"8 



effect of nitrogenous manures, the following summary may 

 be chosen, showing the average produce obtained from the 

 use of nitrogen on various crops at Rothamsted ; each crop 

 being grown continually on the same land, in most cases for 

 a long series of years. 



TABLE I. 



Comparing the produce with mineral manures, which in- 

 clude, phosphates, potash, &c., and that with mineral 

 manures and ammonia salts, we at once see the immense 

 gain from the use of nitrogenous manures. In the case of 

 three of the six crops mentioned viz., wheat, oats, and man- 

 gels the nitrogen added to the mineral manures more than 

 doubled the crop, and with barley it nearly doubled it. 



It will be noticed, however, that the produce with ammonia 

 salts only was not nearly so large as with mineral manures 

 and ammonia salts together, and in two cases was below 

 that from minerals only. Neither minerals alone nor 

 ammonia salts alone could produce such a good crop as 

 when they are used together. Indeed, this follows from 

 what has been already said. The mineral manure supplies 

 all the essential constituents of plant food except nitrogen, 

 while ammonia salts supply nitrogen, but nothing else. 

 Therefore, with the application of either for a series of years 

 on the same land and crop, some essential substance will 

 soon become deficient, and the produce will fall off in propor- 

 tion. When both minerals and ammonia salts are used, how- 

 ever, everything necessary is being supplied, and a full crop 

 may be obtained. It is not strictly accurate to say that the 

 difference between the produce with minerals and that with 

 minerals and ammonia salts is due entirely to the ammonia 

 salts. It is rather due to both manures being able to give 

 their full effect when used together. 



In passing, it may be noticed that many misleading 

 results have been obtained in trials of manures^particularly 

 in field experiments for want of proper appreciation of the 

 interdependence of the different classes of manures. 



In ordinary farm practice, however, the soil is usually 

 more deficient in nitrogen than in minerals, and then, from 

 the farmer's point of view, the increase shown by the addition 



