GYPSUM AS A FERTILISEE FOR SUDAN SOILS 65 



them may prohably be included the accumulation of products excreted by the plant, these 

 (like all excretory products) being poisonous to the organism ])ro(luciiig them. 



The beneficial effects of fertilisers has been thought by some to be due, in part at least, 

 to their direct action on these harmful excretory products. It is not impossible that 

 gypsum may have an action of this kind. Be this as it may, the sterility of some soils has 

 been shown by Shreiner and Shorey of the United States Department of Agriculture to 

 be connected with the presence of certain organic compounds among which is, e.g., dihydroxy- 

 stearic acid. An interesting point in this connection is that while some of the combinations 

 of this acid with bases are as harmful as the acid itself, the potassium and calcium salts are 

 apparently without injurious action. Soils containing this or similar bodies might therefore, 

 it appears to the writer, reasonably be expected to be made more productive by the 

 application of gypsum as a fertiliser. 



((/) Lastly, gypsum has long been known to act favourably in promoting the growth 

 of leguminous crops, notably the clovers. The rationale of its action in this case appears not 

 to be fully understood. It may be due to effects not yet discovered or to a combination of 

 those already detailed. 



Amount to be applied. — The usual dose of gypsum is about 200 pounds to the acre. 

 In special cases, where there is a notable proportion of sodium carbonate present and 

 the soil well puddled in consequence, 500 pounds or even more may have to be employed. 



It should be remembered that lime either as such or in the form of gypsum is not 

 a plant food in the ordinary sense but that it acts rather as a " tonic.'' It would not 

 be good practice to continirally fertilise with gypsum since the effect might be to impoverish 

 the soil too rapidly. The gypsum should, in most cases, simply be added to aid in bringing 

 the soil into good working condition, and its maintenance in this state can be effected 

 only by having regard to the requirements of each special case. 



Gum Eeseakch 



The disadvantages under which the laboratories have been labouring in connection 

 with research on gum jjroducts have already been mentioned. Many lines of investigation 

 have had to be abandoned for lack of efficient aid in the field work. Some advances 

 have nevertheless been made, and it may not be without interest to state, briefly, some 

 of the points which appear to have been established, as well as to indicate the lines 

 along which investigation should be directed in future work. 



(Jriijin of i/uin. Mr. Edie's researches, both those detailed in the Third Report, 

 and those carried out during the winter of 1908-9 {vide par/e 73), appear to confirm the 

 earlier work of Greig Smith, which indicated that gum production is the result of the Microbial 

 activity of a specific micro-organism. The mode of infection probably varies in different ong'" of g*"" 

 cases, but there is evidence to show that ants, and possibly other insects, frequently 

 act as carriers. During the course of some experimental tappings of talk trees (Acacia 

 seijal) in the Sennar Province, the freshly exuded sap was observed to attract streams 

 of ants to the cut surface, and it seemed extremely likely that this fact was directly 

 connected with the infection of the tree by the gum-producing microbe. 



The attention of Mr. K. S. Edie. who was at this time at work on hashdb trees 

 (A. senega!) in the Kordofan forests, was called to this point, and his observations 

 {aide page 81) appeared to amply confirm the conclusion arrived at. Such a view would 



