PRELIMINARY AND MINOR PAPERS 



DECOMPOSITION OF SOIL CARBONATES 



By W. H. MacIntire, 

 Soil Chemist, Tennessee Agricultural Experiment Station 



Investigations recently conducted at the University of Tennessee 

 Agricultural Experiment Station have led to the discovery that the 

 composition of soils is such as to make inhibitory any long-continued 

 occurrence therein of magnesium carbonate, and the conclusion has been 

 drawn that magnesium carbonate does not exist as a solid mineral in 

 our humid soils. 



The research has demonstrated that the affinity of magnesia for silica 

 s such that soils long since alkaline from excessive treatments of calcium 

 carbonate are able to dissipate the CO, of magnesium carbonate under 

 sterile, moist conditions. It has been further demonstrated that the 

 affinity of magnesia for silica is so great that precipitated magnesium 

 carbonate is extensively decomposed by pure SiO, and also by the closely 

 allied compound titanium oxid, which occurs almost universally to an 

 appreciable extent in soils. 



Analyses to determine residual carbonates at the end of one year 

 established the fact that precipitated magnesium carbonate in amounts 

 chemically equivalent to 16,070 pounds of CaCOa per acre in excess of 

 the quantity indicated by the Veitch method as necessary to correct 

 acidity had been entirely decomposed by each of three distinct types of 

 unleached soil. A loam, a sandy loam, and a silty loam were used in the 

 study. In substantiating the work the loam soil was subjected to eight 

 check treatments in field rim experiments. In each of these eight 

 instances precipitated MgCOj equivalent to over 1 5 tons of a good grade 

 of limestone per 2,000,000 pounds of soil had entirely disappeared at the 

 end of eight weeks, when the first analyses were made for residual car- 

 bonates. No drainage took place during this 8-week interval. 



Comparisons between residual carbonates from limestone and dolo- 

 mite treatments showed at the end of 9 months' exposure to weather 

 22,000 pounds of CaC03 per 2,000,000 pounds of soil for the limestone 

 treatment as compared to 1 1 ,000 pounds of CaCOj as a residue from the 

 dolomite. 



The investigations have also determined that the absence of carbo- 

 nates subsequent to applications of magnesium oxid has been erro- 

 neously attributed to persistent causticity of the magnesia, which is 

 shown to be very readily converted to the carbonate, while this in turn 

 is decomposed by siliceous substances. 



The use of CaCOj as a check has shown that the affinity of lime for 

 silica is far greater than has been supposed, and the work has demon- 

 strated that the lime-silica reaction in soils is an important factor in the 

 consen-ation of lime applied in practice. While the lime-silica reaction 



Journal of Aericultural Research. Vol. III. No. i 



Dept. of Agriculture. Washington, D. C. Oct. 15. 1914 



Teen.— I 



(79) 



