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JOUENAL OF THE ROYAL HORTICULTURAL SOCIETY. 



action enter into circulation and are carried to the places in which 

 they can be used as building materials in forwarding growth. Prob- 

 ably during this period the differential septa become more or less 

 permeable to substances which do not pass through them during the 

 period of assimilation. 



Apparently ammonia is the most active ** natural" stimulant but 

 carbonic acid is also effective as a hormone. It may well be that there 

 are other substances present in soil, especially when it has been 

 heavily dressed with farmyard manure, which act as more or less 

 powerful stimulants. And let me here again point out, that a sub- 

 stance which is a stimulant when used in proportions not exceeding a 

 certain low maximum at once becomes toxic when this maximum is 

 exceeded. All horticulturists are aware of the danger attending the 

 use of ammoniacal manures. Liquid manure prepared from cow-dung 

 is invaluable when used with discretion but, as all know, any excessive 

 amount is harmful. 



The recommendation of well rotted farmyard manure rather than 

 of the fresh material is explicable on similar grounds : ammonia and 

 perhaps other hormones are given off but very gradually when such 

 manure decays in the soil ; in . the early stages of the putrefaction 

 which fresh farmyard manure undergoes, however, ammonia is pro- 

 duced very rapidly and in relatively considerable amounts — hence the 

 danger attending its use. In the previous number of this Journal 

 (vol, xxxvii,, p, 550), in the discussion on the Streak disease in the 

 Sweet Pea, it is pointed out that: " Another cause of loss of vitality 

 through root-weakening is the too prevalent practice of placing thick 

 layers of manure between the spits in double digging." The explana- 

 tion is to be found doubtless in considerations such as are advanced 

 above ; such a practice may easily give rise to excessive stimulation. 

 I have heard of a lawn being destroyed by a dressing of wood-ashes 

 and sulphate of ammonia — two substances which should not be 

 applied at the same time, as the strongly alkaline wood-ashes neces- 

 sarily liberate ammonia from the ammonia salt. 



The experiments carried out by Dr. E, J. Eussell in conjunction 

 with Mr. Hutchinson and Mr. Petherbridge, at Eothamsted, on the 

 effect of sterilizing soil either by heating it or by the application of 

 substances such as toluene, are of the greatest importance and interest 

 from the point of view under discussion. The general result of the 

 inquiry has been to show that the fertility of the soil is markedly 

 increased by such treatment. It is found that the sterilized soil con- 

 tains a larger proportion of ammonia than the unsterilfzed, and that 

 oxidation takes place more rapidly in soil after it has been sterilized — ■ 

 at least two powerful hormones (ammonia and carbonic acid) are there- 

 fore present in the sterilized soil in larger proportion than in the 

 unsterilized, and its increased efficiency may be ascribed in large part, 

 if not entirely, to this circumstance. In some cases growth is at first 

 retarded, but subsequently much increased in the sterilized soil — a 



