Fertility of some Colonial Soils. 9 



then, that we are to distinguish between at least two kinds of 

 chemical analyses of soil, one of which supplies the farmer with 

 information of value, while the other does not ; the latter may, 

 nevertheless, be of considerable value to the geologist. Those who 

 assail soil analysis generally confuse between these two kinds, and 

 rain their blows upon both without discrimination. As a matter of 

 fact, it would be more correct to divide soil analyses into three 

 classes. Eirst of all the plant-food constituents may be present in 

 the soil in such a condition as to be quite incapable of being absorbed 

 by the plant ; bearing in mind that we are looking at the subject 

 from the agriculturist's standpoint, we may call these the plant-food 

 constituents of the third or lowest grade. The chemist who wishes 

 to ascertain the total quantity of plant-food constituents in the soil 

 needs to employ the strongest chemicals, or the energetic action of 

 fluxes at a high temperature, in order to attain his object, for the 

 plant-food constituents of the third grade are usually silicates or 

 aluminates, and do not respond to any less radical treatment ; in any 

 case they are not plant food. Needless to say that the acids 

 generally employed by the agricultural chemist fail to extract these 

 compounds from the soil, and hence do not give the utterly mis- 

 leading results occasionally attributed to them. The first and second 

 grades of plant-food constituents differ from the third in being avail- 

 able for plants : that is to say, they are actually plant food. These are 

 extractible from the soil by mineral acids, such as hydrochloric acid. 

 The plant-food constituents of the first grade are readily or immedi- 

 ately available to plants, and the chemist can extract them from the 

 soil by means of water or weak organic acids, such as a dilute solution 

 of citric acid : those of the second grade are less soluble, less readily 

 available to plants, and extracted in the laboratory by strong mineral 

 acids, but not by water or weak organic acids. They are not imme- 

 diately removed by the crops, but continue in the soil as a "reserve 

 stock" — a term that I shall have occasion to use again; let it be 

 remembered, therefore, that whenever it is employed in the course of 

 these remarks it signifies plant food of the second grade. 



Thus we have these three grades of plant-food constituents in the 

 soil : — 



I. Soluble in water and in weak organic acids : 

 Immediately or readily available for plants. 

 II. Soluble in strong mineral acids : 



Available for plants only as a reserve stock. 

 III. Insoluble in ordinary acids, and extracted only by fusion or 

 by specially powerful reagents like hydrofluoric acid: 

 Not available for plants. 



