MAINTAINING THE FERTILITY OF THE LAND 113 



bushels, and that the nitrogen supply limits it to 40 

 bushels. The crop will then be 40 bushels. In growing 

 such a crop, the farmer does not make use of all the favor- 

 able conditions; his crop is limited by the "weakest link." 

 In such a case, the addition of a certain amount of nitrogen 

 to the soil may bring the yield up to 70 bushels. Then 

 the phosphoric acid must be added if a greater yield is 

 to be obtained. 



111. The Amount of Plant-Food in the Soil. Forty- 

 nine analyses of soils in different parts of America showed 

 an average of 3,000 pounds of nitrogen, over 4,000 pounds 

 of phosphoric acid and over 16,000 pounds of potash 

 per acre. 



By multiplying the average yield of crops (Appendix, 

 Table 14) by the composition (Appendix, Table 6) we 

 obtain the amount of food removed by each crop. The 

 average wheat crop of the United States for the past ten 

 years (1899 to 1908) has been 13.8 bushels. This, together 

 with the straw, removes about 14.5 pounds of nitrogen, 

 10.6 pounds of phosphoric acid and 14 pounds of potash 

 per acre. The average surface soil would therefore con 

 tain enough nitrogen for 200 crops, phosphoric acid for 

 400 and potash for 1,000. But many soils do not contain 

 so much plant-food as this, and in the great majority 

 of cases the food is too slowly available to produce 

 maximum crops. 



112. Value of Chemical Analyses of Soils. The chemi- 

 cal analysis of soils does not tell what fertilizers are 

 needed. The almost universal opinion is that a chemist 

 can analyze the soil and tell what it needs. The analy- 

 sis may tell how much food there is in the soil, but it 



