20 BULLETIN 158, U. S. DEPARTMENT OF AGEICULTUEE. 
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.^ The action of a number of these nitrogenous compounds 
has been tested in this laboratory in conjunction with the three 
fertiUzer elements and it has been found that in some cases the 
nitrogen compounds are not only used as a source of xiitrogen 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 Idnd 
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- 
tiUzer 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 m 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 ^ 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 ^ are aU bene- 
ficial to plant growth, causmg nitrogen increases in the plant, and 
the two first diamino acids act as nitrate sparers; this may also be 
true of lysme, although this property of lysine has not been studied. 
Leucine is also beneficial to plant growth, and tyrosine, in the light 
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); Schreiner and Skinner, Bui. 87, Bureau of 
Soils, U. S. Dept. Agr., 1912. 
2 Bui. 87, B :reau of Soils. 
2 Unpublished data. 
