NEED OF COMMERCIAL FERTILIZERS 



and it has shown abnormal deficiency in some 

 types of limited area. It has given us more knowl- 

 edge of soils, but as a guide to fertilization in par- 

 ticular instances it usually has no value. The 

 samples used by an analyst are so small that the 

 inaccuracy in his determination may easily be 

 greater than the total amount of plant-food in a 

 very heavy application of commercial fertilizer. 

 A field that has been reduced to temporarily low 

 productive power by heavy cropping or bad farm- 

 ing methods may show a greater content of plant- 

 food than another field that is in a highly pro- 

 ductive condition. This is a fact difficult of 

 acceptance by some who want the aid of science, 

 but such are the present limitations. The weight 

 of a fertilizer application is so small in compari- 

 son with the weight of the surface foot of an acre 

 of land that the use of a ton of fertilizer may not 

 be detected in the analyst's determinations, and 

 moreover his determinations of actual availability 

 in the soil's supplies are not serviceable in the 

 selection of a fertilizer for any particular field 

 and crop. 



Physical Analysis. Chemical analysis is costly 

 and unsatisfactory as a guide to fertilization. 

 Physical analysis by a competent man may have 



[163] 



