawing
- seasonal climatic changes may cause aggregation of soil materials
* percent of stable aggregates higher in sub-humid and semi-arid soils than in wet or dry soils
* soil structure is important in:
- soil erosion susceptibility
- water absorption rates and tilth
- root penetration and permeability
Soil Horizon
* refers to soil layers
* soil properties tend to vary between horizons
* develops due to the vertical translocation of materials by moving soil water
* five major types of horizon:
- O: organic layer (uppermost layer & may be absent in arid areas)
- A: layer with a mixture of inorganic minerals and organic matter.
Dark in color, fertile and forms top-soil
- E: mineral layer or the eluvial horizon. Lighter in color and common in
areas with high rainfall. Clay and organic substances washed downward
- B: illuviation layer or subsoil. May develop zones of clay accumulation
- C: freshly weathered rock material which may extend for >50 m in the tropics
- R: unweathered bedrock
* A, E, and B horizons together form the true soil called solum
* soil profile refers to the vertical slice of the soil and consisting of various horizons
* soil profile is used to finger print soils
* each soil type has its unique profile used to identify it
* presence of well-developed horizons indicate mineralogical maturity
SOIL CHEMISTRY
* soil nutrients in soil solution are retained in the soil through clay and humus colloids
* clay and humus colloids are negatively charged and attract the bases or cations in soil solution
* plants gain needed bases or cations by exchanging other cations (H+) for them
* ability of soil to exchange cations is called cation exchange capacity (CEC)
* CEC is determined by its supply of exchangeable bases or cations and the quantity of H+ ions that can be traded for the bases (cations)
* the quantity of available H+ ions is measured by the ph scale
* soil pH ranges from 3 to 10
* meaning of pH scale:
- neutral pH = 7
- alkaline pH = >7
- acidic pH = <7
* the best pH for most agricultural crops is between 5 and 7
* a highly acidic soil contains an excess of H+ ions and may not have enough exchangeable bases or cations
* when available, the bases or cations become highly soluble and easily leached in an acidic condition
* a highly alkaline soil is infertile due to the lack of H+ ions to exchange for bases and could be toxic
* soils of humid climates, especially tropical humid climates, tend to develop acidic soils
* arid and semi-arid climates tend to develop alkaline soils
Soil Consistence
* refers to the resistance of soil to deformation or rupture
* determined by the cohesive and adhesive properties of the entire soil mass
* it is important for tillage and traffic consideration
* for example:
- sand dunes are easily deformed by automobiles which may become
stuck in it
- clay soils are sticky when wet and difficult to plow
* Types of consistence:
- wet soil: non-sticky, sticky, non-plastic, plastic
- moist soil: loose, friable, firm
- dry soil: loose, soft, hard
- indurated soil: hard requiring hammer blow
