PLANT LIFE. OSMOSIS. FERTILIZERS 415 



change insoluble phosphates of iron and aluminium into more 

 soluble calcium phosphates. 



(3) Some soils are either naturally acidic or sour, or have 

 become so by excessive use of sulphate fertilizers, and are hence 

 unfavorable to plant growth. This acidity is corrected by addi- 

 tion of lime or calcium carbonate, but not gypsum. 



(4) Salts which are injurious to plants, such as soluble magne- 

 sium compounds or " black alkali " (sodium 'carbonate), are 

 converted into less soluble compounds, such as magnesium hy- 

 droxide or carbonate, by addition of lime or calcium carbonate, 

 or into non-poisonous compounds, such as sodium sulphate, by 

 gypsum. 



Adsorption in Soils. As we have seen, fertilizers must 

 contain the elements necessary for plant growth in soluble form. 

 It is undesirable, however, to use salts which are exceedingly 

 soluble in water as fertilizers, since they will obviously be rapidly 

 washed away from the surface soil. A large proportion of all 

 fertilizers is unavoidably wasted in this manner. The minute 

 particles of the soil, however, possess the power of conserving 

 dissolved substances in the soil solution by concentrating and 

 holding them upon then* moist surfaces. This phenomenon, 

 adsorption, is of considerable interest and importance in other 

 connections, and will be taken up in detail in a later chapter 

 (p. 421). 



Exercises. 1. Given an osmotic pressure cell and an un- 

 known substance, soluble in water, to which the membrane is 

 perfectly impermeable, how 'could you determine the molecular 

 weight of the substance? 



2. What would be the effect of putting fresh flowers in a vase 

 containing a concentrated salt solution? 



3. At 20 100 c.c. of water, shaken up with excess NaCl and 

 KC1, dissolve 30 g. NaCl and 15 g. KC1. At 100 the same 



