184 Agricultural Bacteriology. 



that lime stone is made soluble. Phosphorus is thus con- 

 stantly lost from the soil. 



The fertility of soil may be limited through the fact 

 that the soil does not contain phosphorus in any form, or 

 because the phosphorus it does contain is not rendered 

 available by solution. Phosphorus may be added to the 

 scil in the form of bone-meal, phosphate rock (floats, or 

 calcium phosphate), superphosphate (acid phosphate). 

 Of these only the last is immediately available to the 

 plant, as the other compounds are the same as the phos- 

 phorus already present in the soil. Their addition alone 

 to a soil that is lacking in available phosphorus has no 

 effect whatever. If added in connection with a large 

 amount of organic matter, as barn yard manure, or a 

 crop that is plowed under, the insoluble phosphates are 

 made available because the organic material furnishes 

 food for the bacteria, which as a result of their growth 

 form acids that render soluble the calcium phosphate. 

 The addition of rock phosphate to sandy land or to land 

 low in humus is of little use. Sprinkling "floats" on 

 the manure as it is made is an excellent way of adding 

 the phosphate to the soil. The insoluble phosphate is 

 thus brought in intimate contact with decomposing or- 

 ganic matter and is gradually rendered soluble. 



There is need of an abundant supply of available phos- 

 phorus in the soil not only for the green plant but for 

 some forms of bacteria that are very beneficial to the 

 farmer. The different kinds of nitrogen-fixing bacteria, 

 both those that grow in the nodules of the leguminous 

 plants and those in the soil itself, develop most luxuri- 

 antly only when phosphorus is available. 



The fertility of a soil may be low because the phos- 

 phorus it contains is not being rendered available. Un- 



