NATURE OF A WHEAT SOIL. 117 



and straw) 17 million milligrammes ( = 37*41bs.) of phos- 

 phoric acid, 39 million milhgrammcs (=85-8 lbs.) of 

 potash, and 102 milHon milligrammes ( = 224*4 grains) 

 of sihcic acid, then the rye plants growing on a square 

 decimetre ( = 15-3 square inches) receive from the soil 17 

 milligi'ammes(=0-26 grains) of phosphoric acid, 39 milli- 

 grammes (=0-6 grain) of potash, and 102 milligrammes 

 ( = 1-56 grains) of silicic acid. 



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

 wdieat plants growing on it receive 26 milligrammes of 

 phosphoric acid, 52 milligrammes of potash, and 160 mil- 

 ligrammes of sihcic acid. The food-absorbent surface of 

 the rye and wheat plants is not in contact with all the 

 earthy particles wdiich 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 

 tlirive 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 mfer the 

 volume of earth from which it received food, for every 

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

 wall of w^hich 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 phosphoric acid, 39 milligrammes (= 0*6 gr.) of 

 potash, and 102 milligrammes (= 1-56 grs.) of silicic 

 acid, are absorbed from a mass of earth the transverse 

 section of which is 100 square millimetres (=15*3 square 



