10 Transactions of the South African Philosophical Society. 



Obviously the agriculturist has little, if any, interest in non- 

 available plant-food constituents, and chemical analyses — be they 

 of a single soil sample or of a whole series of representative soils 

 from various parts of the country — which give only figures showing 

 the total amounts of plant-food constituents present, and afford no 

 indication as to the quantity of available plant food^whatever 

 interest they may possess for the geologist — have none for the farmer. 

 It is true that by mechanical disintegration and chemical decom- 

 position plant-food constituents of the third grade may ultimately 

 become available, as they very slowly change into those of the 

 second, but the process is so gradual as practically never to have 

 any value for the generation in occupancy. The constituents 

 of Grades I. and II., on the contrary, possess great interest for 

 the farmer ; they comprise the actual plant food ; the former 

 affect the land's immediate productiveness, the latter its permanent 

 value. 



So we are brought to this conclusion, that if a chemical investi- 

 gation of a country's soils is to be made, it is the reserve stock of 

 plant food in the soil (that is to say, the constituents of the second 

 grade) that calls for first attention. They are constantly changing 

 into constituents of the first grade, and are being removed from the 

 soil by plants very much more rapidly than they can be produced 

 from the constituents of the third grade. It may be of some value, 

 either incidentally in certain cases or subsequently to a thorough 

 investigation of the reserve stock of plant food in the soils of the 

 whole country, to inquire into the conditions and quantities of the 

 other two grades as well, but the immediate and pressing necessity 

 is to proceed with all speed along the line just indicated as demanding 

 prime consideration. 



We must not dwell on these points too long, especially as I hope 

 to revert to them again in a pamphlet, now in course of preparation, 

 which will deal more fully with the details of the soil investigations 

 that have been carried out up to tho present in the Government 

 Analytical Laboratory, but, before passing on, it should be said that 

 one reason why fuller data were not to hand at the time of the pre- 

 paration of my last paper on this subject was that a sufficient 

 number of soils had not then been analysed by a method which 

 could be taken as affording a reliable means of ascertaining the 

 reserve stock of plant food in the soil, and it is, of course, in- 

 admissible to compare, say, a granitic soil examined by one mode of 

 analysis with a sandstone soil treated in another way. Even now 

 the data are all too few ; but the need of arousing, in scientific 

 circles, a deeper interest in investigations of this kind, and amongst 



