NOVBMBEB 6, 1908] 



SCIENCE 



619 



amounts which the crop removes. In the 

 latter years of the experiment this stock 

 of available potash in the soil had become 

 somewhat depleted, so that the omission of 

 potash from the fertilizer reduced the yield 

 from 36.3 to 28.0 bushels per acre. The 

 exhausted soil in these latter years causes 

 the crop to respond to the constituents of 

 the fertilizer only when they are aU present 

 together; taken singly, they increase the 

 yield but little and the omission of any one 

 of them reduces the crop almost to the 

 minimum produced on the unmanured 

 crop. The soil has thus become but a small 

 factor in the nutrition of the crop, whereas 

 as regards potash it was a very large one at 

 the beginning of the experiment, and the 

 defect of Liebig's theory was to neglect it 

 entirely. 



These differences in the manurial re- 

 quirements of wheat and barley, differences 

 which would not be apprehended from their 

 respective compositions, may be correlated 

 with the habits of growth of the two plants : 

 wheat is sown in the autumn after but a 

 slight preparation of the ground, nitrifica- 

 tion is thus restricted, especially as the 

 chief development of the plant takes place 

 in the winter and early spring before the 

 soil has warmed up, and as a consequence 

 the crop is particularly responsive to an 

 external supply of some active form of 

 nitrogen. On the other hand, the wheat 

 plant possesses a very extensive root system 

 and a long period of growth, hence it is 

 specially well fitted to obtain whatever 

 mineral constituents may be available in 

 the soil. In ordinary farming the only 

 fertilizer used for the wheat crop will be a 

 spring top-dressing of 100 pounds per acre 

 or so of nitrate of soda or an equivalent 

 amount of sulphate of ammonia or soot. 



Barley is a spring-sown crop for which 

 the soil generally receives a more thorough 

 cultivation, in consequence of which and of 

 the rising temperature there will be suffi- 



cient nitrates produced for the needs of the 

 crop, often more than enough when the 

 barley follows a root crop that has been 

 liberally manured and perhaps consumed 

 on the ground by sheep. But being shal- 

 low rooted and having only a short growing 

 season, the plant experiences a difficulty in 

 satisfying its requirements for phosphoric 

 acid, hence the necessary fertilizer consists 

 in the main of this constituent. Only on 

 sandy and gravel soils, exceptionally defi- 

 cient in potash and subject to drought, is 

 any benefit derived from a supply of potash 

 to the barley crop. 



A still more noteworthy example is pro- 

 vided by the swede turnip crop; the an- 

 alysis of a representative yield would show 

 it to withdraw from the soil about 150 

 pounds per acre of nitrogen, 30 pounds of 

 phosphoric acid and 120 pounds of potash. 

 Yet the ordinary fertilizer for the swede 

 crop will consist in the main of phosphatic 

 material with but a small quantity of nitro- 

 gen and rarely or never any potash; for 

 example, 400 pounds of superphosphate or 

 500 pounds of basic slag according to the 

 soil {i. e., 50 to 100 pounds of phosphoric 

 acid), together with 12 to 15 pounds of 

 nitrogen as contained la 50 pounds of sul- 

 phate of ammonia will form a very satis- 

 factory mixture. The swede is sown late 

 in the season after a very thorough prepa- 

 ration of the soil, so that the nitrification 

 alone of the nitrogenous residue in the soil 

 is capable of furnishing almost all the 

 large amount of nitrogen it requires; it is 

 very shallow rooted and must be supplied 

 with an abundance of phosphoric acid. 

 It was considerations of this kind which 

 led Ville to suggest that for each crop there 

 is a "dominant" fertilizing constituent, 

 e. g., nitrogen for wheat, phosphoric acid 

 for swedes, and that the particular dom- 

 inant is the constituent which the plant 

 finds the most difficulty in appropriating 

 from the soil, and hence which is therefore 



