UREA UNASSIMILABLE BY HIGHER PLANTS. 331 



bodies differs in the various kinds of animals, whilst quantita- 

 tively they depend on the amount and composition of the food. 



In human urine and that of the carnivorous mammalia, birds, 

 and reptiles which has an acid reaction owing to the presence of 

 acid sodium sulphate uric acid is present to a greater extent than 

 hippuric acid. The total amount voided daily by a man of normal 



size is :- 



Urea 35-50 grams. 



Uric acid 0.5-0.75 gram. 



Hippuric acid . . . .0.3 gram. 



On the other hand, the urine of the herbivorous mammals and 

 birds which has an alkaline reaction owing to its content of 

 KHC0 3 exhibits a different substantive ratio, uric acid being in 

 very minute proportion ( T ^<jth per cent.), whilst hippuric acid is 

 comparatively plentiful, e.g. in cows' urine up to J per cent., in 

 horses' urine up to 2 per cent, (in combination with lime). Urea 

 is present to the extent of 2-5 per cent, in cows' urine, and 3 per 

 cent, in that of the horse. 



186. Urea Unassimilable by Higher Plants. 



Assuming the total number of inhabitants in the world to be 

 1500 millions, and estimating the diurnal excretion of urea at an 

 average of only 25 grams per individual, then there results a total 

 daily production of 37,500 tons of urea by the human race alone. 

 This quantity in the solid state would occupy a space of 50,000 

 cubic metres (65,400 cubic yards), and contain 17,000 tons of 

 combined nitrogen. The excretions of the animal kingdom must 

 be estimated at a much higher figure. 



The amount of nitrogen daily excreted in urine and passing 

 into manure is consequently enormous, and the question naturally 

 arises : What becomes of it afterwards 1 Is urea (and also uric 

 acid, &c.) adapted to serve immediately and directly for the nutri- 

 tion of plants, and of cultivated plants in particular 1 



Agricultural practice answers this query by a decided negative, 

 knowing from experience that manuring with fresh urine is at first 

 either entirely useless or actually injurious. No satisfactory ex- 

 planation of this fact has yet been discovered by scientific research. 

 0. KELLNER (III.), for example, sought it in the circumstance that 

 urea is not absorbed (retained) by the soil. We must therefore 

 turn away from research, and fall back on the fact that urea is 

 unsuitable for the nutrition of the higher plants ; and that conse- 

 quently its nitrogen, not being available for this purpose, is lost. 

 If this element is to continue its cycle in the organic world, it must 

 first be converted into other forms and modes of combination ; and 

 the question arises as to which of these involves the least labour 

 and smallest expenditure of energy. 



It must be borne in mind that urea is a derivative of carbon 



