the plant growing upon that soil can withdraw it and turn it to ite own 

 account. The- expression " plant food constituent " is accordingly used 

 throughout these pages with th specific meaning above implied, and to the 

 term " plant food " likewise a very definite signification is assigned. Bear- 

 ing this in mind, it will directly follow that a chemical analysis of the soil, 

 in order to be of value to the farming community, should tell, not the quan- 

 tities of plant food constituents present, but the proportion of plant food. 

 It becomes obvious, 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; it will, however, be more 

 convenient to look upon soil analyses as capable of sub-division into three 

 classes or grades. First 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; remembering that we are considering the subject from the 

 agriculturist's standpoint, we may be justified in calling these the plant 

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

 to include these in his determination of the total quantity of plant food 

 constituents in the soil, needs to employ the strongest chemical reagents, 

 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 arc not plant food. Needless to say, that the acids gener- 

 ally employed by the agricultural chemist fail to extract these compounds 

 from the soil, and hence do not give the utterly misleading results occa- 

 sionally attributed to them. The first and second grades of plant food con- 

 stituents differ from the third in being available 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 constituent of 

 the first grade are readily, or immediately, available to the 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 for plants, and may be extracted in the 

 laboratory by strong mineral acids, but not by water or weak organic acids. 

 They are not immediately removed by the crops, but continue in the soil 

 as a " reserve stock" a term that we 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.* 



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 amounts of plant food 



* The following figures, adapted from Bulletin No. 41 of the Minnesota Agricultural 

 Experiment Station, illustrate the fact that soils contain considerable amounts of 

 plant-food constituents which are not available for plants : 



"Wheat Soil. Heavy Clay Soil. Grass and Grain Soil. 

 Grade II. Grade III. Grade II. Grade III. Grade II. Grade III. 



Lime 2'44 "36 '48 '16 '51 '35 



Potash -54 2-18 '21 3'46 '30 1-45 



Phosphoric Oxide -38 -12 -08 -23 -05 



la the case of the clay soil, it will be noticed, as much as 96% of the potash was in a form 

 unavailable for plants. 



