EXTENSION 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 sufficient 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 pra'ctice 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 till easily, but are coarse grained and really heavier than 
the clays. All 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. This sepa- 
ration is called mechanical analysis. Fine wire sieves, carefully 
constructed, are employed for separating the coarser sands into (lif- 
erent grades, and bolting cloth, such as is used in flour mills, 
21862°— Bull. 355—16 2 
