Some Causes of Unjiroductiveness in Soils. 123 • 



This list might have been much extended, but the preceding 

 examples will suffice in illustration of the deficiency of lime in 

 peaty, clay, sandy, and pasture soils. 



The want of alkalies, more especially of potash, is, I believe, a 

 far more common cause of the comparative sterility which cha- 

 racterises some land than is generally believed to be the case. 

 If we look at the composition of the ashes of all farm-produce, 

 Ave shall find that a very large proportion of their ashes consists 

 of potash. This constituent, of course, must be supplied either 

 by the soil or in the manure with which the latter is dressed. 

 Root-crops especially leave ashes rich in potash ; and as turnips 

 are often grown on land naturally poor in alkalies, with purely 

 mineral superphosphate of lime, and nothing else, and the 

 produce is sometimes sold off the land, or not consumed entirely 

 upon it, the land may thus become drained of its available 

 potash to an injurious extent. Perhaps the failure of roots on 

 land which formerly produced good crops may have something 

 to do with the gradual exhaustion of their available alkalies. 



At any rate it is well to remember that many soils are very 

 poor in alkalies, and that in all fertile soils the amount of potash 

 and soda is always considerable. 



It is a prevailing idea that light sandy land alone is likely to 

 be deficient in potash ; and it is true that this want does not so 

 frequently affect clay land ; but at the same time it is no less a 

 fact that some clays are almost as poor in alkalies as the worst 

 kinds of sandy soils. In proof of this 1 may give the sub- 

 joined recent analyses of mine of a strong clay and a light sandy 

 soil : — 



Composition of a Strong Clay and a Sandy Soil. 



strong Light 



Clay Soil. ' Sandy Soil. 



Moisture 4-01 .. .. 



Organic matter and water of combination 8'51 .... (5"92 



Oxides of iron and alumina 11*24 .... 64o 



Phosphoric acid '06 .... "ll 



Sulphuric acid "19 



Lime none .... '65 



Magnesia '46 .. .. -39 



Alkalies (potash and soda) "45 .. .. "So 



Insoluble siliceous matter 75"08 .... 85"17 



Chiefly (clay) .. .. (sand) 



100-00 100-00 



In these two soils potash and soda are evidently deficient ; at 

 the same time it will be noticed that both are poor in phosphoric 

 acid, that there is no lime in the clay, and but little in the sandy 

 soil. 



Unproductive soils are seldom deficient in one substance only : 



