168 THE PRINCIPLES OF 



stances associated with superphosphate do very little more 

 than superphosphate can do alone. 



In the next place, I must call attention to the very marked 

 effects of ammonia dressings applied to the cereals. Ammonia 

 dressings alone, at the moderate rate of 200 Ibs. per acre, 

 increase the yield from 17J up to 30 bushels per acre in the 

 case of barley, and from 15 to 20 and 23 bushels in the case 

 of wheat. I might further support this evidence by taking 

 the case of oats, showing the immense effect of nitrogenous 

 dressings in the form of salts of ammonia on cereal crops. 

 Now, how are the results affected when you add not only 

 ammonia salts, but superphosphate ? As our previous reason- 

 ing might lead us to expect, the additions of ammonia are 

 rendered much more beneficial when there is likewise added 

 a sufficient amount of mineral plant food. Ammonia alone 

 produces a large amount of chlorophyll in the leaf, and there 

 are symptoms of the plant being over-charged with nitrogen 

 simply because there is an excess of nitrogen in the form 

 of nitrates over the available mineral matter. It is there- 

 fore necessary to add mineral matter, and that may be 

 done most efficiently by mixing ammoniacal manures with 

 superphosphates. By this addition the yield rises to 44 

 bushels per acre, which is not only an immense contrast with 

 the 17J bushels on the unmanured plots, but is strikingly 

 greater than the 22J bushels produced by superphosphate 

 alone, and also strikingly greater than the 30 bushels pro- 

 duced by ammonia salts alone. See therefore the importance 

 of giving the plant all it requires, not only plenty of nitro- 

 genous material, but likewise abundance of phosphates. 



In the next place I will endeavour to show what benefit 

 occurs if, in addition to ammoniacal manures and super- 

 phosphates, you introduce these substances of lesser import- 

 ance already mentioned, namely, sulphate of potash, sulphate 

 of soda, sulphate of magnesia. That question we are able 



