constituents have, therefore been called respectively "available" 

 and "non-available." It would appear from his writings that 

 Liebig regarded this distinction as applying to the "absorbed" 

 or "adsorbed" mineral matter; that is, on the one hand the 

 material held in or upon the soil grains by surface forces, and on 

 the other the chemically combined constituents in the minerals 

 themselves. We know that L/iebig was much impressed by the 

 absorption experiments of Way, and himself did much work in 

 this field. 1 But the great body of soil investigators has evident- 

 ly held to the opinion that there are two general classes of 

 minerals in the soil. Some have held that the "available" 

 potassium is held in zeolites or "zeolitic" minerals, an interesting 

 example often cited being glauconite or "green sand marl," 

 which sometimes contains phosphorus as well as potassium; 2 

 in minerals which are easily broken down by alkaline solutions, 

 as by sodium carbonate solutions or ammonia; or in minerals 

 which are easily broken down by organic acids supposedly ex- 

 creted from the roots of growing plants, or formed by the decay 

 of plant tissue. 8 



With the advent of this idea of a distinction between the 

 available and non-available mineral plant-food elements in the 

 soil, came attempts to distinguish them by analytical methods. 

 Of these attempts we now have a bewildering array, most of 

 1 Way was misled, as we now know, in considering the results of his 

 absorption experiments with soils as merely metathetical reactions; see 

 Absorption by soils, by Harrison E. Patten and William H. Waggaman, 

 Bull. No. 52, Bureau of Soils, U. S. Dept. Agriculture, 1908. 



2 The formation of zeolites in the soil has often been assumed, but 

 has not yet been proven ; see Rocks, rock-weathering and soils, by George 

 P. Merrill, 1906, p. 363. 



3 The classic experiments of Sachs, in producing etchings on marble 

 slabs, and the etchings observed occasionally on rock surfaces are the 

 proofs universally cited. The experiments of Czapek, who substituted 

 slabs of aluminum phosphate and other substances for the marble, and 

 those of Kossowitch, show that the action can be accounted for more 

 satisfactorily and reasonably as due to dissolved carbon dioxide. In fact 

 such etchings can be produced on marble slabs by laying platinum wires 

 upon them and covering with moist soil, or cotton, or mats of filter- 

 paper; see Bull. No. 22, p. 14, and Bull. No. 30, p. 41, Bureau of Soils, 

 U. S. Dept. Agriculture. 



2 



