408 Outlines of HortmiUural Chemistty. 



unctuous constituents will not combine, with even a slight de- 

 gree ofaflfinity, with silica, which they will with alumina and chalk. 

 At the same time, light soils admit rain into their texture, 

 and to carry away their fertile constituents in the drainage 

 waters ; and the same openness of texture likewise permits the 

 free access of air to hasten the putrefaction of the vegetable 

 matters they contain, as well as the easy escape of the gases 

 which are evolved, and all which, we have before shown, are 

 equally beneficial to plants. Silica may abound to a much 

 greater extent in a soil than any other of its usual constituents, 

 without being unfavourable to vegetation. Chalk should 

 never be present in a soil to a greater extent than 6 or 8 per 

 cent ; decomposable animal and vegetable matter to no more 

 than 10 per cent; nor can the saline constituents soluble in 

 water, oxide of iron, &c., amount to more than 6 per cent, 

 without injury proportionate to the excess. 



Foreign impregnations, causing a soil to be sterile or im- 

 pairing its pi'oductiveness, are rare. 



Acids have been ranked among the causes of sterility ; 

 but a soil containing any in a free state never came 

 under my notice, or under that of any other practical 

 chemist of whom I have ever read, or with whom I have 

 ever conversed. Some soils, or certain portions of a field 

 3iot generally so affected, will be found to produce sorrel 

 and other plants abounding in acids : and, as when chalk 

 or any other neutraliser of acids is applied to such spots 

 they cease to produce sour plants; it has been deemed a 

 legitimate conclusion that those plants obtained their acids 

 Irom the soil, which being removed or neutralised by the 

 chalk, consequently destroyed the plants by depriving them of 

 one of their chief constituents. To say the least of it, such 

 an opinion betrays a very great ignorance of physiology and 

 vegetable chemistry. In the first place, the food obtained by- 

 all plants from the soil is perfectly insipid when absorbed, 

 and whilst rising through the vessels in the woods; and no 

 secretion, acid, or otherwise marked, is ever found in it until 

 it has been elaborated in the leaves. It is only to be detected 

 in them, and more manifestly in the bark. The fact seems to 

 be, that plants abounding in acids generally frequent a wet 

 soil, and such soil is rendered less retentive of moisture by 

 chalk : again the contact of chalk with plants containing acids 

 causes decomposition in them, ulcers, and if perpetually pre- 

 sented, death. Lastly, such sour soils, as they are termed, are 

 usually as effectually cleared of acid plants by mixing them 

 with other substances that will render them porous, and by 

 und^i'idraining, them thoroughly, as they are by mixing chalk 



