SURPLUSES AND DEFICIENCIES 195 



Nitrogen. — The results of an excess of nitrogen usually appear the 

 year following the actual surplus nitrogen absorption. They are shown^*"^ 

 in trees by a tendenc}^ in the fruit to physiological decay. Dieback, 

 or exanthema, and gummosis of citrus trees also are attributed to a 

 surplus of nitrogen. '^'■'^ This causes a diseased condition in the growing 

 tissues of the tree characterized primarily by gum pockets, stained 

 terminal branches, "ammoniated" fruits, bark excrescences and multiple 

 buds. The secondary symptoms are an unusually deep green color of 

 the foliage, distorted growth of the terminal branches, frenching of the 

 foliage and thick coarse leaves shaped like those of the peach. Mineral 

 sources of nitrogen, even in great quantities, are not known to produce 

 dieback though they may accentuate the symptoms in trees already 

 affected, ^^ but organic fertilizers containing nitrogen often lead to its 

 development when they are appHed in large amounts. 



Magnesium. — The poisonous action of an excess of magnesium 

 absorbed by the plant is attended by a browning of the roots and of 

 vessels in the wood, cessation of growth in the roots and eventually death 

 of the root hairs, the entire roots and leaves. These toxic effects may be 

 counteracted in large part by calcium through its antagonistic action on 

 magnesium, previously discussed. It should be pointed out that toxic 

 effects similar to those following an excess of magnesium have been 

 observed to develop from oxalic acid and that the toxic effects of other 

 salts and salt mixtures, such as potassium nitrate with potassium 

 phosphate, may be corrected by calcium. 



Copper. — Of the effects of non-essential elements, those of copper are 

 among the most striking. Copper salts are poisonous even in exceed- 

 ingly small concentrations. Water distilled in copper receptacles is 

 frequently toxic. Coupin''^ found that the lethal dose for grains grown 

 in water culture was for each 100 cubic centimeters of nutrient solution, 

 0.0049 gram copper bromide; 0.005 copper chloride; 0.0056 copper 

 sulfate; 0.0057 copper acetate and 0.006 copper nitrate. Copper salts 

 absorbed by the roots of the grape are likely to stop root growth. On the 

 other hand, the stimulating effect of a mixture of copper sulfate and lime 

 sprayed on leaves is well known. Leaf development is stimulated, the 

 chlorophyll content increased, the palisade cells become longer and nar- 

 rower and the spongy parenchyma has smaller intercellular spaces.^" 

 Without doubt the amounts of copper absorbed by the sprayed leaves are 

 less than those which produce toxic effects when absorbed by the roots. 

 Ewert,^^ however, has demonstrated that concentrations of 1 to 100,000,- 

 000 of copper sulfate are toxic to the pulp cells of the apple and that 

 minute quantities entering through the stomata after spraying or taken 

 up by the roots may result in one of the forms of bitter pit. If the con- 

 tention^^^ that copper is an essential nutrient be correct, then the observed 

 effects may be the result of counteracting a limiting factor of assimilation. 



