EXTENSIOlSr COUESE IN SOILS. 17 



REVIEW QUESTIONS, LESSON II. 



1. "What chemical elements are essential to the growth of plants? In what condition 

 are they utilized by plants? 



2. What is a salt? Give example. Name some properties of acids and of bases 

 which you have discovered. Give examples of acids, bases, and salts. 



3. Tell what you understand by the limiting factor in crop production. 



4. What are root hairs? Describe the process by which plants absorb materials 

 from the soil. 



5. Is it possible that plants might not be able to get enough plant-food material for 

 their growth, even though the soil may contain sufBciont quantities of it? Explain. 



6. Mention a special function of potassium in plants; of phosphorus; of nitrogen. 



7. How much nitrogen, phosphorus, and potassium are taken from the soil in re- 

 moving a 100-bushel crop of corn? A 50-bushel crop of wheat? Three hundred 

 bushels of potatoes? Six hundred bushels of apples? Four hundred pounds of butter? 

 (See Table 23, Ref. No. 5, p. 154.) 



8. Of what value is chemical soil analysis to the farmer? Discuss. 



9. Give the means of removing and the means of replenishing plant-food materials 

 in soils. 



10. What is adsorption? 



LESSON III. PHYSICAL PROPERTIES OF SOILS. 



In farm practice the term '' soil " is somewhat loosely used to include 

 the furrow slice. It is commonly about 6 to 8 inches in depth, com- 

 paratively friable and porous, and in humid climates is darker and 

 contains more organic matter than the part beneath, called the sub- 

 soil. These two parts are better designated by the terms surface 

 soil and subsurface soil, both parts being comprehended in the general 

 term ''soil," which usually includes a layer of about 4 feet, or the 

 depth to which the roots of farm crops commonly extend. In con- 

 nection -with tillage, soils are also spoken of as being heavy or light, 

 depending upon whether they are hard or easy to work. Clay soils 

 are hard to till, due to their fineness of particles and their stickiness, 

 Sandy soils tiU easily, but are coarse grained and reaUy heavier than 

 the clays. AU soils are mixtures of different-sized particles. The 

 size of the particles determines the texture of a soil. Structure has to 

 do with the arrangement of the particles of soil and is independent of 

 their size. When the structure of soil particles is such as to be highly 

 favorable to the growth of crops the soil is said to be in good tilth. 



TEXTURE. 



(Ref. No. 2, pp. 70-76, or No. 3, pp. 84-86, 97, and 102.) 



Mechanical analysis.— To study texture the inorganic soil particles 

 are separated into a number of grades according to size. Tliis sepa- 

 ration is called mechanical analysis. Fine wire sieves, carefully 

 constructed, are employed for separating the coarser sands into dif- 

 erent grades, and bolting cloth, such as is used in flour miUs, 

 21862°— Bull. 355—16 2 



