58 CALCAREOUS MANURES— THEORY. 



Chemistry has taught that silicious earth, in any state of division, attracts 

 but slightly, if at all, any of the parts of putrescent animal and vegetable 

 matters.* But even if any slight attraction really exists when this earth is 

 minutely divided for experiment in the laboratory of the chemist, it cannot 

 be exerted by silicious sand in the usual form in which nature gives it to 

 soils ; that is, in particles comparatively coarse, loose, and open, and yet 

 each particle impenetrable to any liquid, or gaseous fluid, that might be 

 passing through the vacancies. Hence, silicious earth can have no power, 

 chemical or mechanical, either to attract enriching manures, or to preserve 

 them when actually placed in contact and intermixed with them ; and soils 

 in which the qualities of this earth greatly predominate, must give out freely 

 all enriching matters which they may have received, not only to a growing 

 crop, but to the sun, air, and water, so as soon to lose the whole. No 

 portion of putrescent matter can remain longer than the completion of its 

 decomposition; and if not arrested during this process, by the roots of 

 living plants, all will escape in the form of gas (the latest products of 

 decomposition,) into the air, without leaving a trace of lasting improve- 

 ment. With a knowledge of these properties, we need not resort to the 

 common opinion that manure sinks through sandy soils, to account for its 

 rapid and total disappearance.! 



Aluminous earth, by its closeness, mechanically excludes those agents of 

 decomposition, heat, air and moisture, which sand so freely admits ; and 

 therefore clay soils, in which this earth predominates, give out manure much 

 more slowly than sand, whether for waste or for use. The practical effect 

 of this is universally understood — that clay soils retain manure much longer 

 than sand, but require much heavier applications to show as much effect 

 early, or at once. But as this means of retaining manure is altogether 

 mechanical, it serves only to delay both its use and its waste. Aluminous 

 earth also exerts some chemical power in attracting and combining with 

 putrescent manures, but too weakly to enable a clay soil to become rich by 

 natural means. For though clays are able to exert more force than sand 

 in holding manures, their closeness also acts to deny admittance beneath 



* Davy's Agr. Chem. page 129. 



t Except the very small proportions of earthy, saline and metallic matters that may 

 be in animal and vegetable manures, the whole balance of their bulk (and the vvliole of 

 whatever can feed plants,) is composed of different elements which are known only in 

 the forms of gases — into which manures must be finally resolved, after goinj; through all 

 the various stages of fermentation and decomposition. So far from sinking in the earth, 

 these final results could not be possibly confined there, but must escape into the atmo- 

 sphere as soon as they take a gaseous form, unless immediately taken up by the organs 

 of growing plants. It is probable that but a small portion of any dressina; of manure 

 remains long enough in the soil to make this final change ; and that nearly all of it is 

 used by growing plants, during previous changes, or carried off by air and water. During 

 the progress of the many clianges caused by fermentation and decomposition, every 

 soluble product may certainly sink as low as the rains penetrate ; but it cannot descend 

 lower than the water, and that, together with the soluble manure, will be again drawn 

 up by the roots of plants. One exception, however, seems probable. Should the soil 

 need draining, to take off water passing beneath the surface, the soluble manure may be 

 carried off by those springs ; and this supposed result receives strong confirmation from 

 the complete loss of fertility which is often observed in spots over sub-soil that is 

 oozy in wet seasons, but which have been kept under tillage, without being drained. 

 We are as yet but little informed as to the particular changes made, and the various 

 new substances successively formed, and then decomposed, during the whole duration 

 of putrescent manures in the soil — and no field for discovery would better reward the 

 investigations of the agricultural chemist. For want of this knowledge we proceed at 

 random in using manures, instead of being enabled to conform to any rule founded on 

 scientific principles ; nor can we hope, without such knowledge, so to manage manures 

 with regard to their fermentation, the time and manner of application, mixing with 

 other substances, &l.c., as to enable the crops to seize every enriching result as soon as it 

 is produced, and to postpone as long as possible the final results of decomposition — which 

 ought to be the ends sought in every application of putrescent manure. 



