274 Journal of Agricultural Research voi. xvni. no. s 



calcium acrbonate when the latter salt is present in the solid phase, the 

 calcium-ion concentration is considerably increased above that of a sat- 

 urated solution of calcium carbonate alone. This increase in the con- 

 centration of the calcium ion, which gives the protective action, is 

 probably the explanation of the failure of calcium chlorid and calcium 

 sulphate to exert a protective action at concentrations corresponding to 

 the solubility of calcium carbonate alone. Calcium nitrate and calcium 

 hydrate in small amounts were also as eff'^ctive as calcium carbonate. 

 Organic matter had no ameliorating effect on the toxic action of sodium 



nitrate. 



SUMMARY 



Seedlings of various citrus stocks, including lemon, grapefruit, and 

 several varieties of sweet oranges, showed no characteristic differences in 

 response in water cultures or in resistance to toxic solutions. 



Very dilute organic extracts from upland peat (lo parts per million or 

 more) produced a marked stimulation of the root growth of citrus seed- 

 lings. Corresponding concentrations of sodium nitrate or potassium 

 chlorid did not stimulate the root development. 



Calcium carbonate stimulated the root growth and exerted a pro- 

 nounced antagonistic action to toxic solutions of nitrates and ammonium 

 sulphate. 



Peat extract in very dilute concentrations (20 parts per million) and 

 calcium carbonate (solid phase present) both protected citrus seedlings 

 to a marked degree against the toxins of distilled water. 



The tolerance of citrus seedlings for alkaline salts is relatively high. 

 The toxic limit for calcium hydrate was 100 to 120 parts per million, for 

 sodium hydrate 250 to 300 parts per million, and for sodium carbonate 

 550 to 600 parts per million. The hydroxy! concentration in the toxic 

 calcium hydrate solution is only about one-third that of the toxic sodium 

 hydrate solution. 



When soluble organic matter which is acid in reaction and stimulating 

 to citrus seedlings in concentrations up to 1,000 parts per million or 

 more is added to a sodium carbonate solution of 400 parts per million 

 which in itself is not toxic, a highly toxic solution is formed which will kill 

 the root tips of citrus seedlings. This reaction appears to be of importance 

 in connection with the toxicity of soils containing small amounts of 

 sodium carbonate. 



