226 POWER OF THE SOIL TO ABSORB SALTS [chap. 



lime present much diminished, but it comes away 

 chiefly as carbonate. With the growth of the crop 

 in spring the nitrates disappear from the drainage 

 waters. 



The amount of nitrates found in the drainage water 

 varies not only with the time of year, but also according 

 to the interaction of temperature, growth of crop, culti- 

 vation, and percolation. Nitrates are only rapidly pro- 

 duced when the temperature of the soil has risen : if the 

 percolation is not excessive the crop may remove the 

 nitrates as fast as they are formed, but a heavy rainfall 

 in the spring before the nitrates have been much drawn 

 upon by the crop, or one just after the land has been 

 broken up in the autumn and is still warm, will result in 

 a considerable washing out of nitrates. At the same 

 time a certain amount of moisture in the soil is necessary 

 for the formation of nitrates, and the crop itself may so 

 dry the soil as to reduce nitrification considerably. The 

 following table (p. 227) shows the estimated loss of 

 nitrates from the same wheat plots at Rothamsted as have 

 previously been dealt with, during two years, each of 

 which has been divided into two periods : firstly, from 

 the date at which the nitrogenous manures were sown up 

 to harvest ; and secondly, from harvest round again to 

 the sowing of manures in spring. 



The diagram (Fig. 15) shows the same results in a 

 graphic form. 



The seasons were rather exceptional, the summer 

 rainfall and drainage in 1879 and the winter rainfall in 

 the following year being both above the average. It 

 will be seen that except on the autumn manured plot 

 15, the loss was greatest from plot 9 receiving 550 lbs. 

 of nitrate of soda, and this excess of loss was chiefly in 

 the summer drainage water of 1879; the figures are, 

 however, exaggerated by the fact that half the nitrate 



