12 



III possess little more than academic interest, while the readily- 

 available or water-soluble constituents of grade I do not urgently need in- 

 vestigation so long as vast fields of exploiation have to be entered upon 

 in respect of the reserve stock, or, as Hilgard terms it, the reserve food 

 store, in the various soils of the Colony. It is this programme that we have 

 K>een endeavouring to fulfil, as far as opportunities and circumstances per- 

 mitted, during the earlier portion of the past dozen years. 



In the investigations conducted in the Government Analytical Labora- 

 tories one method of soil extraction has been adopted as a standard, but it 

 has been supplemented by sundry others for comparison. In deciding upon 

 a standard method, a solvent had to be chosen bearing in mind what has 

 already been said that would represent, not the slight action of a single 

 crop, nor a dissolving capacity far in excess of any action that successive 

 crops could exert during many years to> come. The aim was rather to make use 

 of a solvent that would extract from the soil just as much of the materials 

 composing the food of plants, as lies ready to hand for conversion into con- 

 stituents of the first grade as fast as the latter are withdrawn by cultiva- 

 tion. Obviously an adoption of such a via media necessitates the rejection, 

 on the one hand, of comparatively weak solvents, such as pure water, which 

 would extract only the directly or readily available plant food, and, on the 

 other, such powerful reagents as boiling aqua regia or sulphuric acid, which 

 would, in addition, dissolve out substances totally unavailable as plant food. 



Investigations have, it is true, been made in our laboratory with dilute 

 citric acid, but simply to compare its action as a solvent with the standard 

 method. 



We have yet to face the question of the actual solvent to be employed 

 in extracting the soil; having decided what not to use; e.g., any that would 

 stop short with the extraction of readily available constituent, on the one 

 hand, or that would include in their action constituents that axe not avail- 

 able at all, it still remains to fix on what may be termed a reserve stock 

 solvent; and here again discrimination is necessary. "Of all the mineral 

 acids available," says Dr. Wiley,* " no one possesses solvent powers for soil? 

 in a higher degree than hydrochloric." That acid has accordingly long 

 been used by analysts for the purpose, and it was also resorted to in these 

 investigations for a similar reason. In this connection, however, three 

 points awaited settlement, namely, the strength of hydrochloric acid to be 

 employed, the length of time the acid should remain in contact with the 

 oil, and the temperature at which it should be allowed to act. Lo<ughridge 

 foundt that hydrochloric acid of I 1 115 sp. gr. exerted a greater dissolving 

 effect upon the soil than either a stronger or a weaker acid ; this strength 

 of acid had also been adopted in our investigations. He also found that 

 the solvent action continued to increase for five days and then ceased. 

 "Upon the above basis the standard method generally employed in the Gov- 

 ernment laboratories has been formulated; it is the first of the methods 

 outlined below. One respect in which this method differs- from that of 

 Hilgard and Loughridge is in respect of temperature. In the warm sunny 

 climate of this Colony it would appear that root action is more energetic 

 than in colder lands. It has been found, for instance, that proportions of 

 phosphoric oxide which would be considered inadequate in Europe, in this 

 country often suffice to yield satisfactory returns. But this greater root 

 action results in a more rapid depletion of the reserve stock of plant food 

 in the soil : that reserve stock, therefore, is rendered relatively smaller than 

 under circumstances of lesser root activity. To represent this fact in the 

 laboratory when a soil is to be analysed, it accordingly becomes necessary 

 to employ a weaker solvent so as to indicate a relatively lower maximum 



* Principles and Practice of Agricultural Analysis," Vol. I., p. 345. 

 | American Journal of Science," Vol. VII., p. 20. 



