Vol. XIII. No. 317. 



THE AGRICULTURAL NEWS. 



199 



More recent experiments, however, indicate the proba- 

 bility of the presence of another factor. During the Kew 

 experiments it was noticed that a small amount of bacterized 

 peat often gave as good results as a heavy dressing. This 

 was most evident when the treated soil contained plenty of 

 available plant food. In an experiment with Kichardias 

 plotted up in heavily manured soil, it was found that a top 

 dressing of about A oz. of peat to a 10-inch pot, doubled the 

 weight of the treated plants in a month's time. I)r. Rosen- 

 heim, of King's College, obtained equally striking results on 

 Frinnda malacoides, by treating twice only with the water 

 xtract of 18 gramme (2| grains of bacterized peat). 



Professor Bottomley ends his paper by drawing the 

 conclusion that tuch eflects cannot be due to the small amount 

 of direct food material present in the pf at or its extract added 

 to the soil, and it was realized that in the prepared peat 

 there is something which has the etiect of stimulating and 

 promoting growth in an extraordinary manner. Numerous 

 experiment.s now in progres.-? indicate that this 'something' is 

 probably similar in nature, as Dr. Rosenheim has suggested, 

 to the accessory food bodies recently found to play such an 

 important part in aninul growth and nutrition. If this is 

 confirmed, it will go far towards explaining the specific action 

 of bacterized peat as a fertilizer. 



The Imperial Department has in view the carrying out 

 of similar experiments to those conducted at Kew with culti- 

 vated plants in the West Indies. It is probable that cane 

 megass or trash may provide a suitable substitute locally for 

 peat. At all events, some of Professor Bottomlej's material 

 will be imported for the purpose of making a preliminary 

 trial, and the results of this work will be made public in due 

 course. 



NITRIFICATION AND DISEASE. 



Since the reproduction of Dr. Lipman's article* in the 

 last issue of this journal on the possible relationship in 

 California between a disease of citrus plants and the extent 

 of nitrate formation in the soil by bacteria, another and 

 more detailed papert by Br. Karl F. Kellerman and R. C. 

 Wright has arrived, containing results of intere,-t on the 

 same subject. 



Dr. Lipman's view is that the disease of citrus trees 

 known as 'die back' is induced not merely by the lack of 

 nitrate in the soil, but by the presence of ammonia which is 

 believed to exert a toxic action on an otherwise healthy 

 plant. The theory is, that in the absence of normal nitrifi- 

 cation, and in the presence 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 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. But from experi- 

 ments already conducted, it is confidently believed that the 

 enforced absorption of ammonia compounds is the principal 

 cause of the trouble. Tests are now in progress having for 

 their object the artificial production of the disease in the 

 green-house by the addition to soil of ammonia compounds. 

 If these give positive results, then I'r. Lipman's hypothesis 

 will be regarded as confirmed. 



In regard to the equally disastrous and much more 

 widely spread aflfection called 'mottled leaf of citrus in 

 California, Dr. Lipman considers that his hypothesis also 



* ifcience. May 14. 1914. 



t Jounial of Agiic>iltnral Research, Yo\. II, No. 2. 



explains the cause of this perplexing trouble. From their 

 paper, referred to at the beginning of this article, it would 

 appear that Kellerman and Wright hold a different opinion, 

 and since their views are supported by considerable experi- 

 mental data, they must for the present be accepted as con- 

 stituting the correct explanation. 



Kellerman and AVright assert that the general decadence 

 of citrus trees in small areas scattered throughout the 

 orange belt of California is due to over stimulation amount- 

 ing in the critical cause to distinct poisoning owing to an 

 excessive supply of readily available nitrogen in the form 

 of nitrate Green-house experiments have been conducted 

 at Washington with seedlings of grape fruit, sour oranges 

 and sweet oranges; and although (as the authors point out), 

 the results of laboratory experiments cannot directly be 

 translated into field results, these results do indicate that the 

 nitrate nitrogen in excess does produce the typical .symptoms 

 of the mal nutrition known under the name of 'chlorosis' or 

 'mottled leal'. As well as nitrates, chlorides were found to 

 be fatal after a certain limit of concentration had been 

 reached, hut not as fatal as nitrates. As might be expected, 

 nitrates and chlorides together intensify the symptoms of 

 injury, but moderate quantities of lime — less than 10 

 per cent, of calcium carbonate — exert a more or less pro- 

 tective action. It .should be noted here, that while rather 

 large quantities of calcium carbonate (limestone) are 

 favourable, slight traces of calcium oxide (quicklime) or 

 calcium hydrate (slaked lime) have an immediate toxic 

 efftct. The flocculating action and consequent improve- 

 ment of foil permeability to the circulation of air and water 

 renders it valuable from a physical as well as from a biolog- 

 ical standpoint. 



Perhaps the most significant part of the work under 

 discussion was the study of the efl^ect of ditterent kinds of 

 organic matter upon nitrification in the soil. Barley straw 

 or pure cellulose when ploughed under may entirely elimi- 

 nate nitrates from the soil through the utilization of all the 

 nitrates in the soil by the bacteria which decompose straw 

 and cellulose. Green manures, on the other hand, while 

 causing some actual loss of total nitrogen do not materially 

 disturb the ratio of nitrogenous compounds. 



The total number of bacteria in the so called 'good' and 

 'bad' soil, is apparently without significance. The condition 

 of fundamental importance is to control within reasonable 

 limits the rate of nitrification. 'In pot experiments a normal 

 rate of nitrification is shown to be possible with a green crop 

 turned under, and it seems reasonable to recommend this 

 practice in the field for maintaining the humus supply of the 

 soil. The evidence of the above experiments suggests that 

 the extensive use of mature straw is to be avoided, though 

 light applications of straw to fields too high in nitrate, 

 probably also with the liberal use of ground limestone, might 

 be advantageous.' 



From what has now been said it will be evident that 

 Lipman's explanation does not hold good in the case of 

 'mottled leaf though his view of ammonia poisoning may 

 explain the cause of 'die back', which is an affection more 

 definitely restricted to certain localities than 'mottled leaf. 

 Possibly, therefore, the soil conditions in places where 'die 

 back' occurs are exceptional in the way Lipman describes. 

 Little more can be said until the appearance of results in 

 greater detail; and it would be interesting if a map could be 

 published showing the distribution of these seemingly oppo- 

 site soil conditions which in both their extremes are unfavour- 

 able for the healthy growth of citrus trees. Finally it must 

 be remembered that the investigations of 'mottled leaf have 

 been concerned not with normal, but with irrigated soil. 



