436 SOILS: PROPEETIES AND MANAGEMENT 



A soil low in organic matter usually has a lower bac- 

 terial content than one containing a larger amount, and, 

 under favorable conditions, the beneficial action, to a cer- 

 tain point at least, increases with the content of organic 

 substance; but, as the products of bacterial life are 

 generally injurious to the organisms producing them, 

 such factors as the rate of aeration and the basicity of 

 the soil must determine the effectiveness of the organic 

 matter. 



359. Soil acidity. — A soil having an acid reaction 

 makes a poor medium for the growth of certain bacteria. 

 A neutral or a slightly alkaline soil furnishes the most 

 favorable condition for the development of the forms of 

 bacteria most beneficial to arable land. The activities of 

 many soil bacteria result in the formation of acids which 

 are injurious to the bacteria themselves, and, unless there 

 is present some basic substance with which these can 

 combine, bacterial development is inhibited by their own 

 products. This is one of the reasons why lime is so often of 

 great benefit when applied to soils, and especially to those 

 on which alfalfa and red clover are growing. For the 

 same reason, the presence of lime hastens decay of or- 

 ganic matter in certain soils, and the conversion of nitrog- 

 enous material with a minimum loss into compounds 

 available to the plants. As showing the value of lime 

 in the process of nitrate formation, it has been pointed 

 out that in the presence of an adequate supply of lime 

 the availability of ammonium salts is almost as high 

 as that of nitrate salts, but where the supply of lime 

 is insufficient the value of ammonium salts is relatively 

 rather low. 



360. Fimctions of soil bacteria. — Bacteria have a part 

 in many of the processes of the soil which greatly affects 



