250 Bulletin 275. 



by the destruction of ammonifying bacteria. They confirm Deherain's 

 conclusion as to the inhibiting effect of heat on the nitrifying bacteria. 

 The greater activity of nitrifying than of ammonifying bacteria in normal 

 soils they attribute to the presence of these larger organisms, particularly 

 protozoa, which through their destruction of ammonifying bacteria 

 constitute a limiting factor in the productiveness of soils. 



Although in some instances contradictory, especially in regard to 

 the exact effect of steam sterilization on crops, the work of these 

 various investigators seems to warrant the following conclusions: 



1. The effect on crops. — The result of heat sterilization of soil is usually 

 to produce an injurious effect on plants, but this retarding effect is 

 generally overcome after a short time and a decided gain in crop growth 

 finally results from the treatment. This is true of the total dry weight 

 produced as well as of the total nitrogen content of the crop. The 

 favorable influence may extend to the second crop. 



2. Effect oil the character of the soil. — Sterilization increases the solu- 

 bility of organic matter, nitrogen and the mineral plant food of soils. 

 Ammonia is formed to a limited extent by the treatment, while carbon 

 dioxid is produced in large quantity. The absorptive power of the soil 

 for water is also increased somewhat. 



EXPERIMENTS .\T CORNELL UNIVERSITY 



Soils and methods used 



In our experiments the soils were heated in an autoclav at two at- 

 mospheres for two hours in the case of Soils i, 2, and 7, and for four 

 hours in the case of Soils 5 and 6. 



Soil No. I was a clay of medium fertility, and Soil No. 2 was taken 

 from the same field and had the same texture but slightly less organic 

 matter. Soil No. 5 was a silt loam, very unproductive but containing 

 a fair quantity of organic matter. Soil No. 6 was a sandy loam in an 

 excellent state of fertility, but containing less organic matter than Soil 

 No. 5. Soil No. 7 was m;ich like soils Nos. i and 2, and was taken from 

 the same field. 



Aqueous extracts were made of both the untreated and the heated 

 soils by stirring one part of soil with five parts of distilled water for 

 twenty minutes, and filtering. Determinations were made of the total 

 soluble matter by evaporation; inorganic matter by ignition of the resi- 

 due; nitrogen in organic matter and ammonia by the Gunning method; 

 nitrates and nitrites by colorimetric methods, and of the total ammonia 

 in the unleached soil by extraction with weak hydrochloric acid. 



