262 POWER OF THE SOIL TO ABSORB SALTS [chap. 



The amount of potash lost is still smaller, from 

 3 to 12 lbs. per acre per annum, but it is distinctly 

 dependent on the amount supplied as manure, being 

 at a maximum with the dunged plot (2) and the plot 

 receiving minerals only (5), and greater from all the 

 other plots receiving potash than from those without 

 it, i.e., 3, 10, ii, 12, 14. The use of sulphate or 

 nitrate of soda increases the amount of potash in the 

 drainage water, not so, however, the use of sulphate 

 of magnesia. Practically all the soda, chlorine, and 

 nearly all the sulphuric acid, that are applied in the 

 manure pass through into the drainage water. 



A comparison of the drainage waters in winter 

 and spring shows that they are more concentrated 

 in the winter, because the manures (excepting the 

 nitrate of soda) have then been recently applied : the 

 chlorides wash out first, then the sulphates, and as 

 the season advances not only is the total amount of 

 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 



