44 



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 Conser- 

 vation 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 infor- 

 mation 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 properties 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 

 percent 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 carbon between 

 12 and 18 percent if the clay content of the mineral 

 fraction is between 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 sim- 

 plicity in writing definitions of taxa, a distinction be- 

 tween 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 hectare. The defi- 

 nition of a mineral soil, therefore, is based on thickness 

 of the horizons or layers, but the limits of thickness 

 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 per- 

 mitted 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 thickness 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 upper 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 overlying organic soil material; or 



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

 material immediately above the bedrock is 10 cm or 

 more thick, and either 



a. Organic soil material of < 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 

 undecomposed 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 frac- 

 tion 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 



