428 GROWTH OF PLANTS 



and nitrogen content of the leaves of plants did not decrease significantly 

 with distances from the source of SO 2. 



Manganese injury. Certain steam-sterilized soils'*- used at the Institute 

 proved toxic to plants because of the high content of soluble manganese 

 salts. Soils steamed at 240° F (116° C) showed a manganese content rang- 

 ing from 384 to 22.9 ppm. High organic content seemed to increase the 

 amount of manganese rendered soluble. Other factors that affect the solu- 

 bility of manganese in soils were studied and the data obtained were dis- 

 cussed in relation to previous literature on the subject. 



Soybeans, snap beans, and tobacco *^ were grown in untreated soil and 

 in soil to which 400 and 600 ppm of manganese sulfate were added under 

 four different intensities of light: full sunlight, 78, 58, and 35 per cent of 

 full sunlight. In general, the injury to the leaves by manganese salts, as 

 shown by brown-spotting, chlorosis, and the amount of manganese salt 

 absorbed by the leaves, decreased as the light intensity decreased. The 

 salt decreased the yield of plants in all light intensities, except perhaps 

 tobacco under heaviest shading. The amount of manganese absorbed by 

 roots and stems was much less affected by light intensity than was that of 

 the leaves. 



Colloidal phosphate. Florida pond or colloidal phosphates^® were no 

 more effective for crop production than finely ground Tennessee brown 

 rock phosphate. 



Fungi for determining soil fertility. Mehlich ^ (joint contribution of 

 Boyce Thompson Institute and Tennessee Agricultural Experiment 

 Station) extended the earlier research by himself and associates and by 

 other investigators on the use of fungi to determine deficiency of potash, 

 nitrogen, and phosphorus in soils for the growth of crops. Use of these 

 organisms requires much less space and less time than the growth of crop 

 plants. Mehlich considers Aspergillus niger to be well adapted to deter- 

 mine potash deficiency, and within limits Cunninghamella blakesleeana 

 is of value in detecting phosphorus and nitrogen deficiencies. 



Thallium effects. It is established that use of thallium sulfate for 

 rodent control leaves patches of soil where it is applied incapable of pro- 

 ducing plants for a considerable period. McCool '^^ carried out a series of 

 researches to put the knowledge in this field on a quantitative basis. He 

 found that it was not possible to render thallium sulfate-treated soils 

 capable of growing plants even by leaching with large quantities of water, 

 partly because of the base exchange of thallium with calcium, aluminum, 

 and other bases. Commercial fertilizers and lime did not lower measurably 

 the toxicity of thallium in the soil. Crop plants were badly injured by con- 

 centrations of thallium in the soil that were too low to affect nitrification; 

 hence it is more toxic to green plants than to nitrifying microorganisms. 

 The nature of the soil determines to a degree the amount of thallium salt 

 that must be added to injure plants; sandy loam requires less than silt 

 loam and silt loam less than fibrous sedimentary peat. Very low concen- 



