(129) 
SOME INVESTIGATIONS EEGAEDING " BEACK " (ALKALI) 
IN CAPE COLONY SOILS. 
By C. F. Juritz, M.A., D.Sc, RLC. 
(Eead at a Meeting of the Eoyal Society of South Africa, held on 
June 17, 1908.) 
Plates XIII-XX. 
Descending rain constantly carries clay particles from the surface 
down to the subsoil, and so enriches the latter with a supply of the 
available plant food which is generally contained in the finer soil grades, 
i.e., the silt and clay. On the other hand, surface evaporation, causing 
an upward movement of the soil water, brings to the upper soil layers a 
constant supply of readily available plant food. Bearing in mind, then, 
the references made in a former paper to the different grades of plant 
food, it may be deduced that the subsoil is constantly being supplied frohi 
the surface with plant-food constituents of Grade II., while the surface 
soil receives an equally continuous increment from below of the plant-food 
constituents of Grade I. The former of these two processes takes place 
more especially during the wet season, while the latter becomes more 
marked during the dry, but in arid regions this increase of salts in the 
surface soil may proceed so far, and result in an accumulation of salts of 
such a nature as to cause distinct injury to plant growth. 
The extent to which these salts have accumulated in the surface soil 
can be definitely estimated only by chemical analysis,! but such analysis 
is of totally different nature from that which investigates the proportions 
of plant food or of plant-food constituents present, for we are now dealing 
not with beneficial but with harmful substances in the soil. 
Thus the agricultural chemical analysis of soils, although as a rule 
confined to ascertaining whether they contain a sufficiency of plant food, 
may at times have to proceed beyond this limit and determine whether 
there be not an excess of plant poison. This generally resolves itself into 
^in investigation of the brackness or alkalinity of the soil. 
"What is generally called " brack," or alkali, in soil consists in the pres- 
ence of excessive quantities of certain sodium salts. These, under favour- 
* S. A. Phil. Soc. Trans., vol. xviii., p. 9. 
t For the present purpose this term may be considered to include the electrolytic 
process of the United States soil chemists. 
9 
