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<br />66
<br />SOIL SURVEY
<br />tion. These factors are climate, living organisms, time,
<br />relief, and parent material. All of these factors are
<br />highly complex. There are many kinds of climate and
<br />many combinations of living organisms. Parent mate-
<br />rials vary widely in physical, chemical, and mineralogi-
<br />cal properties, and there are great differences in the
<br />length of time that these factors have been active.
<br />Although the five factors have been traditionally
<br />accepted as those that influence soil development, a
<br />sixth factor-man and his activities-must be added
<br />to complete the list. Man's activity is all too frequently
<br />destructive, but nevertheless he drastically alters the
<br />character of the soil by such physical processes as
<br />mixing, removal, and fertilization, or he alters the
<br />soil's natural environment by controlling water move-
<br />ment or the plant cover.
<br />The history of the development of soil characteristics
<br />and the study of the interaction of the formative forces
<br />is called soil genesis. The characteristics themselves
<br />constitute the soil's morphology. Thus, the color of the
<br />soil is one feature of soil morphology. The reason that
<br />such a color developed is part of the soil's genesis.
<br />It is impossible precisely to reconstruct the history
<br />of a soil's development from the limited data available
<br />at any one location. To do so it would be necessary to
<br />observe the soil and its environment throughout the
<br />entire period of its development-in most cases, sev-
<br />eral thousand years. Since this is impossible, any recon-
<br />struction of the soil's genesis must be based on inter-
<br />pretations. These are drawn from the soil's morphology
<br />and our accumulated knowledge of how such mor-
<br />phology could most logically have developed.
<br />The system of soil classification used in the United
<br />States is based entirely upon the morphological fea-
<br />tures of the soil. These are features that can be
<br />observed or measured and that can be used to group
<br />similar soils or separate those that are dissimilar. The
<br />selection of what kinds or magnitudes of properties
<br />are to be considered as definitive between soils is
<br />guided by our understanding of what is a significant
<br />indicator of a major difference in genesis. Thus, the
<br />two are closely interrelated and both are essential to
<br />a good classification system.
<br />In the following sections a general evaluation of
<br />the factors that influence soil development in the
<br />Chaffee-Lake Area is attempted, and the manner in
<br />which soil morphology has been used to group the soils
<br />into the units of classification is outlined.
<br />Climate
<br />The climate of the Chaffee-Lake Area is of the
<br />semi-arid, continental type (3), but differences in
<br />temperature and precipitation vary widely and covari-
<br />antly with elevation. Variations in elevation are
<br />extreme and occur over relatively short distances. The
<br />elevation ranges from about 7,050 feet in the valley
<br />areas to more than 12,000 feet on some mountain peaks.
<br />Temperatures within the area vary inversely with ele-
<br />vation, growing colder as elevation increases. Yearly
<br />precipitation varies directly with elevation and
<br />increases as elevation increases. Because of the rapidly
<br />changing character of soil climate, generalizations rel-
<br />ative to the entire survey area are meaningless; how-
<br />ever, by understanding the trends and something of
<br />the magnitude of change in both temperature and pre-
<br />cipitation as they relate to elevation, a general evalua-
<br />tion of the climate at any point of interest can be
<br />constructed.
<br />The average annual air temperature at the Buena
<br />Vista weather station (elevation 8,020 feet) is 44° F.,
<br />and the average summer soil temperature is about
<br />59°. The average annual air temperature at the Lead-
<br />ville weather station (elevation 10,177 feet) is 36.2°,
<br />and the average summer soil temperature is 52°.
<br />For the purpose of evaluating soil climate, soil tem-
<br />perature measurements are more useful than air
<br />temperature measurements. Ten sites in the survey
<br />area and adjacent areas were measured to determine
<br />soil temperature at a depth of 20 inches. At the site
<br />of the lowest elevation (7,550 feet), Dominson gravelly
<br />sandy loam has, at a depth of 20 inches, an average
<br />annual soil temperature of 48.5° and an average sum-
<br />mer soil temperature of 62.7°. At the site of the
<br />highest elevation (11,325 feet), Bross gravelly sandy
<br />loam has, at a depth of 20 inches, an average annual
<br />soil temperature of 36° and an average summer soil
<br />temperature of 44.5°. This represents a decrease of
<br />about 3° in average annual soil temperature and 4.5°
<br />in average summer soil temperature for each 1,000
<br />feet rise in elevation.
<br />The 47° average annual soil temperature isotherm
<br />falls on north slopes at about 7,500 feet contour and on
<br />south slopes at about 8,000 feet contour. The 59° average
<br />summer soil temperature isotherm occurs at about
<br />8,500 feet on south slopes and about 8,000 feet on
<br />north slopes. In the southernmost parts of the valley,
<br />soil temperatures are warmer than 47°, but the area
<br />is small and is isolated from areas of similar soil
<br />temperatures by abruptly rising mountain ranges.
<br />Rather than introduce a group of new mapping units
<br />in the survey to accommodate the small acreage of
<br />soils having average annual soil temperatures warmer
<br />than 47°, they have been included with the colder soils
<br />that predominate in the survey area.
<br />Soil temperature has a pronounced influence on the
<br />activity of biological, chemical, and physical forces
<br />affecting soil development. During periods when the
<br />soil temperature is near or below 32°, chemical
<br />reactions in the soil are drastically slowed, and mechan-
<br />ical movement and physical weathering because of
<br />freezing and thawing are at a maximum. At an
<br />elevation of 7,750 feet, the period of time in which the
<br />soil temperature is near 32° or below at a depth of
<br />20 inches is about 50 days each year. At an elevation
<br />of 9,800 feet, this period is about 120 days, and at
<br />11,325 feet, it is about 140 days.
<br />Biological life, though still active, is drastically
<br />slowed when soil temperatures are 41° or lower. At an
<br />elevation of 7,750 feet in this area, temperature of the
<br />soils of the Dominson series at a depth of 20 inches is
<br />more than 41° for about 243 days each year. At 10,000
<br />feet, this figure is about 171 days for the Pierian series,
<br />and at 11,325 feet, it is about 99 days for the Bross
<br />series.
<br />Precipitation increases with increasing elevation and
<br />at the Leadville station (elevation 10,1?7 feet) the
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