70 GEOLOGY OF AqRICULTURE. 



food. This office will appear the more important, 

 when we consider that all growing plants perspire 

 largely. They take np large quantities of water from 

 the soil, appropriate to their own growth the nutritive 

 matter dissolved in it, and then throw it off from 

 their leaves, by insensible perspiration. The benefi- 

 cial effect of irrigating grass lands is probably owing 

 mainly to the fact, that as the water passes over the 

 field, it is constantly absorbing gases from the air and 

 conveying them to the roots of the grass. If the wa- 

 ter be impure, as happens with many streams, its im- 

 purities operate as fertilizers ; and the irrigation may 

 in this way be regarded as a sort of liquid manuring. 



108. Oxides of Iron (FeO and Fe'O').— The protox- 

 ide of iron (FeO) seldom exists in soils, except in 

 those which are low, wet and boggy. This, as before 

 stated, turns to the sesquioxide (Fe'^O^), under the in- 

 fluence of cultivation. This latter is red, and it is 

 this which gives that color to so many soils. In a 

 rich and productive soil, such as we are now consid- 

 ering, it may be found in proportions varying from 

 1 or 2 to 6 or 8 per cent.. 



109. Oxides of Manganese. — Of these, there is but 

 one that deserves to be mentioned as a constituent of 

 soils, the black oxide (MnO'') ; and this would seldom 

 be found to exceed one-half of one per cent. 



110. Potash (KO). — One per cent, df potash would 

 be considered an indication of great fertility so far as 



