1917] Forbes: Irrigation Effects of Copper Compounds Upon Crops 479 



6. Conditions opposing toxicity of copper compounds are 

 the presence of copper in the form of chrysocolla and chalcocite ; 

 adsorption through contact with finely divided soil particles; 

 reactions with carbonates, silicates, and organic matter tending 

 to precipitate coj^per from its solutions; the presence of certain 

 soluble salts in the soil that overcome toxic action ; and increased 

 resistance of old plant roots. 



7. The stimulation by copper of vegetative growth in pot 

 and water cultures has been observed. Stimulated growth of 

 crops under field conditions is a possibility. 



8. Pot cultures may be used for comparative determinations 

 of toxic effects upon plants of copper in soils, if conducted under 

 rigidly uniform conditions. The copper content and the physio- 

 logical response to copper of such material will be much greater 

 than for similar cultures grown under plot or field conditions. 



9. Copper injury in field soils containing doubtfully toxic 

 amounts of copper may be diagnosed by a combination of symp- 

 toms. Facts which indicate such injury in a soil containing 0.1 

 per cent of copper (more or less) are: yellow tops (for winter 

 grains) in absence of other conditions that cause yellow tops; 

 crinkl}'^ root systems (in absence of excessive amounts of alkali 

 salts) ; and a high copper content in dry matter of root systems. 

 Combined evidence of this character, which may be observed 

 in the district studied, indicates toxic copper effects. 



10. Field observations before and following the exclusion of 

 tailings from the irrigating water-supply indicate that conditions 

 in the district studied are gradually improving, due to the culti- 

 vation of alfalfa and to the incorporation of river sediments with 

 accumulations of tailings. Noticeable toxic effects in the field 

 exist only where the roots of young, growing crops are exposed 

 to surface soils containing maximum amounts of copper. The 

 general tendency in the district is probably toward decreasing 

 rather than increasing percentages of copper in irrigated soils. 



11. Methods of analysis have been developed for the purpose 

 of determining reliably small amounts of copper in vegetative 

 material, particularly in root systems of plants grown in soils 

 containing copper. 



