VIII.] 



NITRATES IN DRAINAGE WATERS 



269 



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 

 I 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 the 

 rainfall was low 



The difference between the results set out in 

 the table on p. 268 and those obtained upon the 

 corresponding plot at Rothamsted, where the dressing 

 of ammonium salts every other year seems to leave no 

 residue for the following year, may perhaps be set down 

 to the different texture of the two soils. The ammonium 

 salts are converted which are washed down into the 

 subsoil ; at Woburn they can rise again by capillarity, 

 as the soil, though sandy, is still fine in texture ; at 

 Rothamsted the soil is too close-grained to admit of 

 any considerable movement of the subsoil water back 

 to the surface. 



