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V. Morphology of Organic Soil Horizons

V. Morphology of Organic Soil Horizons

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TABLE IX

Diagnostic Morphological Properties of Organic Soil Horizons



Horizon type



Range in

organic

Fiber content

( % oven dried,

content

( % oven dried) 0.1 mm. + )



sodium

pyrophosphate

extract

( Munsell)



Fiber characteristics

Change in color

chroma (Munsell)

rubbed wet



Change in color

value (Munseu)

when dried



PH

range,

1.0 N KCI



Fibric

Sphagnofibric



95-98



Over 66-2/3



None



Increase of

4 or more units



2.5-3.5



Hypnofibric



85-95



Over 66-2/3



None to slight



Increase of

3 or more units

Increase of 2

or more units

Increase of 2

or more units



5.1-6.5



'



>



3.5-5.0



Dysfibric



75-95



Over 66-2/3



None to 1 unit



Eufibric



70-92



Over 66-2/3



None to 1 unit



Mesic

Dysmesic



60-90



33-1/3-66-2/3

Over 66-2/3



Variable

Increase 2 or

more units



4.0-5.0



'



Eumesic



50-90



33-1/3-66-2/3

Over 66-2/3



Variable

Over 50% fibers

decrease 2 or

more units

Variable

Over 50% fibers

decrease 2 or

more units



Variable

Increase 2 or

more units



5.1-6.5



t



Sapric

Dysapric



50+



Less than 33-1/3



Slight



4.04.9



1



Eusapric



50+



Less than 33-1/3



Slight



Dry color value

5 or less

Dry color value

5 or less

Dry color value

more than 5

Dry color value

more than 5



Clastidysapric



30-50



Less than 33-1/3



None



Clastieusapric



30-50



Less than 33-1/3



None



Higher value or

lower chroma

than lOYR 7/3



5.1-6.5



Variable



I



5.1-7.5



.



4'e4'9



5.1-8.5



I

I



Lower value or

higher chroma

than lOYR 7/3



2;

a



TABLE X

Properties of Organic Soil Horizonsa



Horizon type



Average

fiber

content

(>0.1 mm.%

dry wt.

basis )



Fibric

Sphagnofibric

Hypnofibric

Dysfibric

Eufibric



91 (10)

85 (10)

79 (10)

76 (10)



Mesic

Dysmesic

Eumesic



51 (6)

48 (5)



Average

ash

content

( % dry

wt. basis)



Average

water-holding

capacity

( % dry wt.

basis)



Average

bulk density

( g./cc. 1



pH (H,O)

( range )



Nitrogen

range in

values

( % dry

basis )



Sodium

pyrophosphate

solubility,

range ( % ) b



7.83 (12)

9.20 (10)



1920 (20)

1190 (10)

905 (10)

850 (10)



0.06(6)

0.12 (5)

0.15 (6)

0.17 (5)



2.9-4.5

5.5-7.5

4.5-5.0

5.0-7.5



0.4-1.1

1.5-2.5

1.2-2.0

1.8-3.0



0.1-0.5

0.25-0.75

0.25-0.75

0.25-0.75



11.4 (10)

21.2 (10)



660 (10)

625 (10)



0.21 (4)

0.23(5)



4.5-5.0

5.0-7.5



1.5-2.5

2.0-3.0



0.50-1.2

0.50-1.5



0.31 (4)

0.34 (5)

0.41 (2)

0.45 (3)



4.5-5.0

5.0-8.0

4.5-5.0

5.0-8.0



1.5-3.0

1.8-3.5

1.0-1.8

1.5-2.5



0.75-2.0

0.75-3.0

1.23.0

1.2-3.5



2.51 (20)

7.90 (10)



Sapric

418 (6)

20 (6)

35.1 (6)

Dysapric

473 (3)

18 (5)

26.2 (3)

Eusapric

Clastidysapric

15 (5)

66.5 (5)

187 (5)

249 (10)

12 ( 6 )

59.4 (10)

Clastieusapric

Figures in parentheses indicate number of samples.

6 Figures given are for the per cent solubility in a saturated sodium

organic material.



n

m



.zG

8

0

9



z

i;

m



@



pyrophosphate solution based on a standard extract from



8



138



R. S. FARNHAM AND H. R . FINNEY



organic soil horizons is a three-stage decomposition scale based on field

morphological evidence supported by laboratory analyses. This scale is

much preferred to a multistage scale method such as the ten-stage degree

of humification ( H ) widely used in Europe (Kaila, 1956),or the fivegrade scale used by some Russian workers (Varlygin, 1924). These

systems with multigrade decomposition stages are subjective and require

considerable experience and skill. Standardization between results of

investigators in one country and those in another is difficult at best

because of the subjective nature of the method. Also, it is believed that

a method using only three stages of decomposition corresponding to

the three diagnostic horizon types is easier to define, is reproducible, is

unusually simple, and is exceedingly well adapted to a wide variety of

uses.

In the classification of organic soils presented here, only three types

of horizons are considered diagnostic in the highest level (Suborder)

in the system. These are the fibric, mesic, and sapric horizons, listed in

order of increasing decomposition. They are characterized in the field

on the basis of morphological properties, many of which reflect genetic

processes indicating various degrees of physical disintegration and biochemical decomposition.

Subtypes of these major kinds of horizons are distinguished on the

basis of base status, of fiber type in the case of two relatively unaltered

fibric types, and of the content of inorganic matter. These subtype horizons are diagnostic at the second highest level (Great Group) in the

classification scheme. A summarization of diagnostic morphological properties of the various kinds of horizons are presented in Table IX, and

some supplemental properties are presented in Table X.

A. FIBRIC

HORIZONS

The morphological features of fibric horizons are principally related

to the nature of the original plant material from which they were formed.

These horizons contain the highest content of plant fiber and the most

well-preserved fiber. The essential morphological properties of a fibric

horizon must include the following:

1. Over two-thirds of the fibers in the total mass of organic material

must exceed 0.1 mm. in size (will not pass 140-mesh screen when sieved

wet).

2,. More than 50 per cent of fibers must be so well preserved as not

to decrease in chroma ( Munsell standard color chart) when rubbed wet

or must resist becoming disintegrated or greasy on rubbing.

3. Increase in color value ( Munsell) is at least 2 units when pressed

wet.



CLASSIFICATION OF ORGANIC SOILS



139



4. Sodium pyrophosphate extract on white filter paper is higher in

value or lower in chroma than l O y R 7/3 (Munsell color chart).

Subtypes of fibric horizons are distinguished on the basis of kind of

fiber and base status as determined using pH (KCl) as the indicator.

1. Sphagnofibric

The sphagnofibric subtype horizons are the most unique horizons occurring in organic soils. They are extremely acid, often with pH (KCl)

ranging as low as 2.53.0. The very raw moss fiber comprising over 75 per

cent of the total fiber content greater than 0.1mm. is readily identified

and consists of the remains from several species of the genus Sphagnum,

which is one of the best-known bryophytes. Individual fibers are often

2 to 3 inches long, very thin, and usually well preserved. The leaves

around the tip of the stem form a whorl which distinguishes Sphagnum

species from other kinds of mosses. In Canada, Ireland, and Northern

Europe, the surface horizons of a great many organic deposits commonly

consist of Sphagnum moss peat. It is this kind of peat that is sold in the

United States in bales and bags as peat moss and is used primarily as

a soil conditioner for garden soils. In the United States, peat bogs containing surface layers consisting of relatively pure Sphagnum moss peat

(sphagnofibric horizons) are located principally in the cooler and moister

areas of the country, especially in Washington, Minnesota, and Maine.

The thickest deposits of Sphagnum moss peat occur on convex positions

in large bogs. The elevation of these areas is due to the rapid growth of

Sphagnum. In northern Minnesota the Sphagnum-covered raised portions

of the extensive organic soil areas often are as large as 1,500 to 3,000 acres

(see Fig. 2), and they generally occur in large glacial lake plains.

Deposits of economic importance contain from 5 to 8 feet of relatively

pure Sphagnum moss peat at least 1,OOO acres or more in size.

In their natural wet condition Sphagnum moss fibers vary in color

from dark tan to dark reddish brown, but when pressed firmly between

the fingers the color value (Munsell) generally increases 3 or 4 units.

It is not unusual for value changes to be as great as 4 or 5, and often

there is a change in hue on the Munsell chart. When squeezed firmly in

the hand the wet sphagnofibric material readily releases large amounts

of almost colorless liquid.

Table X shows the extreme acidity of sphagnofibric horizons as well

as their low bulk densities, high fiber contents, very low ash content, and

low nitrogen contents. The low nitrogen content reflects both the low

degree of decomposition and the ombrogenous source of water. Nutrientrich waters do not saturate the raised bogs where sphagnum occurs

because of the elevation difference.



140



R. S. FARNHAM AND H. R. FINNEY



2. Hypnofibric

Hypnofibric horizons are organic horizons containing over two-thirds

total fibers exceeding 0.1 mm. in size, at least 75 per cent of these fibers

being derived from the Hypnales group of mosses. The mosses of this

group include a number of the more lime-loving genera, such as

Dicranum, Calliergon, Hylocomium, and Hypnum. These are commonly

called feather mosses. The fibers are easily distinguished from sphagnum

moss fiber because they lack the whorl of leaves at ,the growing tip. Well

preserved fibers are darker brown in color than Sphagnum, and commonly the stems are serrated. Properties of these horizons useful in their

identification include their dark-brown color, relatively high pH varying from 5.0 to 7.5, higher ash content than that of Sphagnum, and high

nitrogen content, usually double that of Sphagnum mosses (Table X ) .

Hypnofibric horizons rarely occur on the surface of Fibrist organic

soils, but generally constitute the subsurface portion of these soils.

Horizons are commonly 2 to 5 feet in thickness, and always they are

found in depressions located in enclosed or semienclosed basins surrounded by higher-lying lime-rich mineral soils. In the Midwest this

type of organic soil occurs in moraine bogs associated with calcareous

glacial drift.

3. Dysfibric

Fibric horizons composed chiefly of fibers other than those containing over 75 per cent Sphagnum or Hypnum moss are further divided

into two subtypes: one with pH (KCl) of 5.0 or less and one with pH

(KCI) of more than 5.0. The former is called dysfibric, the prefix dys

(from the Greek prefix dys, which means bad) implies low base status. The

other subtype is called eufibric, the prefix eu (from the Greek prefix

eu, meaning good) implies high base status.

The principal fibers originate from a diversity of herbaceous bog

plants such as sedges, reeds, grasses. The flat platelike structures of

many of these leaves and the fine fibrous nature of many of the grass

blades and roots are the characteristic features of the organic remains

of these horizons. Plant remains are sufficiently preserved to be readily

identified both macroscopically and microscopically. Wood particles from

black spruce, pine, white cedar, and other bog trees and shrubs also are

considered to be fiber. The specific kinds of fiber in these horizons is not

as significant to their classification and utilization as their base status

and other properties.

Dysfibric horizons occur in organic soils throughout the world. The

low base status, which is the most characteristic feature, is due either

to formation in isolated bog localities where base-rich waters are absent



CLASSIFICATION OF ORGANIC SOILS



141



or in areas where the acidifying influence of surface Sphagnum moss

has materially lowered the pH of the organic material in these horizons.

In Minnesota, for example, organic soils with dysfibric horizons occur

most frequently in northern and northeastern Minnesota in association

with the acid mineral soils and low-calcium bedrock areas, particularly

on the sandy glacial outwash plains and the pre-Cambrian shield areas.

Properties of these horizons which help distinguish them from others

include a low pH that is only slightly higher than in sphagnofibric

horizons, volume weights about double those of sphagnofibric horizons,

a color dominantly dark brown in hue, and a water-holding capacity

intermediate between the moss subtype horizons and the mesic or intermediate type of major horizon types.



4. Eufibric

Eufibric horizons are similar in appearance, origin, and major properties to dysfibric horizons, but differ in having a higher base status. The

pH value for these horizons is more than 5.0. These horizons commonly

occur in bog environments that are adequately supplied with bases.

Thus, the distribution of organic soils containing these horizons essentially

corresponds to the distribution of hard water containing large quantities

of calcium and magnesium carbonates. In Minnesota, the large glacial

lake plains such as the western portion of glacial Lake Agassiz and the

wet depressional areas and pot holes in the drier sections in the western

and northwestern part of the state contain organic soils with these

horizons. Many of the large organic soil areas in the Everglades of

Florida underlain with marl ( calcium carbonate ) contain entensive areas

with e d b r i c organic horizons as the dominant horizon type.



B. MESICHORIZONS

Mesic horizons are considered to be intermediate in degree of decomposition between the less decomposed fibric horizons and the more

decomposed sapric horizons. These horizons may have a semifibrous

structural make-up which is both physically altered ( disintegrated) and

partially biochemically altered ( humified ) .

The principal requirements of these horizons are as follows:

1. They must have a total fiber content exceeding one-third of the

total organic mass.

2. The fiber content may exceed two-thirds of the total mass if the

rubbed color of the wet fiber decreases in chroma (Munsell) by at least

2 units.

3. Also the fiber content may exceed two-thirds of the total organic

mass of the horizon if it does not change color upon rubbing but over



142



R. S. FARNHAM AND H. R. F I " E Y



50 per cent of the fibers are readily disintegrated and/or become very

greasy when rubbed wet.

4. Or, if upon drying the chroma of the rubbed organic mass increases by 2 or more units.

Commonly the wet organic material when rubbed has chromas of

1 or 2, like the unrubbed color of many sapric horizons that are well

decomposed. In these horizons plant fiber identification and characterization is not a requirement although botanical identification is possible using a microscope. The emphasis, therefore, has been put on those

morphological features that are indicative of at least partial alteration.

1. Dysmesic



The dysmesic subtype horizons are mesic horizons with a low base

status. The pH (KCl) is 5.0 or less. Those types of horizons generally

occur in bog areas where the surrounding mineral soils are noncalcareous or in calcareous soil areas where the organic material has been

acidified from growth of surface Sphagnum moss.

2. Eumesic



The horizons are essentially like the dysmesic types but differ in

having a higher base saturation. The average pH ( E l ) is more than

5.0, and they generally are located in areas containing calcareous soil

parent materials which after runoff affect the basicity of the lower-lying

organic soils.

C. SAPRIC

HORIZONS

The sapric horizons are the most highly decomposed of the major

diagnostic organic horizon types. They contain the least amount of

fiber, have the highest bulk densities, and exhibit the least color changes

with varying moisture contents.

The basic requirements of these horizons are as follows:

1. Fiber content must be less than one-third of the total mass of the

horizon.

2. There must be little or no color change when rubbed wet,

3. The sodium pyrophosphate extract on white filter paper is lower

in value or higher in chroma than 10YR7/3 (Munsell).

These horizons commonly occur as the surfaces of most drained and

cultivated organic soil areas. Saprist organic soils also occur in undrained

bog areas constantly nourished by lime-rich and well-aerated water. For

example, bogs containing thick stands of white cedar in the Northern

Lake States always contain sapric organic horizons. These horizon types

are widely distributed and readily recognized throughout the areas of



CLASSIFICATION OF ORGANIC SOILS



143



occurrence of organic soils all over the world. Organic soils containing

these horizons even occur in areas of Alaska and northern Canada where

permafrost is present. The most important properties of sapric horizons

are the low fiber content, high bulk densities, and comparatively low

water-holding capacity.



1 . Dysapric

These are highly decomposed organic horizons (sapric) that are quite

acid and high in organic content. Otherwise they are essentially like the

other subtypes of sapric horizons. These horizons occur in relatively flat

large bogs where conditions for decomposition were favorable because of

good aeration, and water movement was rapid enough to prevent stagnant

conditions from developing.



2. Clastidysapric

These are acid subtype sapric horizons which contain between 50 and



70 per cent mineral matter. Actually these horizons approach mineral

soil horizons in that the high mineral content affects the physical and

chemical properties of this material. They will absorb less water and

they have higher bulk densities than the dysapric horizons.



3. Eusapric

These subtype horizons differ from the dysapric only in being less

acid. They probably have formed under the continual influence of limerich waters. Also they contain less than 50 per cent mineral matter as an

admixture with the organic matter. Hence, these horizons are distinguished from the clastieusapric horizons.



4. Chtieusapric

These are subtype horizons that are both higher in mineral content

(50 to 70 per cent) and contain more exchangeable bases than the

dysapric horizons, They differ from eusapric horizons in having higher

contents of mineral matter, These horizons frequently occur in bogs

surrounded by calcareous mineral soils where erosion runoff water

periodically brings both bases and mineral soil particles into the bog

as sediments.

VI. Bases for Proposed Classification System



The classifkation system proposed for organic soils is a classification

scheme based essentially on certain morphological properties distinguishable in the field. The principal differences between this system and

other systems for classifying peat or organic soils are as follows:



144



R. S. FARNHAM AND H. R. FINNEY



1. The main emphasis in this system is on morphological properties

of certain diagnostic horizons, not on botanical remains, geology, topography, or chemistry of bog waters.

2. The object classified is a three-dimensional body occurring on

organic terrain which has certain designated thickness limits.

3. The system uses as many semiquantitative to quantitative evaluations as necessary so that good precision is both possible and reproducible.

4. Classes are carefully selected which are clearly and repeatedly

distinguishable.

5. The system is basically designed for use in making detailed soil

surveys but is also useful for broad groupings of organic soils. This is

not necessarily an objective in many other organic soil classification

systems.

A. NOMENCLATURE

The system of nomenclature for a new classification system is important since it greatly affects its acceptance and utility. The nomenclature used in this system is based on that proposed by the Soil Survey

Staff (1960), which, according to G. D. Smith (1963), is based on the

following principles:

1. The formative elements were to come from the classical languages

insofar as possible, so that the names might be mnemonic and connote

some of the properties, and so that they would fit readily into as many

modern languages as possible and be distinctive.

2. The name should indicate the place of a taxon in the system.

From the name, one should be able to recognize both the category of the

taxon and the taxa in any of the higher categories to which it belongs.

3. The names should be as short as possible. This is critical in the

higher categories if the names of taxa in lower categories are to be

manageable in speech.

4. The names should be as euphonic as possible.

5. Existing terms were to be avoided.

Names used for organic horizons are taken from the classical Ianguages (Greek and Latin) with the suffixic to form adjectilles. The name

fibric comes from the Latin wQrd fibru (fiber containing); the name

mesic comes from the Greek word meso (intermediate), which implies

intermediate stage of decomposition; and the name sapric comes from

the Greek word supros (rotted), which implies a high degree of decomposition.



B. BASICPRECEPTS

The basic precepts or guidelines for the proposed organic soil classification system include the following:

1. Definition of organic soil (Histosol): a soil with over 20 to 30



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