58 ELEMENTARY AGElCtJLTURAL CHEMISTRY 



The proportion of the rainfall which drains away varies with 

 many circumstances — the distribution of the rainfall, the re- 

 tentiveness of the soil, the rapidity of evaporation from the 

 surface, and others. The amount evaporated depends largely 

 upon whether the soil is bare or covered with vegetation, being 

 very much greater in the latter case. 



At Rothamsted, as the average of twenty years (1877-78 to 

 1896-97) with a rainfall of 29'5 inches, the drainage through 

 5 feet of bare soil amounted to 14*7 inches. In very wet 

 seasons the amount and proportion of drainage is greater ; 

 e.g.^ in 1878-79 the total rainfall was 41 inches and the drainage 

 24*4 inches, while in the very dry year 1897-98 the numbers 

 were 19*5 and 6 5 respectively. 



From cultivated soil bearing a crop the drainage is much 

 less. In experiments conducted in France, fallow soil gave a 

 total drainage of 11"5 inches, while similar soil with a crop of 

 potatoes gave only 5 •83 inches. 



Losses caused by Drainage. — The water draining from 

 land always carries with it dissolved matter. The substances 

 whose removal in this way is the most important are the 

 nitrates. The loss is greatest from uncropped soil^ for several 

 reasons — 



(1) Because of the greater amount of drainage ; 



(2) Because no absorption of nitrates by the roots of plants 

 occurs ; 



(3) Because the land, being free from crops, dries more 

 slowly, and so the moisture favourable for nitrification is 

 retained for a longer period, especially in dry weather, when 

 the temperature is often high, and, therefore, most favourable 

 for nitrification. 



The average annual loss of nitrogen as nitrates from un- 

 cropped land at Rothamsted for the twenty years 1877-78 to 

 1896-97 amounted to 33*8 lb. per acre (equivalent to 216 lb. 

 of commercial nitrate of soda). The loss, of course, will vary 

 greatly with the nature of the soil. At Grignon, near Paris? 

 in the year 1896-97 the loss of nitrogen from fallow soil was 



