1889.] MICROSCOPICAL JOURNAL. 109 



Now, if decomposition is caused by bacteria it is important to find out 

 whether all bacteria cause it or whether there are only certain kinds 

 which do so. Also, whether they do so under all circumstances. It 

 has been discovered that all sorts of bacteria do not produce decomposi- 

 tion, at least, not entire decomposition, and those which do require 

 certain conditions. Prof. Wollny states that those bacteria which are 

 concerned in the process of decomposition require abundance of air and 

 moisture, and also warmth. Applying this knowledge to every-day life, 

 we see it is necessary to keep vegetable and animal matter moist and 

 warm and allow abundance of air to prepare it as a fertilizer. It fol- 

 fows that if your compost heap is too dry it will not decompose, and if 

 it is too cold it decomposes very slowly. If the compost heap is too 

 wet it will not thoroughly decompose for reasons to be given presently. 

 For the same reason, crops which are turned under have to be kept 

 moist and warm and allowed to have plenty of air. Of course the 

 exact preparation of the compost of the soils which are turned under 

 must be varied to suit each case. If soil is sandy, the crop should be 

 turned thoroughly under and the earth above it rolled so as to retain as 

 much moisture and heat as possible. With clay land of course the 

 opposite course is indicated. Again, if a compost pile is too much 

 packed to allow access of air, it should be opened up ; at the same time 

 it must not be too loose, otherwise it will dry out too much and loose 

 heat. 



So we have to bear in mind that the bacteria which prepare the food 

 for our crops will only do so when they have the conditions favorable 

 to their growth. If the conditions are unfavorable they either do their 

 work imperfectly, or they are supplanted by other bacteria which split 

 up vegetable and animal matter into substances which are for the most 

 part of no use to plants as food. This takes place when the air is par- 

 tially or wholly cut off. If a crop is turned under and the air is not al- 

 lowed free access there will be very little benefit derived, and in some 

 cases even injury to the soil may result. It can readily happen in a clay 

 soil that the earth may become so packed that the air is excluded. 

 Even in a sandy soil the pores may become so clogged with water that 

 the air is kept out. 



Pasteur first called attention to the fact that there are two kinds of 

 bacteria, one not requiring free access of air, and the other unable to 

 grow in full oxygen. To repeat : those which require the presence of 

 free oxygen cause decomposition, and those growing without oxygen 

 cause only partial decomposition. 



But besides the influence exerted by the presence or absence of oxygen 

 there are other things which influence the growth of bacteria.. Certain 

 metallic salts interfere with it. The concentrated solutions of common 

 salt act in this way. Corrosive sublimate, which is a salt of mercury, 

 even in very. dilute solutions kills bacteria. The same is true, to a 

 greater or less extent, of mineral acids or other substances. 



So a soil must not only have abundance of moisture, warmth, and 

 air, but it must also be free from substances injurious to bacteria. This 

 can be tested even without a chemical analysis, by simply taking cul- 

 tures derived from the soil, and seeing whether the bacteria from them 

 grow in the soil to be examined. This is a more direct test than a 

 chemical examination. 



Not only do these principles underlie the process of decomposition, 



