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

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

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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

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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

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24. The nitrogenase consists of

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

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25. The crops which are involved in nitrogen fixation are

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

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