24 ON THE USE OF LIME, MARL, AND SHELL-SAND. 



It will be observed therefore that No. I. can hardly be called 

 a marl. 



There are many similar minerals, which are called by practical 

 men clay marl, since they are used with marked benefit on some 

 soils. 



The so-called clay marl from Bridgewater appears to be an 

 alluvial clay. It occurs in reddish brown lumps, readily breaks 

 down on exposure to the atmosphere, and possesses considerable 

 fertilizing qualities. It will be seen that this mineral fertilizer 

 contains a considerable quantity of phosphoric acid and soluble 

 potash, but is very poor in lime. The fine clay which enters 

 into its composition, moreover contains a great deal of potash in 

 an insoluble state, from which, however, it is gradually liberated 

 on exposure to the atmosphere. 



There can be no doubt that the fertilizing effects which this 

 clay marl produces are due to the large amount of soluble potash 

 as well as to the phosphoric acid which it contains, and not to the 

 lime. The use of this clay marl will not supersede the applica- 

 tion of lime, or of a marl rich in carbonate of lime. 



This marl thus presents us with the curious fact that the very 

 best thing that can be mixed with this marl is lime, and that 

 wherever lime is really wanted, as is the case in many soils near 

 Bridgewater, the clay marl occurring in the neighbourhood of 

 this town does not answer the purpose for which lime is usually 

 employed in agriculture. 



No. II. occurs in gray lumps at the foot of the Wiltshire 

 Downs. It is largely used by the Wiltshire farmers, both on 

 pasture and arable land. 



The proportion of phosphoric acid in this marl is not large, 

 but still it helps to add to its fertilizing effects, which are due 

 principally to the large quantity of carbonate of lime, and perhaps 

 also to the finely divided or soluble silica, which also occurs in 

 abundance in it. 



Still larger than in No. II. is the proportion of carbonate of 

 lime in the chalk marl from Gloucester, which is by far the least 

 valuable of these three marls. 



Chalk marls on the whole are good fertilizers for all soils de- 

 ficient in lime, and some produce very beneficial results even 

 when applied to soils containing superabundance of lime. It 

 appears thus clearly that lime in the latter instances cannot be 

 the cause of the fertilizing properties of some kinds of chalk 

 marls. Careful analytical researches indeed have traced the 

 fertilizing effects of some chalk marls chiefly to the phosphoric 

 acid which they contain in the form of phosphate of lime. Phos- 

 phatic nodules occur frequently in Wiltshire marls and in some 

 places in Somersetshire. The occurrence of these phosphatic 



