230 Plant Physiology 



extensive fertility experiments throughout the world lead 

 to the conclusion that the maintenance of an adequate 

 nitrate supply in the soil, or of conditions leading to the 

 transformation of nitrogenous matter into nitrates, is an 

 important principle in production. 



The nitrates are readily soluble, and while this character 

 enhances the rapid action of such substances as fertilizers, 

 it is at the same time a quality making possible constant 

 loss through percolation and leaching. It is evident, 

 therefore, that unless the nitrate supply is maintained by 

 natural or artificial means, exhaustion of this requisite 

 element would sooner or later occur. Fortunately, there 

 are both natural means as well as conditions of cropping 

 which may suffice to maintain and to increase the nitrogen 

 content, as developed later. 



124. Compounds of ammonium. Prior to the latter 

 quarter of the last century the prevailing view was to the 



FIG. 59. Fertilization of grass land, all plats given muriate of potash 

 and acid phosphate ; also, from left to right, no nitrogen, J nitrogen 

 ration, and full nitrogen ration. [Photograph from the Rhode Island 

 Exp. Sta.] 



effect that compounds of ammonium (such as the sulfate, 

 chlorid, and nitrate) should be considered the important 

 sources of nitrogen for plants, and the weight of Liebeg's 

 opinion was upon it. At that time the cycle of changes 

 involving nitrogen was incompletely known. Unques- 



