320 CROP ROTATION 



and potassium are obtained by the growing plant only from the 

 soil; it is, therefore, self-evident that no simple system of crop 

 rotation can maintain the phosphorus and potassium, since the 

 quantity within the soil must of necessity be reduced with each 

 crop removed, the extent depending upon the specific crop grown. 

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

 by crop rotation. But this is the element which is found in the 

 soil in smallest quantity and removed by most plants in larger 

 quantities than the phosphorus or potassium. Moreover, large 

 quantities of this element are at times lost from the soil by leach- 

 ing, while the loss of the others is comparatively small. It is of 

 the greatest importance, therefore, that this nitrogen be supplied 

 to the soils in sufficient quantities for crop production and in the 

 cheapest manner possible. The total quantity of these three ele- 

 ments found in an acre-foot section of two Utah agricultural soils, 

 assuming one acre-foot to weight 3,600,000 pounds, is given below: 



Greenville Farm (Utah), Nephi Farm (Utah), 

 pounds per acre. pounds per acre. 



Nitrogen 4,904 3,744 



Phosphorus . . 2,700 8,388 



Potassium 60,560 87,840 



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 forty- 

 nine 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-seven 

 years. It would, however, require a 50-bushel crop one hundred 

 and seventy years to remove the phosphorus from the Greenville 

 soil and five hundred and twenty-five years to remove it from the 

 Nephi soil. Of course a crop would never remove all the nitrogen 

 or phosphorus from a soil, but in actual practice the elements are 

 slowly removed, the crop yields beng reduced each year until 

 a certain minimum is reached. When crops can no longer be 

 produced economically then the owner abandons his soil, moves 

 on to virgin soils, or if it be in an old district he resorts to the 

 expensive commercial fertilizer. The illustration is, however, suffi- 

 ciently accurate to make it clear that the limiting factor, in so far 

 as soil fertility is concerned in both of these soils, is the nitrogen. 

 And it is true of the great majority of all soils that an increased 

 nitrogen supply means an increased yield. This principle is one 

 of the fundamentals of soil fertility. 



Nitrogen. Nitrogen exists in the atmosphere in inexhaustible 

 quantities, every square yard of land having seven tons of nitrogen 

 lying over it, or if the quantity covering one acre could be combined 

 into the nitrate it would be worth as a fertilizer $125,000,000. 



