1905.] 



On the Retention of Bases by the Soil. 



27 



root which aids in bringing about the solution of nutrient materials in the 

 soil,* but no evidence could be found that anything except carbon dioxide 

 passed from the roots into the culture solution. After growth a considerable 

 bulk of the culture solution was evaporated to dryness and gently heated, 

 very slight signs of charring were observed, no more than could be attributed 

 to the dust, and the residue showed the same alkalinity before and after 

 ignition, as would not be the case had any organic acid excreted from the 

 root been present. On one occasion freshly precipitated ferric hydrate was 

 introduced into the solution as a source of iron; although this was distributed 

 all over the young growing roots so that it could not be shaken off, the plants 

 began to suffer from lack of iron, and continued to do so until a trace of 

 ferric chloride was introduced. Despite the well known acidity of the root- 

 sap there seems no evidence that in normal cases it ever passes outside the 

 cell wall, as long as the roots are unbroken. 



These experiments then afford experimental justification for looking to the 

 growth of the plant as an explanation of some of the difficulties raised by the 

 rate of loss of calcium carbonate on the different plots. The analyses of 

 crops already quoted serve to show that the return of base to the soil may be 

 large, quite sufficient to make up for the calcium carbonate required each 

 year for nitrification. Hence soils which start with very small proportions of 

 calcium carbonate may yet preserve their healthy condition and permit of 

 nitrification, the losses caused by which are thus repaired. 



Again it becomes intelligible that the use of ammonium salts as a manure 

 only occasions the loss of one molecule of calcium carbonate for each 

 two molecules of ammonia, since the second molecule required for nitrification 

 will be more or less restored during the growth of the plant. It has already 

 been shown that the actual loss of calcium carbonate to the soil caused by 

 the use of 200 lbs. of ammonium salts approximates to 161 lbs., and not the 

 322 lbs. which would be required if the calcium nitrate produced by nitrifica- 

 tion were wholly removed from the soil. 



Further, when nitrate of soda is used as a manure, from the neutral 

 solution in the soil of calcium or sodium nitrate an excess of acid will be 

 taken by the crop, leaving the soil richer in base. Hence the conservative 

 action of sodium nitrate on the calcium carbonate of the soil that is visible 

 in the analyses of both Broadbalk and Hoos Fields. It is possible to 

 calculate the amount of base restored to the plots receiving nitrate of soda 

 on the assumption that they possess the same average composition as 

 the wheat and barley in Table XII, and that the amount of base returned 



* See Czapek, 1 Pringsheim's Jahr. f. wiss. Botanik,' vol. 29, 1896, p. 321 ; Kossowitsch, 

 ' Ann. de la Science Agronomique,' 2nd Series, vol. 1, 1903, p. 220. 



