'5446, 447, 448, 449] effects of marls. 231 



during the brief period in which we can observe its action, it is clear that in the 

 case of marls, where lime has been in action from times immemorial, the greater 

 part of the nutritive ingredients must be at once in an available condition. And 

 hence it is, that the comparatively small amount of potash and other useful 

 ingredients which, besides the lime, a dressing of marl introduces into our soil, 

 exercises such a powerful effect ; for the greater part of them is at once ready 

 to serve as food for the vegetahle economy, as in the case of stable manure. 



446. Limited Duration of the Effects of Marling.— This consideration leads 

 us to understand also, the cause of a phenomenon which has caused many 

 discussions, and fruitless attempts at exp anation, so long as marls were consid- 

 ered as owing their efficacy merely to the lime they contain. It was found that 

 the period during which the effects of a dressing of marl continued, was much 

 more limited than the slight waste of lime from the soil would justify ; analysis, 

 moreover, proved an abundant supply of lime still to be present in the soil; 

 and nevertheless it appeared to have lost its efficacy. 



A higher degree of accuracy in chemical analvsis, which we have since 

 attained, has enabled us to detect in marls formerly regarded as purely limy, 

 others of the nutritive elemenis of plants, the presence of which had before 

 been overlooked. Their quantities, it is true, are sometimes extremely minute ; 

 but not more so than arc the respective amounts of the ingredients withdrawn 

 from our fields by crops, when divided out over the whole surface and mass of 

 the soil ; or those" which, when furnished to the soil in the form of stable manure, 

 are well known to be eminently effective, because they are in a soluble condition. 



If a dressing of 200 bushels per acre, of marl containing 40 per cent of lime, 

 and }i of a per cent of potash, and the same amount of phosphoric acid, ceases 

 to be effective, (and therefore requires to be repeated) in the course of ten years, 

 as might be the case in practice ; it is very plain that its want of efficacy cannot 

 be attributed to the slight diminution of the lime introduced, by the small 

 amount which our crops (cotton or corn) have withdrawn. Not so, however, 

 with respect to potash and phosphoric acid ; for in case of heavy cropping, the 

 amount of these substances contained in the crops would form a large percentage 

 of the whole quantity introduced in the Kiarl, quite sufficient to explain the in- 

 efficacy of the latter beyond the period stated. 



447. Cases in point have been very commonly observed in the "Northeastern 

 Prairie Region" of our State. Some of the prairie soils in cultivation contain 

 from 5 to 1°5 per cent, of carbonate of lime ; consequently they are not deficient 

 in that substance. Yet it is found, that a dressing of the bald prairie marl, or 

 Rotten Limestone, sensibly increases their productive capacity for several years 

 thereafter— obviously not on account of the increase of the (already very large) 

 supply of lime in the soil. The analysis (IT 149) teaches us that the Rotten 

 Limestone contains, not only Carbonate of Lime, but also, one quarter of a per 

 cen*. '0.248) of Potash, besides other important ingredients ; which circumstance 

 explains, very simply, the cause of the phenomenon observed. 



448. Classification of Marls. — Geologically, the calcareous marls of Missis- 

 sippi occupy, in the main, four different positions, viz : 



1. Cretaceous marls, of the northeastern portion of the State, including the 

 greater portion of the counties o! Tippah, Tishomingo, Pontotoc. Itasvamba, 

 Chickasaw, Monroe, Ocktibbeha, Lowndes, Noxubee and Kemper (H 140 to 146). 

 There is a great number of varieties of these marls, but they may on the whole 

 be distinguished into three classes, namely : 



449. a. Oreensand marls of the upper cretaceous formation or Ripley Group, 

 (128, ff.), occurring in Tippah, Pontotoc and N. Chickasaw, between the prairies 

 proper and the Post Oak Flatwoods (U 143 to 146). They are generally of a sandy 

 character, sometimes largely mixed with scales of mica (popularly termed 

 isinglass), and of a bluish or greenish-gray tint. The earthy and loose varieties, 

 which are easily worked with the spade or mattock, contain from 10 to 20, oa 

 an average, 15 per cent, of carbonate of lime, and form one half to one per cent. 



