20 PLANT PHYSIOLOGY AND ECOLOGY 



of water content, and has but an indirect or obscure effect upon 

 the water present. The texture of the soil is chiefly a matter of 

 size of grains or fineness. The latter can best be ascertained by the 

 use of sieves. Two sieves of 1 mm. and of .05 mm. mesh will make 

 it possible to separate the soil into gravel, sand, and silt-clay. The 

 structure of the soil is then expressed in percents, e.g., gravel 10%, 

 sand 15%. silt-clay 75%o. 



A mechanical analysis of a soil throws little additional light 

 upon its behavior with respect to water. It is much more helpful 

 to know its porosity and capillarity under conditions as nearly 

 natural as possible. Both of these are determined by using a 

 cylinder of the soil concerned and noting the rate with which water 

 moves downward or upward through it. For general purposes, 

 however, a measurement of porosity suffices for both, since capil- 

 larity varies inversely as the porosity. Thus sand is extremely 

 porous, but possesses very little capillarity. Clay, at the other 

 extreme, exhibits strong capillary movement, but is very slightly 

 porous, while intermediate soils approach the one or the other in 

 behavior in direct proportion to the amount of the predominant 

 constituent. The use of soil-cores from typical habitats yields the 

 most satisfactory data concerning porosity and capillarity in rela- 

 tion to actual water content. When these are not obtainable, 

 instructive results can be obtained by using loose soils in the plant- 

 house, provided that the soils are well packed before the experi- 

 ments are made. The presence of decaying vegetable matter 

 increases the porosity of a soil, and correspondingly decreases its 

 capillarity, but after complete decomposition humus tends to 

 increase capillary action, especially in coarse soils. The amount of 

 humus may be readily ascertained by weighing a soil before and 

 after it is thoroughly burned. 



Experiment 3. Porosity and rate of evaporation. Fill three 2-inch 

 flower-pots respectively with sand, loam, and clay that are nearly dry 

 but not powdery. Pack each soil firmly until its surface is a half-inch 

 below the edge of the pot. Place each pot in a tall Stender dish, and 

 slowly pour water upon the clay until the former runs into the dish 

 through the hole in the pot. Pour the same amount upon both sand 

 and loam, and measure the amount of water that percolates through 

 each. Weigh each pot of soil, and, together with a graduate of water 

 having an equal surface, put them in a sunny place. Weigh each every 

 day or two for a week or more. As soon as the sand shows no further 



