Cultural Experiments 191 



ley, wheat and oats were collected in sets of two in a place. One 

 of each set was green, healthy growth, the other more or less 

 yellow and unthrifty in appearance. The object of this method 

 of sampling in soils found to contain small amounts of copper 

 was, if possible, to relate unthrifty appearance of plants exam- 

 ined to copper found in roots and surrounding soil. Table 26 

 (p. 441) contains the results of the determinations made. 



As may be expected under field conditions, which are more 

 complex and variable than those of plot or pot cultures, these 

 data are considerably contradictory. Roots of yellow barley, 

 wheat and oat plants, for instance, in 5544a and 5548a contain 

 less copper than roots of strong green plants grown alongside ; 

 although the average copper content (314 parts) of yellow and 

 more or less unthrifty plants is seen to be greater than in green 

 plants alongside (236 parts). So far as observed, the larger 

 percentages of copper found in soils shaken from roots of the 

 plants are always associated with yellow plants. The average 

 copper content of soils from roots of yellow plants is 0.048 per 

 cent, while that from green plants is 0.023 per cent. These 

 observations indicate that in a general way the larger amounts 

 of copper found in these field soils are associated with larger 

 amounts of copper in root systems and with yellow color in young 

 plants. The percentages of copper observed in the soil, ranging 

 up to 0.073 per cent in one instance, is surprisingly high, but 

 toxic effects must be qualified by the character of the compounds, 

 soluble salts in tlie soil, and other factors noted on preceding 

 pages. 



Yellowness of foliage also may be due to other causes than 

 copper. Among these are: (1) too much water, as in low places; 

 (2) alkali accumulations; (3) cold weather; (4) too much 

 nitrogen in improper form, as in some old barnyards; (5) too 

 little available nitrogen, as on new ground; (6) shade, and (7) 

 insect pests and plant diseases. Malnutrition from any cause, 

 in fact, usually expresses itself in the yellow or striped appear- 

 ance of the leaves of these crop plants. Such appearance, there- 

 fore, cannot be attributed to copper present in the soil, without 

 exclusion of other causes and sufficient confirmatory evidence. 



As in the case of plot and pot cultures, corn roots are ob- 



