228 THE LIVING ORGANISMS OF THE SOIL [chap. 



The nature of the results obtained may be seen 

 from the following table, which gives the percentage 

 recovered in the crop of the nitrogen supplied in the 

 manure, when used alone, or in conjunction with fresh 

 horse-dung : 



Percentage of Nitrogen Recovered (Wagner}"- 



Nitrate of Soda . 

 Sulphate of Ammonia 

 Urine . 

 Grass . 



Per cent. 



recovered when 



used alone. 



77 

 69 

 69 



43 



When used 



with 

 Horse-dung. 



52 

 50 

 40 

 20 



Numbers of similar experiments in pots have been 

 recorded. In some cases the use of fresh dung has 

 even resulted in a smaller crop than was obtained with- 

 out any manure at all ; but it should be noted that 

 very large amounts of the organic manures were used, 

 equivalent to 100 tons or more per acre. Similar results 

 have, however, been recorded in field trials, as in some 

 experiments of Kriiger and Schneidewind's, where fresh 

 cow-dung was applied at the rate of 23 tons per acre, 

 horse-dung, 21 tons per acre, and wheat straw at 5-8 

 tons per acre, on 10th July. These three plots were 

 in part cross-dressed with urine or with nitrate of soda, 

 each supplying 43 lbs. of nitrogen per acre. Two suc- 

 cessive crops of mustard were immediately grown, and 

 the amount of nitrogen removed by the crop was 

 ascertained. Compared with the wholly unmanured 

 plot, the cow-dung alone slightly depressed the crop, 

 about ih lbs. per acre less nitrogen being recovered ; 

 the horse-dung produced a depression of nearly double 

 this amount ; the wheat straw produced the greatest 

 depression, its crop containing about 18 lbs. per acre 



