122 THE SOIL. 



Now, from the same area of a good wheat soil, the 

 wheat plants growing on it receive 26 milligrammes of 

 phosphoric acid, 52 milligrammes of potash, and 160 

 milligrammes of silicic acid. The food-absorbent sur- 

 face of the rye and wheat plants is not in contact with 

 all the earthy particles which contain food in a square 

 decimetre of the field downwards, but only with a small 

 volume of the soil ; and it is quite evident, that if the 

 seed is to thrive in every spot, the earthy particles, 

 which do not happen to come in contact with the roots, 

 must contain as much nutritive matter as the others. 



If we could ascertain with any certainty the root- 

 surface which absorbs nutriment, we might infer the 

 volume of earth from which it received food, for every 

 root-fibre is surrounded by a cylinder of earth, the inner 

 wall of which facing the root is 'as it were gnawed off by 

 the extremities of the root which press downwards, or 

 by the cell-surfaces which are deposited in a downward 

 direction. But in no plant are the diameter and length 

 of the root-fibres determined, and we must rest satisfied 

 with an approximative estimation. 



Let us assume that the 17 milligrammes (=0*26 gr.) 

 of phoshporic acid, 39 milligrammes (=0*6 gr.) of pot- 

 ash, and 102 milligrammes (=1*56 grs.) of silicic acid, 

 are absorbed from a mass of earth the transverse sec- 

 tion of which is 100 square millimetres (15*3 square 

 inches), then the rye-field in each square decimetre 

 (10,000 square millimetres) will contain 1700 milli- 

 grammes ( = 26 '2 grs.) of phosphoric acid, 3900 milli- 

 grammes ( = 60 grs.) of potash, and 10,200 milli- 

 frammes ( = 15*7 grs.) of silicic acid ; that is, a hun- 

 red times as much as an average rye crop requires. 

 Now, as the wheat plant, to thrive equally well, must 

 receive half as much again of phosphoric and silicic 

 acid, and 0*4 more potash, from the same portions of 

 the soil, it follows that if a hectare (2J acres), to produce 

 an average rye crop, contains 



1700 kilogrammes = 3740 Ibs. of phosphoric acid, 

 3900 " = 8580 " potash, and 



10200 " = 22440 " silicic acid, 



