148 ELEMENTS OF AGRICULTURE 



trouble ascertained; then the proper course of treat- 

 ment may be determined on. It is no simple matter 

 to restore the fertility of a worn soil, and there are no 

 fixed rules to be followed; each individual case must 

 be carefully studied and treated. Very often the proper 

 treatment can be determined only by experiment. 



139. Special Manuring. — In using special manures 

 the manure is added for the benefit of some one par- 

 ticular crop, and it is of more importance to know the 

 needs of the crop in question than it is to know the 

 needs of the soil. For instance, clover and cowpeas are 

 crops which require large amounts of nitrogen com- 

 pounds for their production, but which are able to draw 

 on the atmosphere for a large part of their needs. It is 

 therefore useless to supply such crops with nitrogenous 

 manures. The soil may be very poor in nitrogen, but 

 that is of little consequence so far as these crops are 

 concerned; they are to a certain extent independent of 

 the soil for their nitrogen supply. Again, 1,000 pounds 

 of tobacco leaves require for their production 40 

 pounds of potash and only 3.4 pounds of phosphoric 

 acid. It is evident that tobacco requires for its pro- 

 duction more potasri than phosphoric acid, and we may 

 use this knowledge in preparing a fertilizer for use on 

 this crop. 



When manures are applied to increase the yield of any 

 particular crop, it is the crop that is to be fed, and not 

 the soil. By analyzing the crop in question we may learn 

 the amounts of plant food it requires for its produc- 

 tion. Now, it would seem an easy matter to analyze 

 the soil and find out what it lacks in the way of plant 



