Growing season - The portion of the year when soil temperatures are above biologic zero (41° F) as de- 

 fined by "Soil Taxonomy;" the following growing season months are assumed for each of the soil temper- 

 ature regimes: (1) thermic (February-October); (2) mesic (March-October); (3) frigid (May-September); (4) 

 cryic (June-August); (5) pergelic (July-August); (6) isohyperthermic (January-December); (7) hypenher- 

 mic (February-December), (8) isothermic (January-December) and (9) isomesic (January-December). 



Hardpan - A very dense soil layer caused by compaction or cementation of soil particles by organic matter, 

 silica, sesquioxides, or calcium carbonate, for example. 



Hemists - Organic soils (mucky peats and peaty mucks) in which plant remains show a fair amount of de- 

 composition; between one-third and two-thirds of the fibers are still visible upon rubbing the material be- 

 tween the fingers. 



Herb - Nonwoody (herbaceous) plants including graminoids (grass and grasslike plants), forbs, ferns, 

 fern allies, and nonwoody vines; for the purposes of this manual, seedlings of woody plants that are less 

 than three feet in height are also considered herbs. 



Herb stratum - Any vegetative layer of a plant community that is composed predominantly of herbs. 



Hisric epipedon - A 8- to 16-inch soil layer at or near the surface that is saturated for 30 consecutive days 

 or more during the growing season in most years and contains a minimum of 20 percent organic matter 

 when no clay is present or a minimum of 30 percent of organic matter when 60 percent or more clay is 

 present; generally a thin horizon of peat or muck if the soil has not been plowed. 



Histosols - An order in "Soil Taxonomy" (Soil Survey Staff 1975) composed of organic soils (mucks and 

 peats) that have organic soil materials in more than half of the upper 32 inches or that are of any thickness 

 if overlying rock. 



Horizon - A distinct layer of soil, more or less parallel with the soil surface, having similar propenies such 

 as color, texture, and permeability; the soil profile is subdivided into the following major horizons: A- 

 horizon, characterized by an accumulation of organic material; B-horizon, characterized by relative accu- 

 mulation of clay, iron, organic matter, or aluminum; and the C-horizon, the undisturbed and unaltered par- 

 ent material. (Note: Some soils have an E-horizon, characterized by leaching of organic and other materi- 

 al.) 



Hue - A characteristic of color related to one of the main spectral colors (red, yellow, green, blue, or pur- 

 ple), or various combinations of these principle colors; one of the three variables of color; each color chan 

 in the Munsell Soil Color Charts (KoUmorgen Corporation 1975) represents a specific hue. 



Hydric soil - A soil that is saturated, flooded, or ponded long enough during the growing season to devel- 

 op anaerobic conditions in the upper part. 



Hydrology - The science dealing with the properties, distribution, and circulation of water. 



Hydrophyte - Any macrophyte that grows in water or on a substrate that is at least periodically deficient in 

 oxygen as a result of excessive water content; plants typically found in wetiands and other aquatic habitats. 



Hydrophytic vegetation - Plant life growing in water or on a substrate tiiat is at least periodically deficient 

 in oxygen as a result of excessive water content. 



Hypertrophied lenticels - An exaggerated (oversized) pore on the stem of woody plants through which 

 gases are exchanged between the plant and the annosphere; serving to increase oxygen to plant roots dur- 

 ing periods of inundation or soil saturation. 



69 



