10 



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 decomposition, plant food constituents of the third grade may ul- 

 timately 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 other hand, 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. In an investigation such as that under re- 

 view, then, we may dismiss grade III from further consideration. The 

 first grade, or immediately available plant food, is of less account than in a 

 country where agricultural lands are usually held upon short tenancy; 

 and, in any case, where the object is to gain information respecting the 

 agricultural potentialities of extended areas, insufficient date would be> 

 afforded by determinations of the readily available plant food in the soil : 

 individual farmers may profit by such investigations, but even then, to be 

 of lasting value, they would have to be constantly repeated, and upon very 

 much more comprehensive lines than the utmost range of practicability^ 

 if they are to meet all needs. 



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

 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 continually changing into plant food of the first 

 grade, and are being removed from the soil by plants very much more 

 rapidly than they can be produced from the third grade constituents. 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 1 

 proceed with all speed along the line just indicated as demanding prime 

 attention. 



Different plants have, as already remarked, different absorbing powers, 

 and hence we may almost say that the immediately available plant food in 

 a soil varies in amount with the crop to be cultivated. If, therefore, we 

 ascertain the maximum amount of plant food that can become available to 

 any plant from a given soil we shall gauge the utmost limits of that soil'a 

 natural fertility, as far as it is affected by chemical considerations. If that 

 be his purpose, the chemist should employ as a solvent not one which will 

 simulate a single crop of weak extractive power, that is to say, a solvent 

 that will take out little more than the constituents of grade I, leaving the 

 bulk of those of grade II untouched. Of course, he is well aware that the 

 soil which contains a large stock of plant food of the first class will in any 

 case be chemically rich ; it will therefore be fertile, provided that other 

 factors the physical character of the soil, the supply of water, and so on 

 are not defective. But he knofws equally well that if the maximum of 

 available constituents fall below a certain limit, the soil must of necessity 

 be poor, whatever other conditions may be favourable. Hence the first ob- 

 ject of a systematic soil survey along chemical lines should be to ascertain 

 the maximum proportions of plant food likely to be available to average 

 crops within a reasonable period of time in the area under examination. 

 For this reason, especially in a country like this, consisting of immense 

 tracts of virgin soil, amongst which there are many poor soils, the writer 

 considers that a great deal more information will be gained by the extrac- 

 tion of soils by means of Hydrochloric acid than by applying Dyer's citrio 

 acid solution. The latter may be employed subsequently, as supplemen- 

 tary, but the former method should certainly take precedence. 



