310 A.NNTJAL. REPORT OF THE Off. Doc. 



food is illustrated in tlie following table, in which in the first column 

 are given the percentages of lime magnesia potash and soda in an 

 original gneiss rock, and in the second column the quantities of the 

 same present in the residual soil. 



Lime (Ca 0), 4.44 Trace 



Magnesia (Mg 0), 1.06 0.40 



Potash (K 2 0), 4.25 1.10 



Soda (Na 0), 2.42 0.22 



The question might here be asked why are not all of these elements 

 of plant food entirely leached from the soil, and in what form are 

 these residual materials held. In most soils a portion of them are 

 locked up in the form of undecomposed mineral particles and frag- 

 ments of rock, and it is the continued decomposition of these latter 

 which furnish fresh stores of available plant food. 



Another important chemical process going on in the soil is the 

 formation of so called zeolitic compounds. As the alkalies, such as 

 soda and potash, are dissolved out of the minerals by carbonated 

 waters the carbonates thus formed possess a certain solvent action 

 upon silica. This dissolved or gelatinous silica combines with the 

 alkalies, resulting in the formation of zeolites. These secondary 

 zeolites thus fix as it were the alkalies, notably potash, which might 

 otherwise be leached from the soil. Furthermore, the especial affin- 

 ity which potash has for zeolites fixes this, the most important of 

 mineral nutriments, above all others. Thus if a zeolite be composed 

 of silica and soda or of silica and lime, the potash in preference will 

 enter into combination with the silica and the less valuable soda or 

 lime will be set free. 



Zeolites differ from the more insoluble silicates found in rock- 

 forming minerals in the fact that they are readily decomposed by acid 

 soil waters, thus setting free to plants their valuable nutrients. 



III. THE SIGNIFICANCE OF SOIL BACTERL'V. 



Active and Potential Fertility of the Soil. 



Since the different chemical changes taking place in soils, by which 

 plant food is elaborated and rendered available, are in large meas- 

 ures the result of bacterial action, it is assumed that the larger their 

 numbers, up to certain limits, the greater must be the rate of elabora- 

 tion of plant food. 



This is instanced by the fact that soils which are under active 

 fertilization and cultivation, and which in the popular sense are con.- 



