92 BACTERIA AND SOIL FERTILITY 



(l) The microscopic animals which we refer to as the micro- 

 fauna of the soil. These are either unimportant or else detrimen- 

 tal. (2) The microscopic plants of the soil which we often 

 speak of as the microflora. These are either injurious or bene- 

 ficial. The problem of soil bacteriology is to know how to sup- 

 press or eliminate the injurious bacteria and at the same time 

 increase the number and activity of the beneficial. 



iNumber of Bacteria in Soils — The number of bacteria in 

 soils varies with the climatic conditions of the soil, the composi- 

 tion of the soil, the quantity of organic matter and water, and 

 the methods of tillage and cropping. A sandy soil practically 

 devoid of organic matter may have only a few thousand in a 

 gram of soil. On the other hand, a silty loam containing large 

 quantities of organic matter may have as many as fifty million 

 in one gram. The number of bacteria in good arable soil well 

 supplied with organic material usually ranges from three to forty 

 millions. Tight non-porous soils, as well as alkali soils low in 

 humus and moisture, have fewer bacteria. The number of bac- 

 teria in light, sandy, very tight clay, desert, and forest soils is 

 usually much smaller, other things being equal, than in normal 

 cultivated soils. The average of several hundred determinations 

 made on cultivated soils of the arid regions gave a bacterial count 

 in round numbers of 4,500,000 per gram. This appears to be 

 an enormous number of bacteria to be crowded into such a small 

 space. One may think they would not have elbow room, but if 

 we recall that one gram of soil contains from two to twenty 

 billion particles then each microbe in the soil of finer texture 

 would be lord over about 5,000 particles; in the coarser texture soil 

 each organism would be master of 500 particles. This would give 

 to each microorganism more room in proportion to size than human 

 individuals often have in large cities in proportion to their size. 



Factors Influencing Numbers.-The quantity and kind of 

 organic matter is one of the greatest factors governing bacteria 

 in a soil. Large quantities of easily decomposed material increase 

 the bacteria of a soil. The addition of five tons of barnyard 

 manure to one of the arid soils increased the numbers 50 per 



