Analyses of Ashes of Plants. 



203 



than wheat; but it must be remembered that the potash of clay 

 soils exists in them, as siUcate of potash derived from the felspar, 

 &c., of the disintegrated granitic rocks, to which the clay owes its 

 origin. The silicate of potash, in felspar, is composed of silica 

 and potash in tolerably equal quantity ; but a crop of wheat takes 

 off 83 parts of silica for every 14 parts of potash, so that, to ob- 

 tain all the silica it requires, it liberates more potash than it has 

 any need of. A crop of beans just reverses this process. It re- 

 moves from the soil 70 parts of alkali for every 5 parts of silica. 

 It is then almost indifferent which of the plants comes first. The 

 one which follows in the rotation finds potash (if it be beans), or 

 silica (if wheat), ready prepared for it. We venture to suggest 

 that this may in great part be the reason why Avheat and beans 

 follow each other without either appearing to suffer from ex- 

 haustion produced by the crop preceding it. 



It will be seen that weight for weight beans require more potash 

 both in the straw and grain. This is partly perhaps the reason 

 that they thrive on stiffer land than is necessary for peas. 



Oil the Methods of supplying the Mineral Matters for the 

 Root Crops. 



We have no intention of discussing under this head the different 

 methods of raising crops of roots by artificial manures : such a 

 subject belongs more to a treatise on manures than to the present 

 report. It is only here necessary to allude to the quantities of 

 different compounds which must be added to the soil if we wish 

 to furnish the crop artificially with all the substances it requires. 



Let us suppose, then, that we have a soil " at zero " (so to 

 speak) in mineral constitution, — in other words, that it is so poor 

 as regards phosphoric acid, sulphuric acid, magnesia, and the 

 alkalies that we could not depend upon it to supply any portion 

 of these bodies to the growing plant. This is a purely supposi- 

 tious case, for it may well be doubted whether such a soil exists 

 on the face of the earth ; but it will serve to illustrate our meaning. 

 Let us imagine, then, that we have to provide on a soil so barren 

 as this for the growth of a crop of turnips, mangold-wurzel, or 

 carrots, and that we seek to raise a large crop of either of these 

 roots, say 20 tons of clean bulb ; the question is, Wliat mineral 

 matters must we add to the soil to supply the requisite food for 

 the expected crop ?" It is not necessary here to repeat the argu- 

 ments by which we have arrived at the conclusion that the plant 

 requires these inorganic matters, and will not grow without 

 them. This is taken for granted. 



Twenty tons of bulbs and four tons of tops will require the 

 following quantities of the different mineral matters : — 



