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SCIENCE 



[N. S. Vol. XXXIX. No. 1011 



the scientific work would not len^hen tlie 

 voyage more than a week. 



The ships sent by the North Sea countries 

 and by Russia, coming through the English 

 channel, could study a section of the Atlantic 

 north of the Plateau of the Azores. The open- 

 ing of the Panama Canal, which should stand 

 for universal traffic, would likewise form an 

 epoch in the study of the sea, and introduce a 

 future of international cooperation in the 

 scientific activity of nations. 



If this opportunity is neglected, it is not 

 likely soon to come again. Experience has 

 taught the difficulties involved in setting in 

 motion an international undertaking of such 

 dimensions; and the opinion is expressed that 

 the time remaining would just about suffice 

 for the diplomatic, scientific and technical 

 preparations. 



In preparing the above synopsis the words 

 of the authors are frequently used. 



H. C. Jones 



SPECIAL ARTICLES 



THE POOR NITRIFYING POWER OF SOILS A POSSIBLE 



CAUSE OF " DIE-BACK " (eXANTHEMa) 



IN LEMONS 



The disease known as " die-back " in citrus 

 trees has, for many years, worried the citrus 

 growers of Florida and California in this coun- 

 try and has thus far baffled the efforts of the 

 agricultural scientist to discover its cause. 

 The writer has recently made some observations 

 and experiments on several citrus soils bear- 

 ing trees affected with " die-back," which lead 

 him to believe that a poor nitrifying power on 

 the part of the soil, with the ammonifying 

 power remaining normal, may be the cause of 

 the peculiar manifestations which are char- 

 acteristic of the disease and which, for the 

 purposes of this preliminary report, need not 

 be described. The theory upon which I am 

 working at the present time, looking toward 

 the solution of this problem, is that in the ab- 

 sence of normal nitrification and in the pres- 

 ence of sufficient ammonification, the tree does 

 not obtain a sufficient quantity of nitrate for 

 its development and is sooner or later forced 



to assimilate ammonia compounds as produced 

 by a mm onifying organisms in the soil ; or in the 

 presence of a sufficient amount of bases in the 

 soil even the ammonia may be set free, thus 

 causing the plant to starve for want of nitrogen. 

 While it is true that some plants can use am- 

 monia compounds just as well as nitrates as a 

 source of nitrogen and further, that some of 

 them even prefer the ammonia compounds, as 

 Kelley has shown is the case with rice, it is very 

 possible that we have in the citrus tree, a plant 

 which is deleteriously affected by ammonia 

 compounds when it is forced to absorb them. 

 As above explained, however, when a soil's 

 power to fix and hold ammonia is very feeble, 

 owing to the presence of bases in excess, a poor 

 nitrifying power and a strong ammonifying 

 power may mean nitrogen starvation for plants 

 on that soil. The writer has examined and tested 

 the nitrifying power of four citrus soils in vari- 

 ous parts of California, on which trees were suf- 

 fering from " die-back," and has found in every 

 case a very slight nitrifying power or none at 

 all. The tests were made by adding to soils, 

 kept at optimum moisture conditions at a 

 temperature of 26 to 28 degrees C. for approxi- 

 mately a month, both dried blood and sulfate 

 of ammonia, but only slight or no increases of 

 nitrates over the amount in sterile checks or 

 dry soil were obtained. The dried blood was 

 used also in varying quantities from 1 per 

 cent, up to 5 per cent, of the dry weight of the 

 soil, but the same results were obtained in all 

 cases. In some of these soils, particularly, the 

 ammonification of the blood proceeded so 

 rapidly as to give an intense odor of ammonia 

 when the Petri dish cover was raised from the 

 tumbler in which the soil cultures were kept. 

 This theory of the writer's which inclines to 

 account for the " die-back " by the fact that 

 too much ammonia is assimilated by the tree 

 under compulsion in the absence of nitrates, 

 or, under certain circumstances, because am- 

 monia is set free and therefore there is scarcely 

 any nitrogen for the tree to assimilate, would 

 also seem to be in part confirmed by the 

 observations made by Florida investigators on 

 the disease in question, in which it was noted 

 that in all cases the application of organic 



