120 FUNDAMENTALS OF FRUIT PRODUCTION 



Table 14 presents data showing the toxic limits of citrus seedlings 

 for various nitrate salts and for ammonium sulphate and the toxic limits 

 for these salts in the presence of lime. Their lesson in connection with 

 the use of commerical fertilizers in the orchard is well summarized in the 

 words of Breazeale;^^ 



" 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 concen- 

 tration 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 — that is 

 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 from 1,400 to 2,000 parts per 

 million — that is, it would approach the toxic limit. The surface crusts in citrus 

 groves are often highly toxic to citrus seedlings." 



Table 14. — Toxic Li.mits of Nitrates and Ammonium Sulphate p^or Citrus 



Seedlings^* 



Toxic Limit 

 Salt Parts per Million 



Sodium nitrate 1 , 800 



Potassium nitrate 3 , 500 



Calcium nitrate 10,000 



Ammonium sulphate 1 , 000 



Sodium nitrate and calcium carbonate (solid phase) 6,000 



Ammonium sulphate and calcium carbonate* (solid phase) 2,000 



Some Effects of Soil Alkali. — The effects of excessive concentration 

 produced by "alkali" on citrus trees are described by Kelly and 

 Thomas. 10^ 



"Different varieties and species of citrus trees are affected differently by 

 alkali. Lemon trees show the effects by a pronounced yellowing of the margins 

 and burning of the tips of the leaves, followed by unusually heavy shedding of the 

 leaves in the latter part of the winter and spring. The subsequent new growth 

 may appear to be quite normal and vigorous for several months, but later a large 

 portion of the leaves turn yellow in irregularly shaped areas around the margins 

 and fall excessively. In the presence of excessive concentrations of salts, espe- 

 cially chlorides, complete defoliation may take place. Mottle leaf frequently 

 occurs, and sometimes chlorosis. Both the quality and quantity of the fruit are 

 impaired. 



"It has been found that orange trees affected by alkaU are unusually sus- 

 ceptible to injury from adverse cUmatic conditions. Hot winds burn the young 

 leaves and frosts produce more serious injury than with normal trees. Alkali 



