SOIL EXHAUSTION 



85 



of exhaustive cropping to reduce the total phosphorus supply of 

 a fertile silt loam one-half. 



The effect of exhaustive cropping on the phosphorus supply 

 is well illustrated in the following study of two pairs of soils which 

 represent two different silt loams. In each case the " virgin" and 

 the "cropped" samples are the same soil; the one has never been 

 under cultivation and the other has been subjected to exhaustive 

 cropping for over fifty years. 



Effect of Exhaustive Cropping on the Phosphorus Content of Soils 



The following is a brief outline of the history of these two soils : 



I. "Cropped sixty years, largely to wheat during the first ten 

 or twelve years; then corn and oats; clover ten years. The land 

 is now in a depleted condition." 



II. "Cropped sixty-three years. During the first thirty-four 

 years wheat was grown almost continuously. Since 1878 it has 

 been rotated to corn, barley, oats and rye. It has never been 

 seeded down or manured and is in a badly exhausted condition." 



Soil Exhaustion Not Confined to Just One Element orCondition. 

 Generally an exhausted or depleted soil is the result of several 

 unfavorable conditions, which, acting together, bring about a 

 state of infertility. All the fertility factors are necessarily affected; 

 and instead of their acting favorably for good yields, they act 

 unfavorably and thus cause poor crops or prevent any yields at all. 

 The fertility factors are unfavorably affected largely by the 

 removal of organic matter, by the removal of available nitrogen, 

 phosphorus and potassium, by the removal of available calcium 

 in the form of carbonate of lime, and by poor tillage. It is not 

 always easy, therefore, to restore the original fertility of a badly 

 depleted soil. In many instances it has proved a tedious and 

 expensive undertaking. 



