The Soils and Their Nutrients 



225 



TABLE 7-1 



Seasonal A verages of the Percentage of Moisture, Water 

 Potential, and Redox Potential in the Upper 15 cm of Soil, 

 and Soil Temperature During Summer 1972. 



Soil Moisture and Aeration 



Because of their very high content of organic matter and related 

 high porosity, soils of the Biome research area have a high capacity to 

 hold water. Field moisture contents range to over 1000% dry weight. 

 However, the extreme variability in bulk density and porosity makes ex- 

 pression of soil moisture content on the basis of volume, rather than 

 weight, more useful. In terms of soil volume, maximum moisture content 

 in the upper 15 cm of meadow soils averaged 47% during the summers of 

 1970 and 1971. The underlying mineral layers ranged between 55 and 

 60% by volume. In these soils the minimum moisture content in the up- 

 per 15 cm averaged 67% in 1970 and 65% by volume in 1971. During the 

 summers of 1970-73 the soils in much of the nonpolygonal terrain and in 

 the lower-lying areas of polygonized terrain remained almost completely 

 water-saturated within approximately the 5-to 15-cm depth interval. On 

 polygonized terrain, in the relatively warm and dry summer of 1972, only 

 the centers of high-centered polygons had moisture contents of less than 

 65% by volume in the upper 10 cm of soil on 31 July. 



In 1972, moisture tension was measured using tensiometers in soils 

 along a moisture gradient in the polygonal terrain (Table 7-1). In the dri- 

 est soils, on the top of a high-centered polygon and on a polygon rim, 

 water potentials averaged over the 0- to 5-cm depth interval were never 

 lower than -0.070 bar during the summer (Figure 7-5). Potentials at 0- to 

 5-, 5- to 10- and 10- to 15-cm depth intervals at a given location were 

 similar, but with a tendency toward progressively higher potentials with 

 depth. The low water potentials in the soil indicate that most of the water 

 present is available for plant uptake. Variations with time were similar at 

 all three depth intervals and within each microtopographic unit across 

 the entire moisture gradient (Figure 7-5). Despite the general wetness, 



