200 



NATURE AND PROPERTIES OF SOILS 



tarded also. The various forms of water in the soil and their 

 availability to the plant are illustrated diagrammatically in 

 Fig. 35, page 199. 



This diagram may be evaluated in a general way as below, 

 using the sandy and clayey soils for which full physical data 

 have already been given in Chapter VIII. (See footnote on 

 page 179.) 



Table XXXVIII 



THE EVALUATION OF FIG. 35 FOR A SANDY AND CLAYEY SOIL, 

 RESPECTIVELY. 



Hygroscopic coefficient. 



Wilting point 



Maximum field capacity 



Unavailable water 



Available water 



Superfluous water 



106. Optimum moisture for plant growth. — It is very 

 evident that there must be some moisture condition of a soil 

 which is best for plant development. This is usually desig- 

 nated as the optimum content. It is not to be assumed, how- 

 ever, that the total range of the available soil-water repre- 

 sents this condition. Nor is this optimum water content in 

 any particular soil to be designated by a definite percentage. 

 In reality the moisture in a soil may undergo considerable 

 fluctuation and yet allow the plant to develop normally. 1 This 

 is because the physical condition of the soil changes with 

 varying water content and the plant is able to accommodate 



1 Wollny, E., Untersuchung iioer den Einfluss der WachsthumsfaJctoren 

 auf des Produktionsvermbgen der Kulturpflanzen ; Forsch. a. d. Gebiete 

 d. Agri.-Physik., Band 20, Seite 53-109, 1897. 



Mayer, A., t!oer den Einfluss Tcleinerer oder grosser er Mangen von 

 Wasser auf die Entwickelung einiger Kulturpflanzen ; Jour. f. Landw., 

 Band 46, Seite 167-184, 1898. 



