No. 4.] KEEPING UP FERTILITY. 93 



We may, it is true, raise crops for many years by supply- 

 ing the three elements nitrogen, phosphoric acid and potash 

 (incidentally lime is applied with the phospate). Lawes 

 and Gilbert have raised wheat fifty consecutive years on the 

 same land on ammonia salts, superphosphate and sulphate of 

 potash ; and at the present time the yield is nearly as great 

 on this land as on that which has received fourteen tons 

 vearlv of farm-yard manure for the same length of time : but 

 the yield on the fertilizers now begins to show a tendency 

 to fall of, and there can be no doubt that the more complete 

 composition of the manure is proving an advantage. Of 

 course in such materials as common salt, sulphate of mag- 

 nesia, etc., we may replace, if considered desiral)le, the 

 soda, magnesia, etc., carried oiF in the crops; but even such 

 replacement would leave us without the physical eflect of 

 manure, — an effect which we may, it is true, in part secure 

 by turning in a green crop. Under many systems of 

 cropping, however, green manuring is inexpedient. You 

 cannot afford to give up the land for it. You have got to 

 keep the land producing something you can sell. 



For potatoes, because this crop is generally cleaner and 

 freer from disease on fertilizer only, I would make an excep- 

 tion to the general rule ; and I should also except most of 

 the crops involving much hand labor, such as onions, 

 because there are less weeds where fertilizer alone is applied. 

 In the case of a field lying at a great distance from the barns, 

 there is also an evident saving in depending upon fertilizers 

 alone. 



Fertilizers. 



Having thus stated in outline my ideas as to the saving, 

 handling and application of manures, we may take up the 

 consideration of fertilizers as a means of keeping up soil 

 fertility. Under this general name are included a very large 

 number of materials of very diverse characters. You will 

 not expect me to take up each in detail ; here, as with other 

 parts of my subject, I can deal only in outline. It will be 

 an advantage to adopt some classification of fertilizers, and 

 for my present purpose, though not strictly scientific, I shall 

 speak of them under the following heads : phosphates, 

 special complete fertilizers, and raw materials or chemicals ; 



