F. Hardy — Mineral Composition of Fenland Silt. 549 



an analytical method for estimating the percentage amounts of the 

 more easily decomposed (" calcitoid ") carbonate, and of the 

 carbonate of greater stability (" dolomitoid carbonate ") in marine 

 silt soils. Af)p]ying the method to the sample described above, 

 it was found that of the total 8'98 per cent of carbonates in the silt, 

 5'75 per cent was easily decomposed by acids (i.e. soluble in N/3 

 acetic acid), whereas 3"23 per cent (by difference) required a stronger 

 acid for its decomposition, and is assumed therefore to be of the 

 nature of dolomite or allied mineral carbonate. 



The details of the analytical method used in the estimation of 

 calcitoid and dolomitoid carbonate in silt soils, and an account of the 

 agricultural significance of the presence of these two kinds of 

 carbonates in such soils, are given in the writer's forthcoming paper 

 already quoted. 



(B) Potash. 



The chief potassium-containing' mineral in the silt is muscovite, 

 and it is noteworthy that the greater amount of this mineral appears 

 (from the petrographical examination) to occur in the finer fraction 

 of the material. Although orthoclase felspar was identified as a con- 

 stituent of the silt, it was noted that its amount was comparatively 

 small. The only other possible potassium-containing minerals 

 identified were tourmaline and glauconite. The former mineral, 

 however, is so very stable that it is of no importance as a source 

 of potash in soils containing it, and glauconite, which when fresh 

 may contain as much as 8 per cent of KgO,^ is here present chiefly 

 in a much decomposed state and also in such comparatively small 

 amount that it may be left out of consideration as an important 

 source of potash in the silt. 



Thus the chief source of potash for the use of crops grown on 

 Fenland silt similar to that described is muscovite. 



According to the chemical analysis, the total percentage of KgO 

 in the silt examined in r224. This corresponds to about 10 per cent 

 of muscovite (assuming the absence of other potash-minerals). It 

 is relatively high as compared with the K2O content of typical soils, 

 which usually lies between O'l and I'O per cent. Nevertheless, the 

 successful growth of crops (especially potatoes) in the cultivated 

 regions of the reclaimed silt Fenlands frequently necessitates the 

 liberal use of potassic fertilizers. This fact suggests that the potash 

 supply of the Fenland silt soils is only relatively slowly available, so 

 tliat it may fail to satisfy the full needs of certain crops which require 

 an abundance of this plant food. It may be explained by the 

 resistance to weathering which is a known characteristic of muscovite. 



(C) Phosphate. 

 The only phosphatic mineral found in the silt "was apatite. 

 Chemical analysis showed that the total phosphate (P2O5) present 



^ Clarke, "Data of Geochemistry": BuUetiri 616, U.S. Geol. Surv., 1916, 

 p. 516. 



