20 BULLETIN 158, U. S. DEPARTMENT OF AGRICULTURE. 



it was offered to them in the form of nitrates; this idea, however, 

 was modified when it was discovered that under certain conditions 

 plants used ammonia or ammonium salts without their conversion 

 into nitrates quite as well as they used the nitrates themselves. 

 During the past few years it has been clearly demonstrated that 

 plants not only use nitrogen in the form of nitrates and ammonia but 

 that they can also use nitrogen in the form of complex organic com- 

 pounds. 1 The action of a number of these nitrogenous compounds 

 has been tested in this laboratory in conjunction with the three 

 fertilizer elements and it has been found that in some cases the 

 nitrogen compounds are not only used as a source of nitrogen for the 

 growing plant, without any change in the compound, but that these 

 compounds were apparently nitrate sparers; that is, the plant used 

 them in preference to the nitrates. Instead, then, of only one kind 

 of nitrogen compound, nitrate, or at most two, nitrate and ammonia, 

 there appears to be a very large number of nitrogenous compounds 

 which have properties of physiological importance to plant growth. 

 The question of the availability of nitrogen compounds can therefore 

 be answered only when the nitrogen compounds contained in the fer- 

 tilizer can be determined in amount and at the same time classified 

 according to their physiological action on plant growth. It is hardly 

 necessary to state that such a method does not exist at present and 

 that the physiological action of only a part of the total number of 

 nitrogenous compounds present in fertilizers is known. 



The physiological action on plants of all of the nitrogenous com- 

 pounds isolated from base goods has been determined by means of 

 water cultures 2 and the results obtained may be stated briefly, as 

 follows: Both of the purine bases are used by the plant as a source 

 of nitrogen and are beneficial to plant growth; furthermore, the 

 hypoxanthine acts as a nitrate sparer, there being less nitrate used 

 by the plant in the presence of hypoxanthine than when the hypo- 

 xanthine is absent. Histidine, arginine, and lysine 3 are all bene- 

 ficial to plant growth, causing nitrogen increases in the plant, and 

 the two first diamino acids act as nitrate sparers; this may also be 

 true of lysine, although this property of lysine has not been studied. 

 Leucine is also beneficial to plant growth, and tyrosine, in the fight 

 of later investigations, is somewhat doubtful in action. Of the other 

 monoamino acids which may be present in base goods, aspartic acid, 

 glutamic acid, and glycocoll have been found to be beneficial. The 

 action of alanine is somewhat doubtful, it apparently being bene- 

 ficial in low concentrations, and the action of phenylalanine is re- 

 ported as harmful. Thus we see that six of the seven compounds 



1 Hutchinson and Miller, Centralbl. f. Bakt., 30, 513 (1911); Sehreiner and Skinner, Bui. 87, Bureau of 

 Soils, U. S. Dept. Agr., 1912. 

 8 Bui. 87, Bureau of Soils. 

 8 Unpublished data. 



