in the Strata of the Earth. 



525 



The efficacy of this manure may in part be due to the decompo- 

 sition of the phosphates contained by the vegetable rubbish, and 

 by the weeds and roots in the soil of the field, during two or three 

 weeks' contact with quicklime.* 



The practice of manuring with quicklime for a crop of wheat, 

 though not exclusively limited to the red marl districts, pervades 

 all those parts of England in which this kind of red soil prevails ; 

 and as Dr. Lyon Playfair has found phosphate of lime in very 

 many samples of this marl, it may exist more or less through its 

 whole extent. 



Before the introduction of lime as a manure, more than one hun- 

 dred years ago, the preparation for wheat was a coat of red marl, the 

 decomposition of which, by the winter's atmospheric action, must 

 have set free many of its phosphoric and other ingredients. 

 Quicklime produces a similar effect more rapidly. By the first 

 rain that falls upon it, lime-water is formed ; this lime-water (by 

 its affinity for carbonic acid) disengages from the carbonate of 

 lime within the marl, enough cabonic acid to make it dissolve 

 further quantities of carbonate of lime, and also of phosphate of 

 lime, from the phosphoriferous marl, in a state fit to be absorbed 

 by the next crop of corn. 



It has been stated that the quicklime sets free carbonic acid 

 from all the vegetable matter in the soil to which it is applied, 

 and that this acid decomposes and renders soluble not only the 

 phosphoric and numerous other compounds of vegetable matter 

 present in the clods (viz. old roots and stems and dead leaves), 

 but also any mineral phosphoric compounds that may be present 

 in the marl. The practice of laying marl on land under prepara- 

 tion for wheat, was in use a century and a half ago in Devonshire, 

 and through the Midland districts, extending thence N.E. through 

 Worcestershire and Staffordshire, into the south of Derbyshire, 

 and along the valley of the Trent and vale of York, to the mouth 

 of the Tees. Fields on the red marl through this district are 

 full of old deep marl-pits, that were abandoned as soon as the 

 cessation of the use of marl followed the introduction of lime, 

 which was found to be a more quickly acting and more efficient 

 substitute. 



* The accepted explanation of the use of lime has been, that it. hastens the decom- 

 position of all animal and vegetable matter in the soil, and reduces it to a state soluble 

 in water, and fit to be absorbed by the new crop. Now as one of the essential elements 

 of this crop is phosphorus, should any fixed phosphates be present in the soil or subsoil, 

 they also would be decomposed in consequence of the lime setting free carbonic acid 

 from fragments of dead plants present in the soil ; this acid (being absorbed by rain 

 water) enables it to dissolve both carbonate and phosphate of lime from any fixed 

 carbonate or phosphate of lime contained in the marl. 



