THEORIES CONCERNING SOIL FERTILITY 319 



and the more recent prairie soil (brown silt loam) of the late Wis- 

 consin glaciation. Compare also the amounts in the surface and 

 subsoil (in 2 million pounds of each) of potassium, or any other ele- 

 ment which does not accumulate in plant residues. Note whether 

 the calcium carbonate on Broadbalk and Hoos fields at Rothamsted 

 is steadily accumulating at the surface. There are abundant sup- 

 plies in the subsoil " far below where the roots go." Note the com- 

 plete absence of calcium carbonate in very many Illinois soils. 

 (See also Tables 4, 5, and 21 in the preceding pages.) 



Attention is called to the fact that nitrification is a process of 

 biochemical action and not one of mere solution. Director Hall 

 of the Rothamsted Experiment Station makes the following 

 statement in his "Fertilizers and Manures" (1909), page 288. 



" When the Rothamsted soils, with their long-continued differences in 

 fertilizer treatment, are extracted with water charged with carbon dioxide 

 the nearest laboratory equivalent to the actual soil water the amount of 

 phosphoric acid going into solution is closely proportional to the previous 

 fertilizer supply, and this proportionality is maintained if the extraction is 

 repeated with fresh solvent, as must be the case in the soil." 



It should be kept in mind, of course, that a one-crop system 

 followed year after year upon the same land usually encourages the 

 growth of certain weeds whose " habits " are similar to those of 

 the crop grown, that it also tends toward the breeding of insect 

 enemies and to the development of fungous diseases peculiar to 

 that crop, such as " flax sickness," investigated by the North Da- 

 kota Experiment Station, and " clover sickness," which has long 

 been thought to be an actual fact in practical agriculture, concern- 

 ing which the Tennessee Station has recently reported some prom- 

 ising results. The legume plants appear to be especially suscep- 

 tible to such fungous diseases, the cowpea wilt being well known, 

 and " bean-sick " soil is a common expression. It seems probable 

 that bacterial as well as fungous diseases may develop under suit- 

 able conditions. 



While it is possible that inanimate toxic substances may also 

 be formed in the soil from possible plant excreta, or less improb- 

 ably from the decomposition of the crop residues, there is no knowl- 

 edge or evidence sufficient, in the author's opinion, to justify a 

 theory that fertilizers act primarily as antitoxins. It should be 



