42 



APPENDIX D 



Criteria for Distinguishing Organic 

 Soils from Mineral Soils 



The criteria for distinguishing organic soils from mineral 

 soils in the United States (U.S. Soil Conservation Service, 

 Soil Survey Staff 1975:13-14, 65) are quoted here so that 

 those without ready access to a copy of the Soil Taxonomy 

 may employ this information in the classification of 

 wetlands: 



For purposes of taxonomy, it is necessary, first, to define 

 the limits that distinguish mineral soil material from organic 

 soil material and, second, to define the minimum part of a 

 soil that should be mineral if the soil is to be classified as 

 a mineral soil. 



Nearly all soils contain more than traces of both mineral 

 and organic components in some horizons, but most soils are 

 dominantly one or the other. The horizons that are less than 

 about 20 to 35 percent organic matter by weight have prop- 

 erties that are more nearly those of mineral than of organic 

 soils. Even with this separation, the volume of organic matter 

 at the upper limit exceeds that of the mineral material in 

 the fine-earth fraction. 



MINERAL SOIL MATERIAL 

 Mineral soil material either 



1. Is never saturated with water for more than a few days 

 and has <20 percent organic carbon by weight; or 



2. Is saturated with water for long periods or has been 

 artificially drained, and has 



a. Less than 18 percent organic carbon by weight if 60 per- 

 cent or more of the mineral fraction is clay; 



b. Less than 12 percent organic carbon by weight if the 

 mineral fraction has no clay; or 



c. A proportional content of organic cabon between 12 and 

 18 percent if the clay content of the mineral fraction is be- 

 tween zero and 60 percent. 



Soil material that has more organic carbon than the 

 amounts just given is considered to be organic material. 



DISTINCTION BETWEEN MINERAL SOILS AND 

 ORGANIC SOILS 



Most soils are dominantly mineral material, but many 

 mineral soils have horizons of organic material. For simplicity 

 in writing definitions of taxa, a distinction between what is 

 meant by a mineral soil and an organic soil is useful. In a 

 mineral soil, the depth of each horizon is measured from the 

 top of the first horizon of mineral material. In an organic 

 soil, the depth of each horizon is measured from the base 

 of the aerial parts of the growing plants or, if there is no 

 continuous plant cover from the surface of the layer of 

 organic materials. To apply the definitions of many taxa, 

 therefore, one must first decide whether the soil is mineral 

 or organic. 



If a soil has both organic and mineral horizons, the relative 

 thickness of the organic and the mineral soil materials must 

 be considered. At some point one must decide that the 

 mineral horizons are more important. This point is arbitrary 



and depends in part on the nature of the materials. A thick 

 layer of sphagnum has a very low bulk density and contains 

 less organic matter than a thinner layer of well-decomposed 

 muck. It is much easier to measure thickness of layers in the 

 field than it is to determine tons of organic matter per hec- 

 tare. The definition of a mineral soil, therefore, is based on 

 thickness of the horizons or layers, but the limits of thick- 

 ness must vary with the kinds of materials. The definition 

 that follows is intended to classify as mineral soils those that 

 have no more organic material than the amount permitted 

 in the histic epipedon, which is defined later in this chapter. 

 To determine whether a soil is organic or mineral, the 

 thickness of horizons is measured from the surface of the 

 soil whether that is the surface of a mineral or an organic 

 horizon. Thus, any horizon at the surface is considered an 

 organic horizon, if it meets the requirements of organic soil 

 material as defined later, and its thickness is added to that 

 of any other organic horizons to determine the total thick- 

 ness of organic soil materials. 



DEFINITION OF MINERAL SOILS 



Mineral soils, in this taxonomy, are soils that meet one of 

 the following requirements: 



1. Mineral soil material <2 mm in diameter (the fine-earth 

 fraction) makes up more than half the thickness of the up- 

 per 80 cm (31 in.); 



2. The depth to bedrock is <40 cm and the layer or layers 

 of mineral soil directly above the rock either are 10 cm or 

 more thick or have half or more of the thickness of the over- 

 lying organic soil material; or 



3. The depth to bedrock is>40 cm, the mineral soil material 

 immediately above the bedrock is 10 cm or more thick, and 

 either 



a. Organic soil material is <40 cm thick and is decomposed 

 (consisting of hemic or sapric materials as defined later) 

 or has a bulk density of 0.1 or more; or 



b. Organic soil material is <60 cm thick and either is un- 

 decomposed sphagnum or moss fibers or has a bulk density 

 that is <0.1. 



ORGANIC SOIL MATERIALS 

 Organic soil materials and organic soils 



1. Are saturated with water for long periods or are arti- 

 ficially drained and, excluding live roots, (a) have 18 percent 

 or more organic carbon if the mineral fraction is 60 percent 

 or more clay, (b) have 12 percent or more organic carbon 

 if the mineral fraction has no clay, or (c) have a proportional 

 content of organic carbon between 12 and 18 percent if the 

 clay content of the mineral fraction is between zero and 60 

 percent; or 



2. Are never saturated with water for more than a few days 

 and have 20 percent or more organic carbon. 



Item 1 in this definition covers materials that have been 

 called peats and mucks. Item 2 is intended to include what 

 has been called litter or horizons. Not all organic soil 



