254 GENERAL SCIENCE 



the soil friable and fine, but upon this decaying material the "nitrogen- 

 fixing" bacteria thrive. Through their agency there is left in the soil 

 various soluble nitrogen compounds available for use by plants. 



While the farmer and gardener may not be able to make the soils 

 of their lands just what they may desire, it is always possible to so 

 cultivate and fertilize most soils that if well watered they will yield 

 good returns for the labor put upon them. 



Exercises 



1. For what several purposes is soil plowed? To about what depth? Why 

 may fall plowing be somewhat deeper than spring plowing? 



2. Aside from an unsightly appearance, what two serious objections are 



there to allowing growths of weeds among crops? 



3. What is meant by a mulch for soil, and what is its purpose? 



4. What characterizes the so-called "dry farming" of the semi-arid regions? 

 6. Account for the yellow, brown, and red colorings of clay banks. What 



commonly gives the black color to soils? 



6. What is true of the relative rate of decay of organic matter in soils open 

 and porous, and in compact water-soaked soils? Why so? 



7. By what means is nitrogen that is in a free state in the air within the soil 

 converted into compounds suited for plant food? In what way does the 

 free oxygen of the air get into plants? 



8. What is the especial function of the root-hairs of plants? Explain the 



effect on many plants of having the spaces of the soil filled with water. 

 Name any plants that grow in (and under) water. 



9. What is (a) humus; (b) silt? 



10. By calculation fill out the following table showing the amount of surface 

 in one cubic inch volume when subdivided smaller and smaller. (Cubes 

 are used for ease in calculation.) 



Side of cube i inch o.iinch o.oi inch o.ooi inch 



Number of cubes i 1000 



Amount of surface 6 sq. in. 60 sq. in. 



TREE PLANTING 



A deplorable waste of natural resources in the United 

 States has taken place in a wholesale wanton destruction of 

 the forests. Succeeding generations will long mourn these 

 unchecked practices as an enormous economic loss both direct 

 and indirect. Millions of acres, unfit for any use other than 

 timber growing, have been so devastated that instead of 



