194 THE FARMER OF TO-MORROW 



of the nature of available plant food. Every 

 installment of "interest" that the soil liberates 

 in the form of crops draws on its own capital, 

 and when it has run its course the soil is in the 

 condition of the gold mine which continues 

 paying a 5 per cent, dividend until the last 

 ounce of gold is mined. 



From this viewpoint the problems of deter- 

 mining the extent of soil fertility should be 

 simple. No matter whether the potash and 

 phosphorus found in the soil are in "available" 

 form or not, they will be available to-morrow 

 or the next day. Sooner or later every gram 

 of plant-feeding minerals would have passed 

 into solution. Then, most truly, would the 

 soil have been "mined" of its fertility, and 

 would possess nothing but a figurative hole in 

 the ground. 



It is easy to find out how many grams or 

 parts per million of any one of the "plant 

 foods" exist in a given soil. Take the soil 

 one, two, three, or ten feet deep, according 

 as you believe the plants draw their food. The 

 usual method is to take the first twelve inches 

 of soil and subject it to analysis. Let us ex- 

 amine a series of specimen figures, to illustrate. 

 Hellriegel, a German scientist to whom is at- 



