now much Fertilizer is Needed 



97 



The chemical analysis of the plant, while of the great- 

 est use to the chemist in giving him suggestions, is of 

 no practical use to the farmer in determining the kind 

 of fertilizers or what amount shall be applied. The 

 chemical contents of plants vary in the different 

 seasons and in the different parts of the plant, and 

 also with the soil in which the plant grows: the plant 

 may take up more than it needs when some element is 

 abundant. Even the widest variation in the amount of 

 any one ingredient will be amply covered by the large 

 amount of fertilizer which is ordinarily applied. Con- 

 sider, for example, that the fruit of a tomato comprises 

 .05 per cent of phosphoric acid and .27 per cent of 

 potash. If the crop is ten tons of fruit per acre, it is 

 probable that more than the average amount of phos- 

 phoric acid required is ten pounds and of potash fifty- 

 four pounds. It is safe to assume that the land 

 itself should supply at least three -fourths of these 

 amounts. We will assume that one -fourth is to be 

 supplied by the addition of fertilizer. We should then 

 apply to the acre two and one -half pounds of phos- 

 phoric acid and about fourteen pounds of potash. As 

 a matter of fact, however, the smallest amounts which 

 are ever applied are many times in excess of these 

 amounts. Fertilizers must always be applied in excess. 

 It is impossible to distribute a very small quantity ; 

 roots do not occupy every part of the ground. Much 

 is risked in the chance that some may be used. 



FoUowing are figures which show that the best advice as to 

 the use of fertilizers does not closely follow the chemical con- 

 tent of the crop. The table gives the average amount of 



a 



