CONDITIONS AFFECTING SOIL FERTILITY 53 



Physiologically dry soils. A physically dry soil is one in 

 which there is no moisture, but even in a soil containing much 

 water, if the plants are unable to absorb it, it is physiologically 

 dry to them. In winter, though the soil may be saturated, it 

 is physiologically dry because the moisture is locked up in the 

 form of ice. Strong salts or acids of any kind in the water 

 may also prevent absorption. It is probable that many plants 

 which grow in bogs find the soil physiologically dry to them, 

 though at the same time it may be soaked with water. 



PRACTICAL EXERCISES 



1. Take two bottles of equal size and fill with roily water made by 

 shaking up some clay with rain water or distilled water. Add to one 

 bottle one tenth its bulk of a solution of lime and water and stand both 

 bottles where they will not be disturbed. Examine at intervals to see 

 which settles first, and explain the result. 



2. Weigh out three samples of clay of 200 g. each. To one add 

 10 g. of slaked lime, to another add 10 g. of sand, and to the third add 

 10 g. more of the clay. Add enough water to make plastic and form 

 into three balls. AVhen these are thoroughly dry, pile weights, little by 

 little, upon each until it is crushed. Explain the different effects of 

 sand and lime. 



3. Take four straight-sided lamp chimneys and fill one each with 

 sand, clay, peat, and soil from the school garden. These materials 

 should all be dry and well pulverized. Tie a piece of cheesecloth over 

 the bottom end of each chimney and stand them in a rack, with the 

 bottoms dipping into a shallow pan of water. In which does the water 

 rise most rapidly? In which most slowly? Do you conclude that in 

 drying they would follow the same order ? Try it and see. 



4. Visit the nearest barometer and barograph to learn how the 

 pressure of the air is measured and recorded. Examine both centigrade 

 and Fahrenheit thermometers. 



5. . Fill a glass with water, lay a sheet of writing paper over it, and 

 holding the paper in place, quickly invert the glass. When the hand is 

 removed, why does the water not run out ? 



6. Light a piece of paper, thrust it into a glass jar, and invert the 

 jar in a shallow dish of water. Explain the action that takes place in 

 the water. 



