The Composition and Use of Fertilizers, 281 



13. Plant-Food Mixtures for Different Crops. 



"We will now consider in a more specific manner the quantities 

 of different forms of plant food that can be applied to advantage 

 to one acre of land for different crops. 



"We must make it clearly understood at the start that no 

 rigidly fixed formulas can be given for any crop on all soils. The 

 question of quantity of application and of proportion must al- 

 ways, in the very nature of the case, remain more or less a 

 matter of individual experiment. Every field is constantly 

 changing in the extent and character of its needs. The farmer 

 must constantly study results and let the experience of one 

 year suggest to him his plan for the year following. With this 

 preliminary precaution, it may be of interest to consider briefly 

 what has ordinarily been the method of procedure in determin- 

 ing the proportions of nitrogen, phosphoric acid and potash in 

 making fertilizers for different crops. 



In making formulas, it was first proposed to make the formula 

 correspond to the analysis of the plant. This method was prac- 

 ticed for some time, when it was found that there was already in 

 the soil more or less available plant food and that fertilizing 

 material was often applied where one or more constituents could 

 be omitted or reduced in quantity. It was then suggested that 

 soil analysis should form the basis of determining the needs of 

 the soil in different crops, but this failed to produce satisfactory 

 results. The formulas at present used by many have been based, 

 in part, upon the composition of the plant and, in part, upon 

 the actual field-tests. 



The amount of nitrogen called for by analysis of plants is 

 generally reduced because we can depend upon the soil to furnish 

 a considerable amount. In case of leguminous crops, the amount 

 of nitrogen which we need to supply can be reduced to a small 

 fraction of what the plant will use, because such crops can draw 

 their main supply of nitrogen from the air. 



The amount of soluble phosphoric acid is ordinarily increased 

 above what plant analysis calls for, because the solubility is more 

 or less decreased after the fertilizer comes into contact with the 



soil. . - , i 



The formulas given in the pages following have been drawn 

 from such various sources as could be considered reliable. 



It will be noticed that, in giving the amount of fertilizer ma- 

 terial to put on one acre of land, a variable rather than a fixed 



