21. The physical structure of soil is improved by the accumulation of

A. mold mycelium
B. minerals
C. water
D. all of these

22. __________ play a key role in the transformation of rock to soil.

A. Cyanobacteria
B. Pectin decomposing bacteria
C. Nitrifying bacteria
D. De-nitrifying bacteria

23. The groups of bacteria which have the ability to fix nitrogen from air to soil are

A. symbiotic
B. nonsymbiotic
C. both (a) and (b)
D. none of these

24. The nitrogenase consists of

A. dinitrogenase
B. dinitrogenase reductase
C. both (a) and (b)
D. none of these

25. The crops which are involved in nitrogen fixation are

A. alfalfa and clover
B. soybean
C. bean and lupine
D. all of these