CHAPTER XI 



SOURCES OF PLANT FOOD 



IF the productive capacity of American soils is to be maintained, 

 elements of plant food which are present in such small amounts as 

 to limit the crop yields even under good systems of farming must 

 be returned to the soil as needed, and information is given in Table 

 24 to show the average quantities in pounds of the different valu- 

 able elements of plant food contained in one ton of average fresh 

 farm manure, rough feeds and bedding, and other fertilizer ma- 

 terials. 



In computing the value of plant food in these materials, nitrogen 

 is counted at 15 cents a pound and potassium at 6 cents a pound; 

 while phosphorus is counted at 3 cents a pound in raw rock phos- 

 phate, at 10 cents a pound in bone meal, and at 12 cents a pound in 

 acid phosphate, these prices being based upon the usual average 

 market values for the standard fertilizing materials in such quanti- 

 ties as ought to be purchased by farmers, either singly or by two 

 or more uniting. 



From the data given in Tables 23 and 24 it is a simple matter to 

 compute the amounts of average manure or other fertilizers neces- 

 sary to be applied to the land to replace the plant food removed in 

 any rotation of crops. Observe, for example, that a four-year 

 rotation, including corn for two years, oats with clover seeding 

 the third year, and clover for hay and seed crops the fourth year, 

 would require 39 tons of manure to supply the nitrogen, 41 tons 

 to supply the phosphorus, 40 tons to supply the potassium, assum- 

 ing the yields given in Table 23, and counting that the clover se- 

 cures from the air as much nitrogen as is removed in the hay and 

 seed crops. Observe that one ton of raw rock phosphate or one 

 ton of steamed bone meal contains more phosphorus than 100 tons 

 of average manure. Observe that 250 pounds of phosphorus can 

 be purchased for $7.50 in ground natural rock phosphate, for $25.00 



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