THE ASH FROM WHEAT STRAW IS VARIABLE. 225 



at least of the silica, in these two varieties of straw, and this difference 

 can be attributed only to the unlike nature of the soils in which the two 

 samples were grown. But on boggy soils the oat plant is unhealthy, 

 and in general neither fills its ear, nor ripens a perfect seed ; — the dif- 

 ference in the ash in this case, therefore, cannot be considered as entirely 

 opposed to the general proposition, that in a healthy state, plants at 

 the same period of their growth always yield nearly the same weight 

 of ash. 



But that different experimenters have obtained very unlike quantities 

 of ash, from the most common cultivated plants, apparently in a state 

 of health, when grown under different circumstances of soil and climate, 

 — does appear to contradict this general propositior. Thus 100 lbs. of 

 ripe ivheat straw leave of ash 



4-3 lbs. De Saussure ; 

 4 '4 lbs. Berthier; 

 3-5 lbs. Sprengel ; 

 15-5 lbs. Sir H. Davy ; 

 while the straw of one variety of red wheat grown on a clay-loam, at 

 Aykley Heads, near Durham, gave me 6-6 per cent., and that of two 

 other varieties of red wheat, grown near Dalton, in Ravensworth Dale, 

 Yorkshire, a country abounding in limestone — and on the same field — 

 left respectively 12*15 and 16-5 per cent, of ash. The difference of 4 

 per cent, between these last two results, shows that the quantity of ash 

 depends much upon the variety of grain examined — though to what ex- 

 tent all the great differences obtained, as above shown, are to be ascribed 

 to this cause alone, it is impossible to say, until numerous other experi- 

 ments shall have been instituted. 



One thing, however, is manifest, that the quantities of inorganic mat- 

 ter necessarily contained in a crop of wheat, given in a previous page 

 (p. 216) on the authority of Sprengel, must be considered as probably 

 far below the mean proportion, since some varieties yield, in the form 

 of ash, about six times as much as is there stated. 



Every one knows how uncertain general conclusions are, — or expla- 

 nations of natural phenomena, — when deduced from single observations 

 only, and of this truth the above results present us with a useful illus- 

 tration. Thus Liebig, in his Organic Chernistry applied to Agriculture 

 p. 152, to which we have had frequent occasion to refer — explains 

 why land will refuse to grow wheat, and may yet produce good crops 

 of oats or barley in the following manner : — "One hundred parts of the 

 stalks of wheat yield 15-5 parts of ashes (H. Davy) : the same quantity 

 of the dry stalks of barley 8*54 (Schrader), and one hundred parts of the 

 stalks of oats only 4-42. The ashes of all are of the same composition. 

 We have in these facts a clear [)roof of what plants require for their 

 growth. Upon the same field which will yield only one harvest of 

 wheat, two crops of barley and three of oats may be raised." 



In this passage it has been assumed that the ash of wheat and other 

 straws is constant in quantity, that wheat straw always contains much 

 more than that of oats or barley, and that the ash is in each case of the 

 same composition (see above, pp. 216 to 217), — all of which premises 

 being incorrect, the conclusion must of course be rejected. 



But the straw of barley and oats also, accordina: to different authorities, 

 10* 



