The conclusions which may be drawn from the experiments upon the effects of barnyard manure on bacteriological activities of field S'oils presented in this bulletin are as follows:
1. Applications of manure up to sixteen tons per acre increased the numbers of organisms in the soil as shown by the growth on modified synthetic and albumen agar. The ammonifying power of the soil as shown by tests with the casein-fresh soil, albumen-fresh soil, dried blood-fresh soil, and dried blood-air-dried soil methods, and the nitrifying power tested by the ammonium sulfate-fresh soil, and the ammonium sulfate-air- dried soil methods were likewise increased.
2. The greatest increases occurred between the check soil and that receiving eight tons per acre and between the soil receiving the eight tons and that to which twelve tons per acre were applied. In most cases only a very slight increase occurred in the soil on which sixteen tons were used over that where twelve tons were added.
3. Twenty tons of manure per acre caused a depression in numbers of bacteria, in ammonifying power, and in nitrifying power according to all the methods employed, the results being lower than those secured when twelve tons per acre were added.
4. Albumen agar permitted of the development of larger numbers of soil organisms than the modified synthetic agar and also permitted of a greater differentiation between the soils of the various plots.
5. There was a close relationship between the ammonifying power of the soils and the numbers of organisms in them accord-to the methods used in this work.
6. The casein-fresh soil method of testing the ammonifying power of the soil was the simplest, permitted of the greatest differentiation between different soils, and in general was the most satisfactory.
7. The ammonium sulfate-fresh soil method for testing the nitrifying power of the soil showed the greatest differences between the·various soils ,and is recommended as the more rational method.
8. The nitrifying power and the ammonifying power of the soil according to the method's used proceeded in the same direction.
9. Applications of manure up to sixteen tons per acre increased the yield of corn from the plots in this series, the greatest increases occurring between the check plot and that receiving eight tons per acre, and between the latter and the plot to which twelve tons per acre were added. A very slight further increase occurred when sixteen tons per acre were applied.
10. Twenty tons of manure per 'acre depressed the crop yield below that obtained when twelve tons per acre were added.
11. The results of the bacteriological tests and the crop yields coincide almost exactly. Further evidence is thus supplied that there is a close relationship between bacterial activities and the fertility or crop-producing power of soils.
12. The depression in crop yields 'and bacterial activities caused by twenty tons of manure per acre cannot be attributed to denitrification as tests by the Giltay solution method and the soil method give no evidence of losses of nitrogen. The depression must therefore be due to physiological or other causes.
Brown, Percy Edgar
"Bacteriological studies of field soils, III. The effects of barnyard manure,"
Research Bulletin (Iowa Agriculture and Home Economics Experiment Station): Vol. 1
, Article 1.
Available at: https://lib.dr.iastate.edu/researchbulletin/vol1/iss13/1