272 Journal of Agricultural Research voi. xviii, no. $ 



TOXIC LIMITS OF ACIDS 



The toxic properties of only one inorganic acid, phosphoric acid, were 

 investigated. This acid stopped the root development at a concentration 

 of 20 parts per million, with both the grapefruit and the lemon. Attention 

 has already been called to the fact that active root elongation takes place 

 in organic extracts from peat having an acidity equivalent to 80 parts 

 per million of hydrochloric acid. In overripe lemons the seeds are likely 

 to germinate inside the fruit and the plumule and radical be pushed 

 through the skin. Since the juice of the lemon contains 7 or 8 per cent 

 citric acid, it appears that under natural conditions the lemon seeds will 

 sprout in a strongly acid medium. 



TOXIC LIMITS OF NITRATES AND OF AMMONIUM SULPHATE 



It is known that the continued use of sodium nitrate in relatively large 

 amounts tends to produce mottling of the leaves of citrus tress. Culture 

 tests were accordingly made to determine the toxicity of the nitrate salts 

 together with that of ammonium sulphate, which is also used in citrus 

 districts as a source of nitrogen. The results are given in the following 

 table, with the toxic limits of the same salts in the presence of lime. 



Table IV. — Toxic limits of nitrates and ammonium sulphate for citrus seedlings 



Salt. 



Sodium nitrate 



Potassium nitrate 



Calcium nitrate 



Ammonium sulphate 



vSodium nitrate and calcium carbonate (solid phase) 



Ammonium sulphate and calcium carbonate (solid phase). 



Toxic limit. 



. p. m. 

 1,800 



3,500 

 10, 000 

 1,000 

 6,000 

 2,000 



It will be seen that marked differences occur in the toxic limits of 

 the various salts, sodium nitrate being five times as toxic as calcium 

 nitrate. The toxic limits for this group of salts are so high that the matter 

 may appear to be of no practical import. But a simple calculation will 

 show that the surface feeding roots of citrus trees are at times subjected 

 to fertilizer concentrations in field practice so great as to approach toxic 

 conditions. Application of 2 to 3 pounds of nitrate of soda per tree, or 

 200 to 300 pounds per acre, which is not an unusual practice for some 

 citrus growers, would correspond approximately to a concentration of 70 

 to 100 parts per million in the soil of the surface foot. The fertilizer, 

 moreover, is ordinarily applied to the open ground between the tree rows — 

 thz,t is, to not more than one-half the total soil area. If the moisture 

 content of the soil were reduced to 10 per cent of the weight of the soil, 

 the concentration of the sodium nitrate in the soil solution would range 



