308 CITRUS FRUITS AND THEIR CULTURE. 



cent. ; castor pomace, 6 per cent. ; and tankage, 6 per cent. 

 The last three also contain some phosphoric acid. These 

 six substances may be divided into two groups, the first 

 two being designated as chemical sources of nitrogen, 

 the last four as organic sources, vegetable or animal. 



In Florida the use of the chemical sources is always 

 advised, owing to the fact that applications of cotton- 

 seed meal and blood and other organic sources, or of 

 mixed fertilizers containing these materials, have been 

 followed by die-back. This has happened so repeatedly 

 that any organic source of nitrogen is always, and justly, 

 regarded with suspicion. Their use is always attended 

 with considerable risk and it too frequently happens that 

 all the growth added may be lost owing to the disturb 

 ance of the functions of the tree. In sections where die- 

 back does not follow their use, organic sources are good, 

 if used with discretion, but heavy applications may im- 

 pair the quality of the fruit. 



The nitrogen in nitrate of soda is available as soon 

 as it is dissolved in the soil. Most of the nitrogen taken 

 up by plants is in the form of nitrate, which explains 

 why the trees respond so readily to applications of ni- 

 trate of soda. It must be used with discretion, else a 

 considerable part may be leached from the soil, and it 

 is preferably applied in a number of separate dressings 

 of two or three hundred pounds per acre. 



Sulphate of ammonia is of such physical character 

 as to be readily distributed through the soil. The form 

 of the nitrogen has to be changed before it can be used 

 by plants, hence it does not act so quickly as nitrate of 

 soda. It is an excellent form of nitrogen to use on citrus 

 groves, particularly among bearing trees. Lime should 



