ESTIMATION OF FOC^D IN A WHEAT SOIL. 123 



a fertile .wheat soil must contain 



2560 kilogrammes = 5632 Ibs. of phosphoric acid, 

 5200 = 11440 " potash, and 



15300 " = 33660 " silicic acid, 



If a cubic decimetre (1 litre = 1'T pint) of arable 

 soil weighs on an average 1200 grammes ( = 2 '64 Ibs.), 

 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 milligrammes of phos- 

 phoric acid, 3900 milligrammes of potash, and 10,200 

 milligrammes of silicic acid, must be contained in an 

 available form in 2-J cubic decimetres, or 3000 grammes 

 ( = 66 Ibs.) of soil : this makes 0-056 per cent, of phos- 

 phoric acid, 0-034 per cent, of potash, and 0-34 per 

 cent, of silicic 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 (2|- acres) in corn and straw, 

 have been determined by chemical analysis, and are 

 not hypothetical. It is therefore certain that a wheat 

 crop draws from the ground half as much again of 

 phosphoric acid and silicic acid, and one-third more 

 potash, than a rye crop. 



The supposition that a wheat soil, to the depth of 

 10 inches, contains in physical combination 0.056 per 

 cent, of phosphoric acid, 0-034 per cent, of potash, and 

 0-34: per cent, of silicic acid, which makes a hundred 

 times as much as a wheat crop would take in corn and 

 straw from the field, is purely hypothetical ; and the 

 present question is to determine the limits up to which 

 this estimate may be accepted as true. 



If arable soil is left for twenty-four hours in contact 

 with cold muriatic acid, a certain quantity of potash, 

 phosphoric acid, silicic acid, as well as lime, magnesia, 

 &c. is extracted. If the soil is treated for a long time 

 with loiling muriatic acid, the quantities of dissolved 

 silicic acid and potash are much greater. Lastly, by 



