396 NATURE AND PROPERTIES OF SOILS 



pointed out that in the presence of an adequate supply of 

 calcium the availability of ammonium salts is almost as high 

 as that of nitrate salts, but where the supply of calcium is 

 insufficient the value of ammonium salts is relatively low. 



216. Organisms injurious to higher plants. — While the 

 macro-organisms may, under certain conditions, be detri- 

 mental to the growth of higher plants, it is the smaller in- 

 habitants of the soil that attract especial attention in this re- 

 spect. While protozoa may, under special circumstances, be 

 extremely detrimental, injurious organisms are confined 

 mostly to fungi and bacteria. They may be entirelv parasitic 

 in their habits or only partially so, while they may injure 

 higher plants by attacking the roots or even the tops. Those 

 that infest parts of the plant other than the roots are not 

 strictly soil organisms, as they pass only a part of their 

 cycle in the soil. Some of the more common diseases pro- 

 duced by soil organisms are : wilt of cotton, cowpeas, water- 

 melon, flax, tobacco, tomatoes, and other plants; damping-off 

 of a large number of plants ; root-rot ; and galls. 



Injurious fungi or bacteria may live for long periods in 

 the soil, if the conditions necessary for their growth are main- 

 tained. Some of them will die within a few years if their host 

 plants are not grown on the soil, but others are able 'to main- 

 tain existence on almost any organic substance. Once a 

 soil is infected it is likely to remain so for a long time, or 

 indeed indefinitely. Infection easily occurs. Organisms from 

 infected fields may be carried on implements, plants, or rub- 

 bish of any kind, in soil used for inoculation of leguminous 

 crops, or even in stable manure containing infected plants 

 or in the feces resulting from the feeding of such plants. 

 Flooding of land by which soil is washed from one field to 

 another may be a means of infection. 



Prevention is the best defense from diseases produced by 

 such soil organisms. Once a disease has procured a foothold, 

 it is often impossible to eradicate all its organisms. Rota- 



