330 ANNUAL REPORT OF THE Off. Doc-. 



i;t'b iioui uitraU's, and Liie process of coiivertiu^ various oj-^aiiic 

 lorms iuto nitrates is knowu as nitrificatiou. 



As the growtii of plants is so largely dependent upon the activity 

 of soil ferments, it is important for the farmer to know the condi- 

 tions most favorable for their work. A certain amount of humus 

 is necessary, but excessive quantities are unfavorable, as this re- 

 sults in an acid condition which retards the work of the soil fer- 

 ments. Here we note another reason for the use of lime. The 

 activity of nitrifying organisms is increased by an alkaline environ- 

 ment secured by liming, and it is not improbable that, on many 

 soils, this is the most important use of lime. The ferments flourish 

 best near the surface of the soil, where there is an abundance of at- 

 mospheric oxygen. Prof, Frederick D. Chester, of the Delaware 

 Agricultural Experiment Station, found at four inches from the 

 surface, 1,632,U0U bacteria per gram. At six inches from the surface, 

 l,fi23,U()0 bacteria per gram. At twelve inches, 73,000 per gram. At 

 eighteen inches 21,000 per gram. At twenty-four inches only 4,000 

 per gram. We hnd in these figures an argument against the deep 

 plowing of very heavy, compact soils. For the turning up of soil 

 so deficient in nitrifying organisms we cannot expect the plants to 

 be liberally fed. 



The number of bacteria of the soil is largely dependent upon 

 the extent of aeration. It is often necessary to expose to the air 

 for a season or two certain lands, especially pastui'e fields of long 

 standing, before they will respond satisfactory to tillage. This 

 necessity arises from the fact that the number of ferments in some 

 soils is too limited to digest the supply of food needed to make a 

 large crop. Last summer the writer thought he would prepare an 

 ideal plot of ground for late cabbage by plowing a small area 

 heavily covered by a blue-grass sod, which had been pastured for 

 a long series of years. After plowing, harrov>^ing and dragging, the 

 soil was loose and friable, and the workmen and the writer were 

 unanimous in their opinion that the yield would be large. But 

 the crop there was a failure. The nitrifying ferments were not 

 present in sufficient number to feed the cabbage and commercial fer- 

 tilizers alone could not do the work. 



As aeration is necessary for the activity of the nitrifying organ- 

 isms, the advantages of thorough tillage are apparent. From the 

 figures given by Prof. Chester and other investigators regarding the 

 number of bacteria in the soil found at different depths, we learn 

 that good plowing and good harrowing do not consist in the pulveri- 

 zation of only a few inches of surface soil, but we must conclude 

 that thorough tillage to the depth of at least seven or eight inches is 

 necessary to secure the best results. Previous to this time the 

 frecjuent tillagi^ of cultivated crops has been agitated chiefly t)e- 



