No. 6. DEPARTMENT OF AGRICULTURE. 319 



products, a vigorous growth will take place, and a relatively large 

 aiiiouut of decomposition products will be quickly formed; but 

 soon active growth will cease, and that before the full amount of the 

 original pepton has been completely decomposed. In short, the 

 organism has been either killed or its energies have been paralyzed 

 by the products of its own growth, which in this case have been pro- 

 duced in relatively large amount in the more concentrated solution. 

 If on the other hand a solution be prepared containing only the one- 

 hundredth of one per cent, of pepton, and be seeded with the same 

 organism, growth in the latter medium will be relatively slow, with 

 a correspondingly slow development of ammonia; but the decom- 

 position wall continue until all of the pepton has been decomposed. 

 Furthermore, the activity of the decomposition will be as great at 

 the end as at the beginning of the process, showing that the vitality 

 of the organism has not been impared. This is doubtless because 

 in the dilute solution the toxic products are not sufficiently concen- 

 trated to injure the life of the micro-organisms. 



What is true here would be equally true of the state of concen- 

 tration of the organic matter in the soil. If the latter be present 

 in excessive quantity bacterial development will proceed for a time 

 at an excessive rate, but soon products injurious to their best de- 

 velopment will be produced. There is, therefore, a limit to the 

 amount of humus which a soil should' contain. 



In forest and woodland soils the amount of humus in the surface 

 layer is large. In such soils organic acids are generated in quantities 

 too large for the best development of bacteria, and hence, as is found 

 to be the case, the number of bacteria is low. 



It has been shown that nitrification practically ceases in forest 

 soils, due doubtless to the fact that the nitrifying bacteria, more 

 than any other, are injured by high acidity and excessive humus. 



The only condition which renders possible the addition of large 

 stores of humus to the soil is subsequent tillage, which so stimulates 

 bacterial growth as to lead to the destruction of organic acids, or to 

 the production of ammonia, which neutralizes them. Hence when 

 a crop of clover or other legume is plowed under' it is hest followed 

 oy a cultivated or hoed crop. 



4. The Relation of Soil Acidity to the Number of Soil 



Bacteria. 



It is important to know the conditions of the soil which are most 

 favorable to the rapid development of soil bacteria. Among these 

 nothing is so important as to maintain a proper reaction of the soil. 

 Acid soils are infertile because soil bacteria, which are digesters of 

 plant food, cannot grow therein. We say that lime when applied to 



