46 



The Ottawa Naturalist. 



[June 



from 5,000 to 10,000 lbs. of phosphoric acid. Similarly, in soils 

 of g^ood average fertility we find : nitrogen, 2,500 to 5,000 lbs.; 

 potash, 5,500 to 1 1,000 lbs., and phosphoric acid 3,500 to6,ooolbs. 

 From comparing- these figures with those that I gave repre- 

 senting the amounts used by various crops, it would at first sight 

 appear as if adding plant food to the soil were quite unneces- 

 sary, a "carrying ot coals to Newcastle"; that where there is such 

 an abundance of food there would be no economy m supplying 

 more. The explanation lies in the fact that while the vast stores 

 that we have mentioned are truly present, but a xiery small per- 

 centage of ther7i is immediately available to plants. In this we 

 recognize a wise provision of nature, for if it were otherwise soils 

 might soon become exhausted by the leaching of the food consti- 

 tuents below the reach of roots, and by the selfish practices of 

 farmers who care nought for posterity and return nothing to the 

 soil. I have alluded to the agencies and forces instrumental in 

 soil formation; it is by a continuation of these and by the solvent 

 action of root sap that soil constituents are being continually 

 prepared for the use of the higher plants. We have to recognize 

 that the very small proportion of the nitrogen present as nitrates.and 

 those minute percentages ot phosphoric acid and potash soluble 

 in water or in 1% citric acid solution — a solvent of approximately 

 the same activity and strength as root sap — represent all the 

 quantities immediately available to crops, and give a measure of 



the soil's productiveness. We have made determinations of this 

 soluble plant food in many Canadian soils. One instance is given 

 below of a rich and fertile black loam from British Columbia.* 



*CoMPARisON of "Available" with "Total" Amounts of Potash and 



Phosphoric Acid. 



o 



Soil. 



O 



o 



Surface |0-23 



Between 12 and 18 ins. 0.23 

 Between 18 and 24 ins. 0.26 



