Vol. XIII. yo. 316. 



THE AGRICULTURAL XEWS. 



is: 



NITRIFICATION AND DISEASE. 



The disease known as 'die-back in citrus trees ha? for 

 many years worried the citrus growers of Florida and 

 California in this country, and has thus far battled the eftbrts 

 of the agricultural scientist to di-cover its cause. The 

 writer has recently made some observations and experiments 

 on several citrus soils bearing trees atfected 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 characteristic 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 lime, 

 looking toward the solution of this problem, is that in the 

 absence of normal nitrification and in the presence of 

 sufficient amraonification. 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 ammonifying 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 

 ammonia cimpounds ju.'st as Wrll as nitrates as a source of 

 nitrogen, and further, that some of them even prefer the 

 ammonia compounds, as Kelly 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 com- 

 pounds when it is forced to absorb them. As above 

 ej])laiiied, however, when a foil's power to fix and hold 

 ammonia is very feeble, owing to the presence of bases in 

 e.xcess, 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 various parts of California, on which 

 trees were suffering 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 ■2S°C. for approx 

 imately a month, both dried blood and sulphate of 

 ammonia, but only slight or n^j increases of nitrates 

 over the amount in sterile checks or dry soil were 

 obtained. The dried blood was used also in varying 

 quantitiis 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 ammonifica- 

 tion of the blood proceeded so rapidly as to give an intense 

 odour of ammonia when the Petri dith 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 three under compulsion in the absence of nitrates, or 

 under certain circumstances, because ammonia is set free and 

 therefore there is scarcely any nitrogen for the tree to assim- 

 ilate, would also seem to be in part confirmed by the observa- 

 tions made by Florida investigators on the disease in question, 

 in which it was noted that in all cases the application of 

 organic manures to citrus groves invariably made conditions 

 worse or increased the amount of 'die-back'. In accordance 

 with the idea above expressed, and in the absence of nitrifi- 

 cation, this would be, for the reason that either the added 

 organic matter would be responsible for the formation of so 

 much ammonia, which (through its enforced absorption) 

 would poison the plants or because, under some soil condi- 

 tions, most of the ammonia would be set free and pass off 

 into the atmosphere in a gaseous form, thus inducing nitrogen 

 hunger. 



I feel justified in transmitting this brief preliminary 

 note on the subject in question, before the theory has been 



fully tested out, by the fact that the disease known as 'die- 

 back' has been studied in many different ways without giving 

 encouraging results, and because, further, this new method 

 of attack related to the chemical and bacteriological condi- 

 tions of the soil, as affecting plant nutrition, and as applied 

 to a problem of this kind, deserves to be called to the 

 attention of other investigators working along this or 

 similar lines. I am vigorously proceeding to test out 

 the theory above given by simultaneous green house and 

 field experiments with citrus trees which are now under 

 way, in which, on the basis of what I have said above, 

 I am not only trying to overcome the disease by variously 

 treating soils in the field, so as to provide a plentiful 

 supply of nitrates and reduce ammonification, but I am also 

 making an effort to produce th-i disease experimentally in the 

 green house with seedling citrus trees on soils deficient in 

 nitrates by the addition to the latter of various ammonia 

 compojinds, and to overcome it by means of adding stimulants 

 for the nitrifying bacteria. The writer feels confident that 

 a definite relationship may be shown between the abnormal 

 soil conditions mentioned and the 'die-back of the lemon, and 

 perhaps as well of other citrus trees, when the experiments 

 have progressed far enough. The theory above enun- 

 ciated is not only in accord with the Florida observ- 

 ations but also with all other observations with which I 

 have become acquainted in respect to the disease known as- 

 'die-back'. Moreover, it allows for the production of bad soil 

 conditions which will result in a poor nitrifying power 

 through a large variety of causes. Such are, for example, the 

 poor physical conditions of soils, unfavourable drainage 

 conditions, the presence of a vigorous denitrifying flora of one 

 kind or another, and many other chemical, physical and 

 bacteriological conditions, not to mention conditions respect- 

 ing the origin of the soil which would seriously affect th& 

 nitrifying power thereof. 



These lines for the most part were written about a year 

 and a half ago when I first formulated the theory and may 

 need slight revision and some additions in the light of many 

 new facts with which we are now acquainted, but in the 

 main, the theory seems to he better supported in fact at the 

 present lime than it wa.s two years ago. Moreover, it seems 

 very probable now, from a large series of tests which we have 

 been making, that not only 'die back', the true Exanthema, 

 is to be accounted for by the theory under discussion, but 

 also that the equally disastrou.', and much more widely 

 spread disease known as 'mottled leaf of citrus trees is 

 capable of being explained in a similar way, if I may regard 

 a large series of tests on various soils in this State as a reli- 

 able criterion, (Juite unlike the 'die back', the 'mottled leaf 

 condition in citrus trees is to lie found in all citrus districts 

 of California, and is at the pre.sent time regarded as one of 

 the most menacing factors on the production of citrus fruit 

 for profit here. 



The 'die back' and 'mottled leaf conditions of some dis- 

 tricts in this State are becoming so bid as to make it impera- 

 tive to solve these questions as quickly as possible from the 

 practical standpoint alone. One orchard on which, in partic- 

 ular, the writer is working, has a very large proportion of its 

 trees badly affected by the first named disease, and most- 

 trees in the orchard show some niianifestation of the disease. 

 The trees are about four years old and had for a time grown 

 vigorously, and then suddenly began to give symptoms of 

 one of the worst and most widespread cases of 'die-back' of 

 which we have knowledge in the State. It is hoped that our 

 experiments may serve eventually to remedy such conditi.jns. 

 A description of detailed experiments will soon appear. 

 (Chas. B. Lipman in Science, for May 15, 1914.) 



