Exaluation of Soil Fertility 325 



much as 20-50 per cont of tlic total nitrogen present in the manure. 

 \'arious procedures have been utih/ech for preventing such losses. 

 One of the simplest principles is to hasten the activities of the micro- 

 organisms which bring about the destruction of the cellulose and the 

 hemicclluloses in the manure. If conditions are favorable, an active 

 microbiological population will bring this about during the early 

 stages of decomposition, thereby t.'ansforming the soluble forms of 

 nitrogen in the manure into complex organic forms. The supple- 

 mentar\- addition of superphosphate will often tend to neutralize the 

 ammonia liberated, thus preventing its volatilization. 



If the decomposition of the manure has been allowed to proceed 

 too far and if oxidation of the nitrogen to nitrate has begun, there 

 is great danger of this nitrate's being reduced to atmospheric nitro- 

 gen. A compost offers ideal conditions for such a reaction. To pre- 

 vent such losses, the compost must be made anaerobic, so as to 

 hinder the activities of the nitrifying bacteria, thus avoiding the 

 conversion of the ammonia into nitrate and the subsequent reduc- 

 tion of the latter to gaseous forms of nitrogen. 



Evaluation of Soil Fertility by Measuring 

 Microbiological Activities 



Numerous attempts were made during the first decade of this cen- 

 tury, beginning \\'ith Remy and Lohnis and followed by Lipman and 

 Brown and many others, to interpret the fertility potential of a soil 

 on the basis of its microbiological activity. Several methods of ap- 

 proach were usually followed, of which these may serve as illus- 

 trations : 



1. A small amount of a given soil was added to a nutrient solution 

 of known composition and, after a few days' incubation, a single 

 biological change was measured. The reactions most commonly 

 studied were the formation of ammonia from peptone, or ammonifi- 

 cation; the formation of nitrate from ammonium salt, or nitrification; 

 the destruction of nitrate, or denitrification; and the fixation of ni- 

 trogen. 



2. A chemical substance, simple or complex in nature, was added 

 to a given quantity of soil, the moisture of which was adjusted to 

 60 or 70 per cent of water-holding capacity. The soil was incubated 

 at 20-30 ^C for 7-30 days, and changes, similar to those listed above, 

 were measured. 



