2. The formation of soil starts with the ____________ of rock.

3. Soil forms in layers that are called soil ________________ .

4. The soil in the A horizon is also called ______________ and is the soil in which most ______________ grow.

5. The A horizon also contains the decayed organic materials, or _______________ , that makes soil fertile

6. The B horizon, called the ______________ , has lots of fine rock particles but little humus.

7. The C horizon, which rests on _______________ , is mostly large pieces of weathered rock.

8. Soil in a forest has a thin ____________ horizon, while soil in a(n) ______________ may not have this horizon at all.

9. Soil in forests has a thin layer of _____________ , and has little ________________ .

10. Desert soil is sandy and does not hold much _______________.

11. The soil of the prairies and other ________________ in the central United States is rich in humus.

12. Grassland soil is good for _________________ .

13. Plants hold nutrients that return to the soil when the plants die and ___________________ .

14. Chemicals that kill insects and weeds can cause soil to become ___________________ .

15. Farmers can replace humus and nutrients in soil with _________________.

16. When farmers practice ____________________, they plant different crops on the same land in different years.

Farmers can conserve soil on hills when they use ______________ plowing and ___________________.