THEORY OF SOIL FERTILITY 205 



five elements has reduced the fertility of most 

 cultivated soils of the United States, has 

 greatly impoverished the older farm lands and 

 brought agricultural abandonment to millions 

 of acres in the original thirteen States. On 

 the other hand, intelligent attention to these 

 same points will bring restoration and high 

 productive power to such lands." 



Leaving out of the discussion, for the time 

 being, nitrogen, Doctor Hopkins continues: 



"Of the four important mineral elements, 

 potassium is by far the most abundant in com- 

 mon soils. Thus, as an average of ten residual 

 soils from ten different geological formations 

 in the eastern part of the United States, two 

 million pounds of subsurface soil were found 

 to contain: Potassium, 37,860 pounds; mag- 

 nesium, 14,080 pounds; calcium, 7,810 pounds; 

 phosphorus, 1,100 pounds. 



"Even the depleted, and to some extent 

 abandoned, gently undulating upland, 'Leon- 

 ardtown loam,' which was farmed for gene- 

 rations and which, according to the surveys 

 of the Federal Bureau of Soils, covers 41 per 

 cent, of St. Mary's County, Maryland, and 

 more than 45,000 acres of Prince George's 

 County still contains in two million pounds 

 of surface soil corresponding to the plowed 

 soil of an acre about 62-3 inches deep: Po- 



