CHEMISTRY OF SOILS 103 



It will be seen that the total for the whole year amounts 

 to 34 Ibs. of nitrogen per acre. That is about three times 

 as much as is lost by the unmanured plot under wheat crop. 



The loss of nitrogen by drainage depends partly upon 

 the amount of nitrates in the soil and partly upon the 

 amount of percolation. This relation is strikingly illus- 

 trated by the curves (Fig. 16). 



During the first four months of the year the actual loss, 

 in Ibs. per acre, diminishes almost in the s,ame proportion 

 as the amount of percolation ; then it begins to diverge 

 because the proportion of nitrates in the soil gradually 

 increases whereas the amount of percolation remains prac- 

 tically constant till the end of September. The time at 

 which the formation of nitric acid takes place is shown 

 by the amount, in parts per million, of the drainage water. 

 The increase begins in April and continues slowly till 

 about the end of June, cT,fter which it proceeds very rapidly, 

 reaches a maximum in September, and then declines again 

 almost as rapidly (p. 102). The greatest amount of loss 

 (Ibs. per acre) takes place in October, November, and 

 December. The sharp rise in the first of these months is 

 due partly to increased percolation and partly to the much 

 larger amount of nitric acid in the soil where it is 

 formed rapidly during the months of July, August, and 

 September. 



These conclusions are, in the main, confirmed by the ex- 

 periments of Way, Deherain and others. 



Absorption of Salts by the Soil. The phenomena of 

 absorption, to which reference has been made, are inti- 

 mately connected with the question of the fertility of soils. 

 It is owing to this power of absorption that only small 

 quantities of the soluble substances naturally present in 

 the soil, or applied to it as manures, escape in the drainage 

 water. Otherwise, the soluble plant foods would soon all 

 be washed out of the soil and lost. It has been shown that 



