MAGNESIA IMPREGNATED S0IL8. 



Bv G. N. Blackshaw. O.B.E., B.Sc, F.I.C 



Read Juhj IT, 1920. 



This paper deals with certain soils, occurring' on the 

 so-called Great Dyke in Southern Rhodesia, which present 

 features of particular interest to the agricultural chemist on 

 account of their somewhat uncommon chemical character. 



My attention was first drawn to tliese soils in the year 1912, 

 when samples were submitted for analysis owing' to the poor 

 returns of maize which had been obtained. 



These samples comprised red, chocolate and black loams 

 which, from superficial observation, would appear to possess a 

 considerable reserve of fertility. Althoug-h this did not prove 

 to be the case on chemical examination, yet the cause of the 

 particularly poor returns could not be attributed to lack of 

 plant food. The analyses, however, revealed the presence of 

 an abnormally larg-e amount of magnesia in the soils, the ratio 

 between the lime and magnesia being- such as to lead one to 

 suspect that the poor results were due to the excessive amount 

 of the latter constituent. 



From particulars supplied by Mr. H. B. Maufe, Director 

 of the Rhodesian Geological Survey, it was foimd that the 

 .samples had been obtained from areas situated on the Great 

 Dyke, which is about four miles Avide and extends over a dis- 

 tance of approximately three hundred miles from the XTmvukwe 

 Hills, on the Lomagundi-Mazoe border, to the Doro Hills, in 

 Belingwe. This dyke generally forms higher ground than the 

 iieiglibouring country, and consists of norite, serpentine and 

 other basic igneous rocks characteristically ricli in magnesia, 

 the presence of which affords an explanation for the high 

 proportion of magnesia in the soils derived therefrom. 



In the United States and Japan a considerable amount of 

 work upon the relation betwee-i calcium and magnesium with 

 respect to plant nutrition has been carried out, and a selection 

 of the literature has been made in the present discussion with 

 a view to presenting some of tlie ascertained facts as concisely 

 as possible. These data are largely drawn from a A-ery inter- 

 esting review on the subject compiled by McCool.* As early 

 as 1814 Davyt discussed the injury that magnesia sometimes 

 produces on crops. He wrote: "On mixing some calcined 

 magnesia with soil in which different seeds are soAvn, it is found 

 that they either die or vegetate in a very imperfect manner." 

 He also stated " that lime from mag-nesia limestone may be 



* McCool: " The action of certain nutrient and non-nutrient bases 

 on plant giowtli." Cornell University Agricultural Experiment Station, 

 U.S.A., 1913. 



t Davy: " Elements of Agricultural Chemistry,'' 2nd ed., p. 281. 



