68o Journal of Agricultural Research vot. xii. no. w 



less active than elsewhere, the data show that the small amounts of 

 oxidation products formed dissolved tricalcium phosphate. On the 

 other hand, wherever calcium carbonate was also present, the oxidation 

 products not only combined with it, but the initial solubility of the 

 tricalcium phosphate was very materially lowered as well. This was 

 true in the nitrification of both ammonium sulphate and dried blood.^ 



INTERPRETATION OF RESULTS 



In making a practical interpretation of these investigations it should be 

 borne in mind that fertile soils commonly contain at least small amounts 

 of carbonate, that even the so-called acid soils frequently contain con- 

 siderable amounts of bicarbonate, and that the presence of calcium 

 carbonate in soils is generally considered to promote conditions that are 

 favorable for the growth of most crops. Furthermore, large amounts 

 of calcium carbonate are being applied to soils in many localities, 

 especially in the humid sections. In the Central West, for example, 

 ground limestone is being applied on a large scale, and generally it is 

 recommended that the application be repeated every few years. 



Under the conditions that result, chemical reasoning (j) and the experi- 

 mental results reported above agree in suggesting that the action of the 

 biochemical oxidation products, formed in the nitrification of organic 

 nitrogen, would be spent on the carbonate and not on tricalcium phos- 

 phate. Furthermore, it seems doubtful whether this could be avoided 

 by the application of limestone and rock phosphate at different times 

 in a rotation, as was suggested by Hopkins and Whiting. Although 

 it is possible that under this condition the particles of rock phosphate 

 may chance to occur in local centers that are somewhat removed from 

 solid particles of calcium carbonate, and nitrification may happen to 

 take place in these centers, it does not even then necessarily follow 

 that the phosphate would be dissolved. For such centers would probably 

 always be in contact with soil silicates, and the above data indicate 

 that at least some soil silicates may be attacked by the products of 

 nitrification in preference to tricalcium phosphate. When the con- 

 ditions permit the accumulation of considerable acidity, however, such 

 as probably obtain when ammonium sulphate is applied to a soU low 

 in carbonate, it was found that small amounts of the phosphate were 

 dissolved. But then a degree of acidity that is distinctly injurious to 

 crops may soon develop, as has been found by field trials in a number 

 of localities. Furthermore, bicarbonate, arising from the action of 

 carbonic acid on the solid particles of calcium carbonate, would certainly 

 tend to diffuse toward the supposed centers of acidity, thus precipitating 



• Soluble phosphoric acid may have been utilized to some extent in the life processes of the bacteria 

 present, as was found by Tottingham and Hofimann (,8). But the simultaneous losses m soluble calcium 

 also suggest the precipitation of phosphoric acid, a view that is in harmony with the well-known fact that 

 calcium carbonate may precipitate phosphoric acid irom solution. 



