70 AGRICULTURAL BACTERIOLOGY 



decade, and of the coming century. He must so handle his 

 soil that it shall be now and in the future a place in which 

 the green plant will find favorable conditions for develop- 

 ment. In the handling of the soil, the farmer must con- 

 sider the multitudes of microorganisms that are growing 

 therein. 



Unavailable and available plant food. The green plant 

 obtains its carbon from the carbon-dioxide of the air, its 

 oxygen from the same source, its hydrogen from water. 

 Nitrogen, phosphorus, calcium, magnesium, potassium, sul- 

 phur, iron, and silicon must be supplied from the soil. 

 The plant must be furnished with each of these elements in 

 certain definite chemical compounds. One property that 

 all these compounds must possess is solubility in water. 

 The food must pass into the roots through the firm cell wall 

 by the process of osmosis. 



The great agricultural districts of the world are in the 

 humid regions, where the rainfall is sufficient to allow the 

 larger part of the water to pass through the soil until it 

 reaches the so-called ground-water layer. The water, as it 

 passes downward, tends to dissolve and transport with it all 

 soluble soil constituents. The storage of plant food in a 

 form available to the plant is incompatible with this condi- 

 tion. The food must be stored in an insoluble form, and 

 thus be protected from the leaching action of water. This 

 available food must be prepared for use by the plant 

 through the action of some slowly acting factors, such as the 

 solvent effect of water on materials that are ordinarily 

 classed as insoluble. Weathering, the mechanical reduction 

 in size of soil particles under the action of frost, and the 

 effect of biochemical agents such as bacterial activity, are 

 factors that release plant food for use by the green plant. 



The fertility of any soil depends upon the amount of un- 

 available plant food stored therein, upon its potential fer- 



