200 THE LIVING ORGANISMS OF THE SOIL [chap. 



equivalent to ioo 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 1 \ 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 

 less nitrogen than that given by the unmanured plot. 

 Where straw was used with nitrate of soda the two 

 gave a crop containing 23 lbs. less nitrogen per acre 

 than the nitrate alone; where urine was used alone, 

 the produce contained 25 lbs. more nitrogen per acre 

 than when it was used in conjunction with cow-dung 

 and straw. 



In fine, all the results pointed to the same con- 

 clusion — that large amounts of fresh organic manure 

 not only do not themselves help the crops, but 

 diminish the effect of other rapidly acting nitrogenous 

 manures like nitrate of soda, sulphate of ammonia, or 

 urine. 



The action cannot, in the two latter cases at least, 

 be put down to denitrification proper, unless it is 

 supposed that nitrification and subsequent denitrifica- 

 tion can proceed practically simultaneously in the 

 same soil. It must either be attributed to the fact 

 that nitrification is very much checked by the 



