IX AVIIAT MANNER PLANTS ABSORB FOOD. 105 



which falls on the field during the period of vegetation, we 

 find only just so much potash as the turnip crop requires, 

 and no more, the assumption that the potash in the turnips 

 has been derived from this solution would necessarily 

 involve the impossible supposition, that all the watery 

 particles containing potash must have been in contact 

 w^th the roots of the turnips ; otherwise, the latter could 

 not have absorbed so much potash as is actually found in 

 them. This supposition is impossible ; because, during 

 the time when the turnip vegetates, there is generally no 

 water cu-culating in the soil — such, for instance, as might 

 be carried ofi" by tbain-pipes. 



If the examination of the water in the soil shows it to 

 contain half the quantity of jiotash required by a turnip 

 crop, there is no need to explain how the dissolved half 

 of the potash has passed into the turnip-plant, but in 

 what form and manner the plant has absorbed the other 

 half deficient in the water. 



If, again, by the examination of the water in other 

 fields, we find that it contains only ^ ; nay, only J, 20 ^ oi' 

 3L. of the quantity of potash found in a turnip crop grown 

 upon it ; and if we further ascertain that in a soil, favour- 

 able for the growth of turnips, the plant always takes up 

 the same quantity of potash from the ground, no matter 

 how much or how little of that substance the water cir- 

 culating in the soil dissoh'es from the earth ; it follows, 

 that as the water, the soil, and the plant, can alone come 

 into consideration here, the direct power of the water to 

 dissolve potash is of no importance to the plant ; and 

 that the plant itself, by the help of water, must have ren- 

 dered the needful potash soluble. 



What is here asserted of one constituent, holds good 

 for all. If, therefore, we find, that by treating a soil Avith 

 rain-water we can dissolve from it potash, phosphoric 



