The Soils and Their Nutrients 229 



higher than those of basins (Figure 7-7) because of the much higher bulk 

 density of the trough soils. 



Soil pH values from the different microtopographic units generally 

 range from 5.1 to 5.7; thus these soils are moderately to strongly acid by 

 agricultural standards. They are, however, less acid than those of peat 

 bogs, which have pH values between 3.0 and 4.0 (Moore and Bellamy 

 1974). The high concentrations of H* ions in the soil tend to favor their 

 adsorption by the cation exchange complex, and decrease the adsorption 

 of metallic cations. 



Soil acidity varies both spatially and temporally, is generally con- 

 stant with depth, and shows some association between the more basic 

 values and high plant production. Polygon troughs and the rims of low- 

 centered polygons have relatively high soil pH values of 5.6 to 5.7, while 

 the basins of low-centered polygons and the centers of high-centered 

 polygons with peaty soils are the most acid sites, with soil pH values of 

 5.1 to 5.3. In studies of vehicle track disturbance the soil pH in the de- 

 pressions where vegetative growth was abundant was 5.8, while in the 

 control area it was 5.5 (Challinor and Gersper 1975). A drop in the mean 

 soil pH in the mesic meadow from 5.4 to 5.1 between 1970 and 1971 was 

 associated with a 20'^o drop in primary production (Dennis et al. 1978). 



Major Cations 



Over the range of microtopographic units from wet meadows to 

 tops of high-centered polygons, the quantity of exchangeable cations per 

 square meter in the upper 10 cm of the soil ranges from 3.7 to 7.9 g Na, 

 4.4 to 16.2 g K, 19.6 to 76.9 g Mg and 71.3 to 384.5 g Ca. The rims of 

 low-centered polygons have the largest pools of all cations; however, 

 potassium is equally abundant in the centers of high-centered polygons. 

 Wet meadows are lowest in all cations except potassium, which is lowest 

 in the basins of low-centered polygons. Mesic meadows are generally 

 richer than wet meadows, and troughs are richer than basins in ex- 

 changeable cations. 



Patterns of cation concentration in soil solution (Figure 7-8) differ 

 sharply from those of exchangeable pools. Soil solutions used for the 

 analysis of metallic cations were obtained using porous ceramic cups in 

 situ and a mild suction of -0.75 bar. High cation concentrations in the 

 soil solution of polygon basins and the tops of high-centered polygons 

 occur with low plant production while low concentrations in the troughs 

 occur with high plant production (Webber 1978). The properties of the 

 soil solution fluctuate during the summer and range widely between years 

 in response to thaw, precipitation, evapotranspiration, surface and sub- 

 surface flow, nutrient uptake by roots, and microbial activity. 



