The Profitable Limit of Growth. 47 



36J bushels per acre, whilst where 861b. of nitrogen was 

 applied it was only 3| bushels less. The total increase 

 obtained by this large amount of nitrogen (1291b.), in excess 

 of the produce obtained by minerals alone, amounted only 

 to 21 bushels per acre per annum ; hence it is clear that in 

 this experiment the nitrogen applied was in excess of the 

 quantity which could be utilised by the crop ; and even in 

 the best season of growth, 1863, when plot 7, receiving 861b. 

 of nitrogen, yielded 53-|- bushels per acre, the plot receiving 

 1291b. only yielded 2 bushels more ! 



Some important practical inferences are now deducible. 

 Sir John Lawes and Dr. Gilbert estimate the average yield 

 of wheat in Great Britain at 28 bushels, while others put it 

 at 30 bushels per acre. The crop obtained by 861b. of 

 nitrogen appears to have quite reached, if it has not exceeded, 

 the profitable limit of growth, and only a rise in the price of 

 wheat and not lower prices could justify the outlay in 

 manure which would be required to grow a larger crop. 

 Much of the nitrogen of the ammonia-salts has been con- 

 verted into nitric acid, and washed into the drains, chiefly 

 during the winter. At the time of applying the nitrate of 

 soda in the spring, the plots receiving ammonia-salts in the 

 autumn had already lost more or less nitrogen ; and until 

 1878 the trial of the relative crop -producing power of 

 nitrogen as ammonia, and as nitric acid in the form of 

 nitrate of soda, was not carried out on equal terms. For 

 the crops of 1878-83 plots 7a and 7b received the ammonia- 

 salts in the spring, whilst another plot similarly manured 

 received these salts in the autumn ; and, although the spring- 

 sown ammonia has given the largest produce, still, the 

 difference between the two crops is by no means what might 

 have been expected from the known loss by drainage which 

 took place when the manure was autumn- sown, so that the 

 question arises whether there has been loss by destruction 

 and evolution of free nitrogen, or whether any considerable 

 amount of the ammonia has remained unnitrified in the soil, 



