WEATHERING. FALLOWS. 205 



718. Another benefit comes from the fallow, Weathering. 

 The soil, often turned up, is exposed to the influences of 

 the air, and to sunshine, rain, cold, and wind. From the 

 air it receives oxygen, carbonic acid and ammonia, which 

 are either employed in rendering soluble the mineral salts 

 lying in the soil, or are laid up in the geine of the soil for 

 the use of future crops. 



These salts lie concealed in small stones or minute par 

 ticles of the rocks. In mica and felspar, for example, 

 which are ingredients in granite, there are potash, alu 

 mina, magnesia and iron, as well as silica, and sometimes 

 soda and lime, all essential elements in the food of plants. 



719. The old Greeks and Romans often allowed their 

 fields to lie fallow, and found them thereby rendered 

 more fertile ; and the same is done, for the same reason, 

 by many nations in the South of Europe. But the intro 

 duction of Indian corn, potatoes and other roots, has 

 rendered it less necessary, and where land is very valu 

 able, fallows are generally discontinued, the benefits of 

 weathering being secured by deep ploughing and by fre 

 quent tillage between the rows of the standing crops. 



The same rotation is not suited equally to every kind 

 of soil. On the sandy soils of New England, abundant in 

 silica, Indian corn, rye and the grasses naturally occupy 

 more space than they would in a soil rich in lime. In 

 such a soil as the last, wheat might take the place of rye 

 and of Indian corn. 



720. The farmer must find out, from the experience of 

 others or from his own observation, what course is best 

 for the particular soil he cultivates, and the particular 

 object he has in view. 



One may choose to keep sheep, another, only cattle 

 for the market, another, cows for the dairy. A farmer 



