VIII.] 



NITRATES IN DRAINAGE WATERS 



231 



receive 400 lbs. per acre of ammonium salts with mineral 

 manures, only differing in the fact that on plot 7 the 

 ammonium salts are sown in March, and on plot 15 in 

 October ; also the average crops of grain and straw for 

 the twenty-three years, 1875-97. 



1879-1881.— Lbs. of Nitric Nitrogen per Acre. 



That sulphate of ammonia will to some extent 

 persist in the soil, and become available for a succeed- 

 ing crop, after even a whole year has elapsed, is to be 

 seen from the results of the Woburn experiments upon 

 wheat. Some of the plots at Woburn receive mineral 

 manures every year, but ammonium salts or nitrate of 

 soda only every alternate year : in both cases the crop 

 falls very much in the years of no nitrogen, but the 

 decrease is by no means so marked with ammonium salts 

 as with nitrate of soda, which latter seems to leave no 

 residue whatever. 



The soil at Woburn is an open sandy loam ; but in 

 the years for which the results are quoted (see Table, 

 p. 232) the rainfall was low. 



The difference between the results set out in the 

 above-mentioned table and those obtained upon the 

 corresponding plot at Rothamsted, where the dressing 



