and the Origin of Soils. 71 



this locality, the scrapings from the highways were considered 

 good manure, and gardeners often planted melons in it ; but 

 now none consider it good for any purpose. Indeed, the farmers 

 refuse the scrapings from the roads ; the cause is this : the 

 labouring classes are in distress, and the dung which happens to 

 be on the road is eagerly sought by them for their gardens, 

 which renders the scrapings little more than ground stones. 



If I mistake not, the writer alluded to in the foregoing ex- 

 tract mentions that when soil becomes what is commonly called 

 worn out, nothing but rest will restore it. Whatever the soil 

 has lost, he cannot think it is alkalies ; for, in the extract before 

 noticed, he says the disintegrating of rocks makes them pro- 

 duce alkalies. Various are the opinions why soil becomes in 

 the state just mentioned. It being a very important matter, I 

 shall make a few observations upon it. How is it that some 

 soils, well cultivated, refuse, if I may so call it, to produce a 

 series of the same kind of crops, while other sorts do not? 

 Wherefore is it that some plants or trees will thrive in soil 

 where other trees have grown before, and others will not, espe- 

 cially if they happen to be of the same kind of tree ? As 

 regards the first, the common belief that it is owing to the soil 

 is, of course, correct ; but the grand question is, what is the 

 material that the land loses by feeding a series of the same kind 

 of crops? Supposing it to be alkalies, how is it that some kinds 

 of soil will not produce a crop of potatoes, except the seed pota- 

 toes grew on a different kind of land ? This fact is well known 

 to gardeners in the neighbourhood of Edinburgh. It cannot be 

 possible that fresh alkalies were brought by the potatoes. 



With regard to the second thing in question, it is considered 

 that each kind, or rather each family, of plants draws a particu- 

 lar nourishment from the soil : this seems in a great measure 

 correct, and, of course, accounts for the failure of trees, espe- 

 cially on land planted over again with the same kinds. Mr. 

 Lymburn, who is a very accurate writer, mentions that the 

 excretions from the roots of some plants will not let other kinds 

 thrive on land where they grew ; but it is not clear why a series 

 of the same kind of crops cannot be produced from some soils. 

 After all it appears to me that the "whys" and the "where- 

 fores" are still to be learned. 



But to return to the subject of soil from rocks. I have pre- 

 viously stated that the crust of the earth is principally of vege- 

 table remains. It may be asked, If soil from decayed rocks is 

 not favourable for plants, how came their remains at first upon 

 the earth ? To answer this, it will be needless for me to go 

 back to the Mosaic era. I shall merely mention that when 

 islands are raised from the bottom of the sea by volcanic agency, 

 or by coral insects, they remain barren for some time, though in 



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