138 The Potato 



labor and also expensive in the changes it produces in 

 the soil. A soil thoroughly tilled for a potato crop has 

 thereby lost a considerable amount of organic matter. 

 The expense of replacing this must be considered in the 

 final profit-and-loss account of the crop. In many parts 

 of the United States, potato-growing has gained a bad 

 reputation from the rapidity with which it reduces the 

 fertility of the soil. In others, where organic matter is 

 kept up by good rotation and where fertilizers are used 

 heavily, potato-growing is regarded as a sure way to 

 enrich the land. A single tilled crop, like potatoes or 

 corn, places the soil in better physical condition to grow 

 succeeding crops of grain and hay. The alternation of 

 tillage crops, w^hich excite and increase bacterial activity, 

 with the hay crop will provide organic matter as a food for 

 the bacteria and thereby keep up fertility at low expense. 



(4) The increase of the volume of the air-spaces of the 

 soil by thorough tillage greatly increases the chemical 

 and bacterial activities of the soil. The oxygen of the 

 air combined with the organic matter of the soil causes 

 the formation of carbon dioxide. This, in turn, acts 

 with the soil water as a solvent of mineral compounds, 

 such as insoluble phosphoric acid. The beneficial bac- 

 teria of the soil are favored by plenty of air in the soil 

 and increased greatly in number and activity. Their 

 action increases the availability of the soil plant-food. 

 Some species of soil bacteria are injurious. As these 

 mainly thrive where little or no air is present, tillage re- 

 duces their effect. 



(5) Tillage exposes fresh soil surfaces to the air and at 

 first causes some loss of moisture. Loosening the surface 

 spilvby tillage prevents evaporation from the lower parts 

 c4 the soil by breaking up capillary connection between 



