14 THE SOU, SOLUTION 



be real, but as already mentioned, one and the same plant may 

 have a very different ash content as the result of different cul- 

 tural methods, different climatic and seasonal factors, as well 

 as different soils. Generally, a poor crop has a higher per- 

 centage of ash content than a good crop, and sometimes the 

 poor crop may remove from the soil more in absolute amounts of 

 some one or other of the ash constituents than does the good 

 crop. The ratio of the ash constituents is by no means constant 

 for any one crop, and of course varies with different crops. 1 

 Finally, it is now known that the amount of the several mineral 

 nutrients which a soil must furnish to a crop in the earlier stages 

 of growth is greater than the crop contents at maturity, 2 con- 

 sequently an analysis of the ripe crop would not indicate the 

 plant's drain upon the soil at all growing periods. So that, 

 while ash analyses have taught some important things concern- 

 ing plant growth, they have of necessity failed as guides or 

 criteria of the crop-producing power of a soil, its fertilizer re- 

 quirements, or its content of "available" plant-food. 



A third method of soil investigation, also essentially analytical 

 in character, is the plot or pot test. The difference between 

 a plot or pot experiment is mainly one of size, although it is 

 claimed, and with a certain amount of justice, that the plot 

 experiment more nearly approximates actual practice, and 

 should be given a somewhat different consideration than the 

 more readily controlled pot experiment. Here again it has to 

 be considered that seasonal factors and factors other than the 

 soil play a relatively large part in the production of the crop, so 

 that conclusions regarding the productivity of a soil can not be 

 drawn from one season's crop. Also, nowadays it is recognized 

 generally that continuous growing of one crop is an incorrect 



1 For a brief but comprehensive discussion of ash analyses see, The 

 ash constituents of plants, etc., by B. Tollens, Expt. Sta. Rec., 13, 207- 

 220, 305-317 (1901-02). 



1 Uber die Nahrstoffaufnahme der Pflanzen in verschiedenen Zeiten 

 ihres Wachstums, von Wilfarth, Romer und Wimmer, Landw. Vers. Sta., 

 63, 1-70 (1905) ; Plant food removed from growing plants by rain or dew, 

 by J. A. Le Clerc and J. F. Breazeale, Year Book, U. S. Dept. Agriculture, 

 1908, pp. 389-402. 



