102 .CIVIC BIOLOGY 



Losses in plant food due to cropping. Too many have not 

 counted the cost of a crop to Mother Earth, and hence have 

 taken it as a free gift, with no thought of making any return. 

 The table above, modified from Hopkins, 1 shows what a few 

 typical crops actually take from the soil. 



Complexity of the problem. It remains to add that the prob- 

 lem of soil fertility is much more complicated than the above 

 brief statement would seem to indicate. Warren says : " The 

 fertile surface soil may be carried away by erosion, by wind, 

 or water. Probably more soil fertility is lost in this way than 

 by cropping." 2 So the humus may be exhausted, and with it 

 the soil may lose its power to hold moisture, so that it becomes 

 hard and dry, and plant food in any amounts is of no avail. Or 

 soil may be too wet and require drainage, and too free drain- 

 age may rapidly leach away nitrates, potash, and lime. Chem- 

 ical changes are going on within the soil, and additions are 

 being made to it from the air, which lead some authorities to 

 claim that mineral plant foods are practically inexhaustible. 

 Poisonous substances, it is claimed, are excreted by the roots 

 of certain plants, so that proper rotation of crops is all that is 

 needed to maintain fertility indefinitely. That is, the soil is 

 " A bank account which requires for its maintenance only the 

 rotation of the check book among the members of the family ! ' 

 Hopkins sums up the whole matter as follows : 



The possible enormous and irreparable damage of such teaching lies 

 in the fact that even our remaining supply of good land will ultimately 

 be depleted . . . beyond the point of self-redemption, thus repeating the 

 history of our abandoned Eastern lands, where the rotation of crops was 

 the common rule of practice for more than a hundred years. 



Problems in animal industry. Perhaps the most important 

 dairy records are those of Professor Fraser of the University of 

 Illinois. He tested 554 cows in 36 commercial dairy herds, each 



1 Soil Fertility and Permanent Agriculture, p. 154. 



2 Farm Management, p. 184. 



