Chapter IV. 



THE PLANT-FOOD THEORY OP FERTILIZERS. 

 The guiding principle in soil investigations for about three- 

 quarters of a century and until the past few years has been the 

 assumption that the principal function of the soil is to furnish 

 mineral nutrients to the plant, and that, to supply a lack in the 

 soil, fertilizers are added because of the mineral plant nutrients 

 they contain. This theory has apparently much to support it; 

 actually, however, the evidence usually cited accords better with 

 a more comprehensive generalization which will be formulated in 

 a later chapter. It is attractively simple. It will be shown 

 later, however, that this very simplicity is an argument against 

 its validity. 



Those substances which experience has shown to be useful 

 soil amendments usually contain one or more of the constituents 

 necessary to plant metabolism, commonly phosphorus, potassium, 

 nitrogen or calcium. Fertilizers do not always produce in- 

 creased yields of crops, but it has been usual to consider bad 

 results as due to other more or less extraneous causes. More- 

 over, as will appear later, crop yield is as strongly affected by 

 some substances containing no mineral plant nutrient as by 

 ordinary fertilizers. Again, the plant-food theory has been 

 apparently confirmed by the popular misconception that crop 

 yields are decreasing. Government statistics, however, indicate 

 very positively that crop yields are increasing in Europe as well 

 as in America, more in areas where the acreage is stationary 

 than in areas where the acreage is increasing, and in areas where 

 fertilizers are not used as well as in areas where they are used. 

 Analyses of European soils which have been cropped for cen- 

 turies show no characteristic differences from the newer soils of 

 the United States. 1 It is true that, from bad management or 

 other causes, individual fields where crop production has fallen 



*A study of crop yields and soil composition in relation to soil pro- 

 ductivity, by Milton Whitney, Bull. No. 57, Bureau of Soils, U. S. Dept. 

 Agriculture, 1909. 



