On Paring and Burning. 13 



silica, whilst soluble silica is the chief constituent of couch. 

 Again, the proportions of lime and potash in the thistle are much 

 larger than in couch, a fact which is interesting on account of 

 both weeds having been collected from soils of a similar cha- 

 racter. This is quite consistent with our present information on 

 the particular wants of different families of plants. Thistles are 

 evidently lime and potash plants, and couch appears to require 

 much soluble silica for its growth, a fact which perhaps will 

 explain its occurrence in sandy as well as in calcareous and clay 

 soils. The two last-mentioned soils often, it is true, do not 

 contain any silica in the shape of sand, capable of being mechani- 

 cally separated from the other soils' constituents, but most clay 

 soils are rich in alkaline silicates, from which plants un- 

 questionably can take up soluble silica much more readily than 

 from sand. 



The large proportion of phosphoric acid in couch ash likewise 

 deserves to be specially noticed. United with lime we have no 

 less than 20 per cent, of bone-earth in couch ash, a circumstance 

 which throws some light on the experience of many Gloucester- 

 shire farmers, with whom I have conversed on this subject, and 

 by whom I am told that the fouler the land is, the better will be 

 the crop of turnips, grown without any manure upon land simply 

 pared and burned. I remember having walked once over a very 

 foul piece of land full of couch, and was not a little amused by 

 the remark of my agricultural friend who accompanied fne: 

 " What fine healthy couch, Sir, and will give me next year, I don't 

 doubt, a splendid crop of roots, although I do not mean to put a 

 single load of manure on this land." Considering the large 

 amount of couch which is sometimes collected from fields and 

 burned along with other vegetable remains, containing phosphoric 

 acid, a very considerable dressing of bone-material must be given 

 to the turnip crop in couch ashes. Indeed I have ascertained 

 that a much larger proportion of bone-earth is brought within 

 reach of the turnip plant in the red ashes obtained on paring and 

 burning than is contained in a heavy dressing of bone-dust. I 

 shall advert again to this point further on, and only observe in 

 this place that phosphate of lime is the most valuable consti- 

 tuent of couch ashes, and that next to it, potash and sulphate of 

 lime exercise beneficial effects upon the growth of root-crops. 



The ashes left on burning other weeds and other vegetable 

 matters found in soils, no doubt will present similar differences 

 to those just pointed out in the analyses of the ash of couch and 

 the stemless thistle. However great the differences in the pro- 

 portions of phosphate of lime, potash, and other fertilizing con- 

 stituents may be in the various kinds of vegetable remains which 

 are reduced into ashes in paring and burning, every description 

 of vegetable matter produces ashes containing valuable mineral 



