26 



BULLETIN 3 3, POETO EICO EXPEEIMEjSTT STATIOX 



other plants, a cover crop may profitably be planted at the time 

 cultivation ceases. 



In regard to the kind and amount of fertilizer to be used, the fol- 

 lowing suggestions may be helpful : Suppose a planter has found 

 by experience that it pays him to apply, say, 60 pounds per tree 

 annually of a 6-8-10 mixture (3.6 pounds nitrogen, 4.8 pounds phos- 

 phoric acid, and 6 pounds potash),. Rather than divide the mixture 

 into three equal parts, let him use $0 pounds of a 3-2-2, or 10 pounds 

 I of a 6 4 1 mixture in December, 20 pounds of a 2-3-3, or 10 pounds 

 of a 4^6-6 mixture in March, and 20 pounds of a 1-3-5, or 10 pounds 

 of a 2-6-10 mixture in June. The reason for this suggestion is 

 obvious. The aim is to have the fruit mature early and the tree 

 attain as high a stage of dormancy as possible by early December; 

 hence the minimum amount of nitrogen in the June application and 

 cessation of cultivation during the summer. In December the aim 

 is to break the dormancy of the trees as quickly as possible; hence 

 the large amount of nitrogen in the application, f ollowed by intensive 

 cultivation. 



The following observation will serve to emphasize some of the 

 problems in citrus fertilization. It was found during the past Tear 

 that small trees in 25-gallon containers continued growth even when 

 the soil nitrogen content was very low. It was also observed that 

 wild trees produce considerable growth and fruit in soil contain- 

 ing only small amounts of plant nutrients. This indicates that 

 the citrus tree can supply its needs from very weak soil solutions. 

 Field experience, on the other hand, seems to show that for quantity 

 production and early maturity liberal amounts of fertilizers are 

 necessary. 



In regard to methods of application, most growers apply the fer- 

 tilizers broadcast in groves 6 years old or more. Some claim to 

 get better results by applying it in a narrow band around the tree, 

 about as far from the trunk as the spread of the branches. Others 

 who are now irrigating apply the fertilizers entirely under the tree, 

 in a space radiating from the trunk to the tips of the branches. 

 The results obtained from these different methods will soon show 

 which is the superior one. 



In regard to the kind ana amount of ingredients in a fertilizer 

 mixture, variations might be made to meet soil requirements. In 

 very light sandy soils, for instance, there is loss by leaching of some 

 nitrogen and perhaps some potash, but probably not much phosphate. 

 A great deal of phosphate may be found in the upper few inches 

 of such soils at any time during the year. An accumulation of 

 phosphate under such conditions would seem to justify the use of 

 fertilizers containing decreased amounts of the ingredient after a 

 few years. 



In clay soils both phosphate and potash may accumulate, but the 

 phosphate is often present in a form that is not available to the 

 tree. Whenever leguminous cover crops are used as a source of 

 nitrogen the content of this ingredient in fertilizers may usually 

 be diminished. The amount to be applied can be partly determined 

 by the appearance of the tree. Applied in excess of needs, nitrogen 

 will cause the rind of the fruit to be coarse and thick. 



