THE MONTHLY BULLETIN. 229 



cause remains to be determined. The fact that we do not know the 

 cause of the disease renders the results shown hy th&se fertilizer experi- 

 ments of exceptional interest. An estimate of the percentage of leaves 

 showing mottling is given for each plat in Table II for the year 1914 

 in column seven ; while in column eight is given an estimate of the total 

 leaf surface affected. The estimate of the surface affected Avould 

 appear to be the most important data to us. Certain plats are almost 

 free from mottle leaf, while others immediately beside them may be 

 badly affected. The least mottling, 1 per cent of the total surface, is 

 found on plats (manure and raw phosphate rock) and S (blood). 

 The greatest amount of mottling, 50 per cent, is on H. the nitrate of 

 soda plat. All of the plats that received nitrate of soda. A, G, H, L 

 and Q, show a high per cent of mottling except Q. Plat H is in serious 

 condition and will certainly fall very low in comparative yield in the 

 next few years, and the same will apparently be true of all nitrate of 

 soda plats except Q. It has been suggested that mottle leaf might 

 possibly be caused directly or indirectly by alkali in the soil and that 

 nitrate of soda added as a fertili;^er in western soils might in breaking 

 down form sodium carbonate, which possibly would account for the 

 increased mottle leaf on nitrate of soda plats. This hypothesis, how- 

 ever, can not be considered seriously without further and definite proof. 

 The fact that all the plats receiving nitrate of soda, excepting Q, 

 show such a high percentage of mottling seems to place consideral)le 

 suspicion on nitrate of soda as a nitrogen fertilizer for citrus trees. 

 The results, however, can not be taken as final nor as applying gener- 

 ally. Neither is it safe at the present time to venture any explanation 

 of this fact. jMuch further work on this subject will be necessary 

 before a satisfactory explanation of the mottling cpiestion can be given. 

 Summarizing, the results up to the present time indicate very 

 strongly the importance of nitrogen fertilization and the desirability 

 of using nitrogen from organic sources. Phosphoric acid is apparently 

 necessary, but it is doubtful whether such large quantities as ordinarily 

 used are recpiired. The results suggest that a medium amount may be 

 desirable. Potash has given no appreciable effect up to the present 

 time and apparently may be safely withheld in the fertilization of 

 young groves. These conclusions, it must be clearly recognized, apply 

 only to soils such as those on which the experiments have been con- 

 ducted. The experiments, furthermore, must run for a much longer 

 period before the final results can be determined. 



From the results given above, obtained in connection with a series 

 of fertilizer experiments at the Citrus Experiment Station, growers 

 might be inclined to consider that the .station recommended those ele- 

 ments or combination of elements giving the best results as the best 

 fertilizers to use generally in citrus groves. I do not feel, however, 

 that such a conclusion would be safe. It would seem clear, for instance, 

 that dried blood is a very excellent source of nitrogen and possibly the 

 best source used in this series of tests ; yet many materials from which 

 nitrogen might be obtained were not tried in the experiments. Again, 

 if all citrus growers were to use dried blood as a source of nitrogen, 

 there might not be sufficient of this material available to meet the 

 demand. It is quite probable that other .sources of nitrogen, such as 



