386 THE POTATO 



gradually filled up with wash and sediment, and 

 with dust blown from volcanoes. In places this 

 sediment is known to be 1,000 feet deep. Flows of 

 lava from numerous vents, and deposits blown in 

 by the wind, added to the superstructure of the 

 country. The lava flows to the eastward of the 

 lake region have been covered with wash from the 

 mountains, dust blown from the old lake bed and 

 lava dust from old volcanoes. The disintegration 

 of lava rock has also probably added to the present 

 soil. While in one way a plain, the Snake River 

 country is more or less broken, making soil drain- 

 age perfect. The valley is surrounded by moun- 

 tains rising from a few hundred to 6,000 feet above 

 the plains, and 7,000 to 10,000 feet above the sea 

 level. 



The lava ash soil of the Twin Falls country is fine 

 in texture. It is classed as an arid or desert soil in 

 comparison with those of the rain belt. Soils 

 found in a country of light rainfall, where irrigation 

 is absolutely essential for the production of maxi- 

 mum yields of most crops, are high in the min- 

 eral elements of fertility because the soluble 

 salts have never been leached out by the rains. 

 Muck, peat or sour acid soils are not found in the 

 Twin Falls country. A large number of tests of 

 samples of arid and humid soils shows that the 

 average arid soils contain three times as much 

 potash, thirteen times as much lime, and six times 

 as much magnesia as the humid soils. There is 

 less humus in an arid soil. Humus is one of the 

 chief sources of nitrogen, and nitrogen is one of the 

 principal elements of plant food. The humus of an 

 arid soil, however, contains three times as much 

 nitrogen as the humus of a humid soil. Limestone 

 soils are usually rich in phosphorus, and it is 



