MODERN SOIL SCIENCE—KELLOGG 243 
tion; in fact, there is no practical way now known to maintain the 
nutrient supply and productivity of these soils as high as they were 
under continuous grass and still use them for crops under anything 
like present economic conditions. However, these soils can be main- 
tained in cultivation at a slightly lower level of productivity under 
practices that maintain the nutrient supply and organic matter and 
control the water supply to avoid excess run-off and erosion. This is 
to be expected, since these soils were developed under grasslike plants, 
similar to the crops the farmer wants to grow. 
Exchange Capacity (M.E,/100gms.) 
, 40 60 80 100 120 140 pH 
s 46 
- SS 48 
49 
Depth 
DAN AEe MpEtD ORC Beet aD eNO TN CHOOT, RETIN Es teats dh atn iso 
A. Under Spruce 
Exchange Capacity (M. &./100 ms.) 
20 4 60 80 100 120 pH 
fe) OE Oi IL AU TN SS) 
5.0 
n 
2 §.1 
£ 
= ie 
Ss Ba Exchangeable Bases 
« 20 5.2 
= : Exchangeable Hydrogen 
fa : 
30 
Q 5.3 
“B. Under Hardwood 
Figure 6.—These graphs iliustrate a comparison of total exchangeable bases 
and exchangeable nitrogen in two soils developed under similar environmental 
conditions, except for the vegetation. Because the spruce needles and twigs 
decompose slowly there is a considerable accumulation before decomposition 
is equal to the annual drop from the forest. Since the hardwoods drop nearly 
twice as much calcium and other bases each year, soils developed under hard- 
woods have a relatively higher amount of exchangeable bases. The upper 
chart indicates clearly the horizon of extreme leaching lying almost directly 
under the organic mat of a Podzol (8). 
