110 



ELEMENTS OF AGRICULTURE 



CHAPTEE XXI.— How Soils Lose Mineral 

 Matter 



107. How Cultivation Removes Mineral Matter from 

 the Soil. — Soils, become impoverished from careless 

 cultivation, which causes a loss of water and nitrogen. 

 They may also become impoverished through the ex- 

 haustion of their supply of phosphoric acid, potash and 

 lime. How this occurs may be understood from the 

 fact that each crop gathered from the soil uses up for 

 its growth and takes with it just so much phosphoric 

 acid, potash and lime. A crop of cotton, lint and seed, 

 weighing about 1,000 pounds, takes from the soil about 

 6J pounds of phosphoric acid and 10 pounds of potash. 

 One thousand pounds of tobacco leaves take from the 

 soil about 3.4 pounds of phosphoric acid and 40 pounds 

 of potash. One thousand pounds of hay take about 

 3 pounds of phosphoric acid and 12J pounds of potash; 

 and so each crop takes from the soil varying amounts 

 of mineral plant food. 



If year after year so much of these plant foods are 

 taken away and none returned, the supply must become 

 exhausted. As a matter of fact, the available supply 

 does often become exhausted. 



108. Phosphates in the Soil. — Phosphorus exists in 

 the soil in several forms; combined with certain pro- 

 portions of calcium and oxygen it forms a compound 

 soluble in water, and in this form is available as plant 

 food. Were all the phosphorus in the soil in this form, 



