WHY KYE MAY FLOURISH AND NOT WHEAT. 121 



kilogrammes (=110 to 132 Ibs.) of silicic acid, more 

 than the rye crop. 



Before the true cause was known upon which the 

 nutritive power of arable soil depends, it was utterly 

 incomprehensible 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 con- 

 tained 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 demanded 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 can- 

 not of themselves change their place in the ground, the 

 failure of wheat upon a rye field is very simply ex- 

 plained. 



If a 2^- acre field yields to an average rye crop 

 (grain and straw) 17 million milligrammes ( 37 '4 Ibs.) 

 of phosphoric acid, 39 million milligrames (=85'8 

 Ibs.) of potash, and 102 million milligrammes (=224*4: 

 grains) of silicic acid, then the rye plants growing on a 

 square decimetre (=15*3 square inches) receive from 

 the soil 17 milligrammes (=0'26 grains) of phosphoric 

 acid, 39 milligrammes (=0'6 grain) of potash, and 102 

 milligrammes (=1*56 grains) of silicic acid. 



