II.] ACIDITY CAUSED BY AMMONIUM SALTS 63 



refuses to grow barley any longer, though the former 

 fertility is at once restored by the application of a 

 dressing of lime. The soil of the plots receiving 

 ammonium salts is actually acid to litmus paper, and a 

 similar condition prevails on some of the grass plots 

 at Rothamsted, where the soil, unlike that of the 

 wheat field, is deficient in calcium carbonate. At 

 Woburn the soil is a light sandy loam which con- 

 tained in 1876, when the experiments began, only 

 0-074 per cent, of calcium carbonate. For many years 

 the wheat and barley plots manured with ammonium 

 salts gave as good returns as those receiving an equal 

 amount of nitrogen as nitrate of soda. Towards 1895 

 it became every year more difficult to obtain a plant 

 of barley on the plots receiving ammonium salts, the 

 soil was noticed to be acid to litmus paper, and certain 

 special weeds, spurry in particular, invaded the plots. 

 The plots were then divided, and on one portion 2 

 tons per acre of lime were applied, whereupon the 

 soil recovered its healthy condition and the crop was 

 restored. 



Table XV. shows the result on the crop of 1904 

 of the dressing of lime applied in December 1897, 

 and also the destruction of the crop where the 

 soil had remained acid through the use of ammonium 

 salts. 



The effect of sulphate of ammonia upon wheat at 

 Woburn is not so marked as upon barley, probably 

 because of the deeper rooting habit and more robust 

 constitution of the wheat plant. 



The acidity of the soil where the ammonium salts 

 have been used is due to the attack of various moulds 

 and other micro-fungi upon the ammonium salts ; they 

 seize upon the nitrogen for their own nutrition, and set 

 free the acids with which the ammonia was combined. If 



