The Composition and Use of Fertilizers. 253 



other relates to the quantities in which fertilizers should be ap- 

 plied. Granting that we know fairly well whether we need to 

 use nitrogen, or phosphoric acid or potash compounds, or some 

 combination of these, how can we ascertain how much to put on 

 an acre of land? This question is quite as complicated as the 

 preceding one. A variety of conditions must be considered. If 

 we knew how much available plant food there were in an acre of 

 soil, and how much the growing crop would require, the matter 

 would be comparatively simple, provided the mechanical condition 

 of the soil was satisfactory. The form of fertilizing material 

 used would also need consideration. 



There are two extremes which we must strive to avoid. On 

 the one hand, we can assume that the supply of food in the soil 

 is fairly abundant and make only small additions, thus running 

 the risk of using too small amounts for growing a good crop. 

 On the other hand, we may assume that the supply of food in the 

 soil is decidedly deficient and put on quantities of fertilizing 

 material sufficient in itself to grow a good crop. In this case, 

 we run the risk of putting on more than the present crop needs. 

 If we cannot hit upon the desired medium, putting on amounts 

 that will grow the best crop most economically without leaving 

 too much plant food over, it is better to make the mistake of 

 putting on too much than too little. Only individual experiment- 

 ing can determine in each case how much nitrogen, potash, and 

 phosphoric acid can be used to best advantage, and to this line 

 of work attention will now be given. 



Experiments in Learning Plant Food Needed. — In order 

 to ascertain with certainty what food elements are lacking in the 

 soil, the surest way is for each farmer to do some experimenting 

 on his own soil and crops. In fact, if farmers are unwilling to 

 carry out some simple experiments for themselves on their own 

 farms and with their own crops, they cannot expect ever to make 

 use of plant food in the most economical and intelligent manner. 



It is our purpose now to indicate in some detail how such ex- 

 perimenting can be done by individual farmers. 



(a) Selection of Land for Field Tests. — The land selected for 

 testing plant-food needs should be in good condition in respect 

 to tillage, drainage, etc. The soil should be as much alike as 

 possible throughout the test field. It is desirable also that the 



