ACTION OF LIBIE UPON SALINE SUBSTANCES. 41.3 



the absence of air and moisture. If the lime sink, therefore, beyond 

 the constant reach of fresh air, its efficacy is in a great degree lost. 



2°. But the agency of the hght and heat of the sun, though I have 

 not hitherto insisted upon their action — are scarcely less necessary to 

 the full experience of the benefits which lime is capable of conferring. 

 The light of the sun accelerates nearly all the chemical decompositions 

 that take place in the soil — while some it appears especially to promote. 

 The warmth of the sun's rays may penetrate to some depth, but their 

 light can only act upon the immediate surface of the soil. Hence t!ie 

 skilful agriculturist will endeavor, if possible, to keep some of his lime 

 at least upon the very surface of his arable land. Perhaps this in- 

 fluence of light might even be adduced as an argument infavor of the 

 frequent application of lime in small doses, as a means of keeping a 

 portion of it always within reach of the sun's rays ; and this more es- 

 pecially on grass lands, to which no mechanical means can be applied 

 for the purpose of bringing again to the surface the lime that has sunk. 



There are, at the same time, as you will recollect, good reason also 

 why a portion of the hme should be diffused through the body of the 

 soil, both for the purpose of combining with organic acids, already 

 existing there, and with the view of acting upon certain inorganic or 

 mineral substances, which are either decidedly injurious, or by the ac- 

 tion of lime may be rendered more wholesome to vegetation. 



In order that this diffusion may be effected, and especially that lime 

 may not be unnecessarily wasted where pains are taken by mechanical 

 means to keep it near the surface, an efficient system of under-drainage 

 should be carefully kept up. Where the rains that fall are allowed 

 to flow off the surface of the land, they wash more lime away the more 

 carefully it is kept among the upper soil — but where a free outlet is af- 

 forded to the waters beneath, they carry the lime with them as they 

 sink towards the subsoil, and have been robbed again of the greater 

 part of it before they escape into the drains. Thus on drained land 

 the rains that fall aid Hme in producing its beneficial effects, while in 

 undrained land they in a greater or less degree counteract it. 



§ 32. Action of lime upon the inorganic or mineral matter of the soil. 



Though the main general agency of lime is exerted, as we have 

 seen, upon the organic matter it meets with, yet it often also produces 

 direct chemical changes upon the mineral compounds existing in the 

 soil, which are of great importance to vegetation. Thus 



1°. Lime, either in the mild or in the caustic state, possesses the 

 property of decomposing the sulphate of iron, whic> especially abounds 

 in peaty soils, and in many localities so saturates the subsoil as to 

 make it destructive to the roots of plants. Sprengel mentions a case 

 where the first year's clover always grew well, while in the second 

 year it always died away. This;, upon examination, was found to be 

 owing to the ferruginous nature of the subsoil, which caused the death 

 of the plant as soon as the roots began to penetrate it. 



When salts of iron exist in the soil, a dressing with Hme will bring 

 the land into a wholesome state without other aid. The lime will 

 combine with the acid, and form gypsum, if it is the sulphate of iron 

 that is present, while i\iQ first oxide of iron which is set free will, by 



