24 



SOIL CONDITIONS AND PLANT GROWTH 



compounds rapidly change to nitrates in the soil, so that what- 

 ever compound is supplied as manure plants get practically 

 nothing but nitrate as food. This closed the long discussion 

 as to the nitrogenous food of non-leguminous plants ; in 

 natural conditions they take up nitrates only (or at any rate 

 chiefly), because the activities of the nitrifying organisms 

 leave them no option. The view that plants assimilate 

 gaseous nitrogen has from time to time been revived,^ but has 

 not been taken seriously. 



Table III.— Relation between Nitrogen Supply and Plant Growth. 

 Hellriegel and Wilfarth (130c). 



Nitrogen in the cal-"| 

 cium nitrate sup- \ 

 plied per pot, grams J 



Weight of oats ob- 

 tained (grain and 

 straw) 



Weight of peas ob- 

 tained (grain and 

 straw) 



[ -3605 

 I '4191 



{ -551 

 { 3'496 

 '.5"233 



•056 



-5-9024 

 5-8510 

 .5-2867 

 - -9776 



i'3037 

 .4'i283 



10-9814 

 . 10-9413 



■ 4'9i46 



9-7671 



. 8-4969 



-168 



15 "5974 

 5-6185 



•224 



f 21-2732 

 \2i-4409 



/ 9-7252 

 I 6-6458 



-336 

 30-1750 

 11-3520 



The apparently hopeless problem of the nitrogen nutrition 

 of leguminous plants was soon to be solved. In a striking 

 series of sand cultures Hellriegel and Wilfarth (130) showed 

 that the growth of non-leguminous plants, barley, oats, etc., 

 was directly proportional to the amount of nitrate supplied, 

 the duplicate pots agreeing satisfactorily ; while in the case of 

 leguminous plants no sort of relationship existed and duplicate 

 pots failed to agree. After the seedling stage was passed the 

 leguminous plants grown without nitrate made no further 

 progress for a time, then some of them started to grow and 

 did well, while others failed. This stagnant period was 

 not seen where nitrate was supplied. Two of their experi- 

 ments are given in Table III. 



Analysis showed that the nitrogen contained in the oat 

 crop and sand at the end of the experiment was always a little 

 less than that originally supplied, but was distinctly greater 

 in the case of peas ; the gain in three cases amounted to -910, 



^ e.g. see (2246). 



