SOIL ORGANISMS 



319 



table gives the number of bacteria at various depths under different 

 systems of cropping at the Iowa Station : 



Bacteria per Gram of Air-Dry Soil 1 Rotations 



It will be noted that the number of bacteria at four inches in 

 depth is greatest in the rotation which brings the clover crop on 

 the land more frequently. The difference is very striking when 

 compared with continuous corn. 



The Kansas Station found that the number varied directly with 

 the fertility of the soil. 



'3. Conditions for Development. It is generally known that 

 most plants require very favorable conditions for their growth, such 

 as food, moisture, heat, air, light, and the physical condition of the 

 soil. The same conditions that are favorable to higher plants are 

 favorable for the activity of bacteria, with the exception of light. 



(a) Moisture. Bacterial activity involving chemical change 

 ceases in dry soil. The other extreme, a water-logged soil, is almost 

 equally inhibitive of the action of bacteria. When a soil has ap- 

 proximately half of its air space filled with moisture the- conditions 

 are most favorable for bacterial activity and their growth is most 

 rapid. 



(b) Food. Organic matter is a very important food for most 

 bacteria, but some of the beneficial organisms obtain their supply 

 of carbon from carbon dioxide. They develop in great numbers in 

 drained soils having an abundance of organic matter. Small 

 amounts of mineral food are required, but soils usually contain 

 sufficient quantities for the use of these organisms. Soluble organic 

 matter in considerable quantities tends to inhibit nitrification. Xor- 

 mal soils contain very little. Large amounts of sewage are not de- 

 sirable on land because it furnishes soluble organic matter. 



