PLANT-FOOD MATERIALS IN SOILS 111 



9. Explain the availability of absorbed fertilizers. 



10. What two constituents are removed in greatest quantity 

 by drainage water from an unplanted soil ? 



11. Explain how roots aid in the solution of soil. 



LABORATORY EXERCISES 



Exercise I. — Soluble matter of soil. 



Materials. — A very rich soil, filter paper and funnel, evaporat- 

 ing dish, flame, dilute hydrochloric acid. 



Procedure. — Place a small amount of a rich soil on a filter paper 

 held in a funnel and leach with distilled water, catching percolate 

 in an evaporating dish. Evaporate percolate to dryness and exam- 

 ine residue. Is it large or small in amount ? Treat with a few 

 drops of dilute acid. Finally heat over a flame. Explain results. 

 This soluble matter is the most valuable portion of the soil. 



Exercise II. — Absorptive power of soil for dyes. 



Materials. — Soil, filter paper, funnel, solution of gentian violet. 



Procedure. — Place a small amount of soil on a filter paper in a fun- 

 nel and treat with a solution of gentian violet. Note that the water 

 comes through clear for a considerable period indicating the high ab- 

 sorptive power of the soil for this dye. The capacity of the soil to 

 absorb soluble matter prevents heavy losses of plant-food materials. 



Exercise III. — Selective absorption by the soil. 



Materials. — Soil, filter paper and funnel, solution of gentian 

 violet and solution of eosin. 



Procedure. — Proceed in the same way as Exercise II, comparing 

 the absorptive power of portions of the same soil for the two dyes. 

 Note the difference. The soil varies in its absorptive power with 

 different materials. For instance, the soil absorbs acid phosphate 

 much more strongly than sodium nitrate. 



Exercise IV. — Absorptive power of the soil for gas. 



Materials. — A moist loam rich in organic matter, a flask or 

 bottle, concentrated ammonia. 



Procedure. — Place in a flask or bottle a quantity of moist soil. 

 Pour in a few drops of ammonia. Note strong odor. Stopper 

 bottle and shake. Allow to stand for half an hour with several shak- 

 ings. Open and note odor. 



The absorptive power of the soil for ammonia, oxygen and other 

 gases is a very important function. Explain why this is true. 



