s 



essary for plant growth in the required quantity and In 

 the proper form. A soil which is lacking in any of these 

 materials, or which does not have them in the proper 

 form, is in no condition to produce a full crop, and must 

 have the deficient material supplied in the proper 

 amount an i form in order to make it productive. As 

 has frequently been stated in these bulletins, there are 

 about a dozen constituents of the soil that are required 

 for plant nutrition. Most of these are found in such 

 quantity in the soil, or are in such little demand by the 

 plant, that the supply of them in the soil is not likely to 

 be exhausted by years of cultivation. Three of the con- 

 stituents, however, nitrogen, phosphoric acid and pot- 

 ash, are in such demand by the plant that their supply 

 is readily exhausted, and it is necessary to restore these 

 exhausted constituents to the soil in order to make it 

 fertile. For the present, then, soil fertilization consists 

 in restoring to the soil nitrogen, phosphoric acid and 

 potash in such quantities in assimilable form as may be 

 required for the proper nutrition of the growing crop. 

 The rational course, therefore, to pursue with reference to 

 a given soil is first to determine its deficiency in these 

 three constituents and then to supply the deficiency in 

 proper form. 



It is not an infrequent occurrence for this department 

 to receive a sample of soil with the request to tell what it 

 needs for its proper fertilization. The correspondent is 

 acting on the hypothesis that a chemical analysis of a 

 soil will determine its fertilizer requirements. Unfor- 

 tunately it will not do so satisfactorily. The chemist 

 can easily determine what constituents are present in 

 the soil and in what quantities, but he cannot so readily 

 determine whether these constituents are present in as- 

 similable form, and if they are not present in assimilable 

 form, they might as well be absent altogether, as far as 



