OUTLINE AND TABLE OF CONTENTS Xlll 



PAGE 



II. Organic constituents of the soil . 119 



a. Sources, derivation and forms (52). 



b. Chemical composition (53). 



c. Amounts present (54). 



d. Some physical properties (55). 



(1) Solubility (56). 



(2) Weight (57). 



(3) Absorptive properties (58). 



(4) Volume changes (59). 



(5) Plasticity (60). 



e. Effects of organic matter (61). 



(1) Physical effects (62). 



(2) Chemical effects (63). 



/. Maintenance of organic matter (64). 



B. The Soil as a Reservoir for Water 133 



I. Functions in plant-growth 133 



II. Amount of water in the soil 135 



Determined by 



a. The supply (65). 



b. Retentive capacity (66). 



1. Statement of water-content (67). 



2. Forms and availability (68). 



3. Amounts of each form (69). 



(a) Hygroscopic water (70). 



(b) Capillary water (71). 

 Determined by 



(1) Texture (72). 



(2) Structure (73). 



(3) Content of organic matter (74). 



(a') Volume of water held by different soils 

 maximum, minimum and opti- 

 mum water-content (75). 



(b') Available water in some field soils (76). 



(e') Relation of surface tension to capil- 

 larity (77). 



(c) Gravitational water (78). 



c. Amount and rate of loss (79). 



