118 THE SOIL. 



inclies), then the lye-fielcl in each square decimetre (1 0,000 

 square millimetres) will contain 1700 miUigrammes 

 (= 26-2 grs.) of phosphoric acid, 3900 milligrammes 

 (= 60 grs.) of potash, and 10,200 milligrammes (= 15-7 

 grs.) of sihcic acid ; that is, a hundred 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 sihcic acid, and 0-4 more potash, from the 

 same portions of the soil, it follows that if a hectare 

 (2 J acres), to produce an average rye crop, contams 



1700 kilogi-ammes= 3740 lbs. of pliosphoric acid, 

 3900 „ = 8580 „ potash, and 



10200 „ =22440 „ silicic acid, 



a fertile wheat soil must contain 



2560 kilogrammes= 5632 lbs. of phosphoric acid, 

 5200 „ =11440,, potash, and 



15300 „ =33660 „ sihcic acid. 



If a cubic decimetre (1 htre = 1*7 pint) of arable 

 soil weighs on an average 1200 grammes (= 2-64 lbs.), 

 and we assume that the greater number of the roots of a 

 wheat plant do not go deeper than 25 centimetres (10 

 inches), then the above 1700 milhgrammes of phosphoric 

 acid, 3900 milhgrammes of potash, and 10,200 milh- 

 grammes of sihcic acid, must be contained in an available 

 form in 2^ cubic decimetres, or 3000 grammes (= 66 lbs.) 

 of soil : this makes 0*056 per cent, of phosphoric acid, 

 0-034 per cent, of potash, and 0-34 per cent, of sihcic 

 acid. 



Before we discuss the inferences which follow from 

 these numbers, we must remember that they involve 

 some hypothetical elements, which ought not to be left 

 out of view. The numbers representing the quantity of 

 ash constituents, which an average rye and wheat crop 

 take from a hectare (2J acres) in corn and straw, have 



