116 THE SOIL. 



The difference in the absolute requirement is therefore 

 very small. The wheat crop received from the soil only 

 9 kilogrammes (= 20 lbs.) of phosphoric acid, about 12 

 kilogrammes ( = 26-4 lbs.) of potash, and 50 to 60 kilo- 

 grammes (:=110 to 132 lbs.) of sihcic acid, more than the 

 rye crop. 



Before the true cause was known upon which the nutri- 

 tive power of arable soil depends, it was utterly incom- 

 prehensible how this trifling difference of a few pounds of 

 phosphoric acid, silicic acid, and potash in the requirements 

 of wheat and rye, could make so great a difference in the 

 quality of a field ; for in comparison with the total amount 

 of these ingredients actually contained in the rye field, 

 the additional quantity required by the wheat plant is 

 inappreciably small. 



This difference would indeed be inconceivable if the 

 nutritive substances required by the cereal plants had 

 any perceptible power of locomotion, for in that case there 

 could not be an actual deficiency of food in any given spot 

 of the soil ; every fall of rain would provide the poorer 

 places with nutriment, if the trifling excess required by 

 the wheat above the rye could really be distributed by 

 the agency of water. 



Thus, although a soil suited for rye but not for wheat, 

 may contain, within a short distance from the roots of the 

 wheat, a large quantity of phosphoric acid and potash, 

 often amounting, in the volume of earth between two rye 

 plants, to fifty times more than the trifling addition de- 

 manded by the wheat, yet, in point of fact, this nutriment 

 cannot reach the roots of the latter. 



But if we consider that the nutritive substances cannot 

 of themselves change their place in the ground, the failure 

 of wheat upon a rye field is very simply explained. 



If a 2 J acre field yields to an average rye crop (grain 



