136 BIOLOGICAL CONDITIONS OF THE SOIL 



that absorption of the sodium carbonate by the organic 

 matter of the soil plays a considerable part in these ex- 

 periments, as the salts were added to the soil, and, as 

 mentioned in Chapter V, loam soils, especially those high 

 in organic matter, do not hold in solution all of the sodium 

 carbonate added. 



The various experiments agree pretty well that about 

 1000 parts per million of sodium chloride is a toxic quantity. 

 Greaves, Carter, and Goldthorpe (6) found a stimulation 

 with this salt up to a concentration of about 1000 parts 

 per million above which there was a marked toxicity 

 Other investigators have found stimulation where the 

 quantities of sodium chloride were lower than this. 



From the available experiments, the toxic limits of 

 sodium sulphate appear to lie between 2500 and 5000 

 parts per million. Small quantities of this salt were found 

 to be stimulating to nitrifying bacteria by Brown and 

 Hitchcock (2), but Greaves and his associates (6) found 

 no stimulation even in soils containing very small quanti- 

 ties of sodium sulphate. 



Greaves found the toxic limits for sodium nitrate to be 

 only a little greater than 200 parts per million, or much 

 more toxic in comparison with its toxicity to wheat than 

 are the other sodium salts. The quantities of sodium 

 carbonate, sodium chloride, and sodium sulphate present 

 in soils producing half the quantity of dry matter of normal 

 wheat plants and those in soils producing half-normal 

 nitrification were found to be nearly the same. The 

 salts which stimulated wheat most also stimulated nitri- 

 fying bacteria. 



From the low quantities of sodium carbonate and sodium 

 nitrate which cause injury to nitrifying bacteria, it appears 

 that the puddling effect of these salts may play an im- 



