AVAILABILITY OF PLANT NUTEIMNTS 345 



based on the assumption that, other things being equals 

 plant growth is a function of the concentration of the 

 plant nutrients in the soil solution. According to this 

 conception, increased or decreased soil fertility is reflected 

 in the composition and concentration of the soil solution, 

 and this in turn in crop yields. The soil solution is there- 

 fore a variable quantity, and, to some extent at least, within 

 the control of man. An elaborate explanation for the 

 responsiveness of the soil solution has been worked out 

 by Van Bemmelen and his school. 



249. Variability in composition and concentration of 

 the soil solution. — The process of rock w^eathering has, 

 according to Van Bemmelen,^ Biltz,^ and others, resulted 

 in deep-seated chemical changes in some of the mineral 

 constituents of _tiie soil, ^yhereby there are formed com- 

 plex colloidal silicates w^hich, in the form of gels, cover 

 the surfaces of the soil particles. These colloidal com- 

 plexes may contain iron, aluminium, calcium, magnesium, 

 potassium, phosphorus, and other substances, which are 

 absorbed from the different electrolytes as ions or as 

 salts and depend in quantity on the concentration of the 

 solution from which they are absorbed. They therefore 

 act like solid solutions, whose composition changes 

 with every change in the concentration of the liquid solu- 

 tion that comes in contact with them. This relation of 

 the colloidal complexes to the soil water with which they 

 come in contact is essentially different from that of the 



^Van Bemmelen, J. M. Beitrage zur Kenntniss der 

 Verwitterungsprodukte der SiKkate in Ton, Vulkanischen, nnd 

 Laterit-Bdden. Zeit. f. Anorganisehe Chemie, Band 42, 

 Seite 265-324. 1904. 



^Biltz, W. Ueber die Gegenseitige Beeinfllussung: Col- 

 loidal Geloster Stoffe. Ber. deutseh. ehem. GeselL, Band 

 37, Seite 1095-1116. 1904. 



