172 BACTERIA AND SOIL FERTILITY 



phosphorus, potassium, magnesium, calcium, iron, and sulfur — 

 are obtained by all plants from the soil. 



Plant-food Added by Legumes.— The majority of agricul- 

 tural soils contain sufficient quantities of all these elements except 

 nitrogen, phosphorus, and potassium. These are low in many soils 

 and used by the growing plant in larger quantities than are any 

 of the other elements obtained directly from the soil. Therefore, 

 it becomes pertinent to ask: Can legumes maintain these ele- 

 ments in the soil in quantities sufficient for maximum crop produc- 

 tion? Phosphorus and potassium are obtained by all growing 

 plants only from the soil. It is, therefore, evident that legumes 

 cannot maintain the phosphorus and potassium since the quantity 

 within the soil must of necessity be reduced with each crop re- 

 moved, the extent depending upon the kind and size of the specific 

 crop. Hence, nitrogen is the only element which we can hope to 

 return by legumes. Usually this is the element limiting crop 

 production as it is found in the soil in smallest quantity and re- 

 moved by the plants in larger quantities than either phosphorus or 

 potassium. Moreover, large quantises of this elerr^ent are at 

 times leached from the soil, whereas the loss of the others is 

 comparatively small. It is, therefore, of the greatest importance 

 that nitrogen be supplied to the soils in sufficient quantities for 

 crop production and in the cheapest available form. 



The total quantity of the three elements-nitrogen, phosphorus, 

 and potassium — found in an acre-foot of two Utah agricultural 

 soils, assuming one acre-foot to weigh 3,600,000 pounds and the 

 rate of renloval by crop, is given on opposite page. 



Both soils contain an abundance of potassium, but the supply 

 of phosphorus and nitrogen is much lower. A study of these 

 results shows that a 50-bushel crop of wheat each year for fifty- 

 one years would remove the equivalent of the total quantity of 

 nitrogen in the Greenville soil to a depth of one foot, while a 

 similar crop on the Nephi farm would accomplish this in just 

 thirty-nine years. It would, however, require a 50-bushel crop 

 169 years to remove the phosphorus from the Greenville soil 

 and 524 years to remove it from the Nephi soil, whereas the 



