OBSERVED EFFECTS OF MARLS. 375 



free and easier worked such soils as are stiff, intractable, and more or 

 less impervious — while either will impart solidity and substance to 

 such as are of a peaty nature or over-bound with other forms of vege- 

 table matter. 



2°. Their chemical effect in actually rendering the soil productive of 

 larger crops. This effect is altogether independent of any alteration in 

 the physical properties of the soil, and is nearly the same in Icind, what- 

 ever be the variety of marl, &c., we apply. It differs in degree, chiefly 

 according to the proportion of calcareous matter which each variety 

 contains. This action of the pure carbonate of lime they contain is 

 supposed to be modified in some cases by the proportion of phosphate of 

 lime, &c. (p. 370,) with which it maybe mixed — it is certainly modified 



7 



als 



s and shell sands. 



The several effects of marls and calcareous sands being dependent 

 upon circumstances so different, it is not surprising that the opinions of 

 practical men should, in former times, have been divided in regard to 

 the action of this or that marl itpon their respective soils. In no two 

 localities was the substance applied to the land exactly alike, and 

 hence unlike results must necessarily have followed, and disappoint- 

 ment been occasionally experienced from their use. And yet the im- 

 portance of rightly understanding the kind and degree of eflfect which 

 these manuring substances ought to produce may be estimated from 

 the fact^ that a larger surface of the cropped land in Europe is improved 

 by the assistance of calcareous marls and sands — than by the aid of 

 burned lime and farm-yard manure put together. 



It is not easy in any case to estimate with precision what portion of 

 the effect caused by a given marl is due to its chemical and what to its 

 physical action. Even the pure limes, when applied in large doses, 

 produce a change in the texture of the soil, which on stiff' lands is ben- 

 eficial, and on light or sandy fields often injurious. In all cases, there- 

 fore, the action of lime applied in any form may be considered as part- 

 ly physical and partly chemical — the extent of the chemical action in 

 general increasing with the proportion of lime which the kind of cal- 

 careous matter employed is known to contain. 



The observed effects of marls and shell sands, in so far as they are 

 chemical, are very analogous to those produced by lime as it is gener- 

 ally applied in the quick or slaked state in so many parts of our islands. 



They alter the nature and quality of the grasses when applied to 

 pasture — they cover even the undrained bog with a short rich grass — 

 they extirpate heath, and bent, and useless moss — they exterminate 

 the weeds which infest the unlimed corn fields — they increase the 

 quantity and enable the land to grow a hetter quality of corn — they ma- 

 nifest a continued action for many years after they have been applied — 

 like the purer limes they act more energetically if aided by the occa- 

 sional addition of other manure — and hke them they finally exhaust* 

 a soil from which'the successive crops are reaped, without the requisite 

 return of decaying animal or vegetable matter. 



* Of shell marl the same quantity exliausis sooner than clay marl (Kames). This is 

 owing chiefly to the larger proportion of hme contained in the former. 



