A. Nitrogen Fixation:

Free-living bacteria live in mutualistic relationships in the root nodules of such plants as the pea family. These bacteria convert atmospheric nitrogen to ammonia using the energy from ATP.

B. Nitrification:

Several species of soil bacteria convert ammonia to nitrate when the soil contains high levels of oxygen.

Several species of soil bacteria convert nitrate to free nitrogen when the soil is waterlogged and thus contains low levels of oxygen. Such bacteria use the nitrate as the final electron acceptor during respiration rather than oxygen. Denitrification depletes the soil of essential nitrate fertilizers.

Both ammonia and nitrate are important fertilizers for plants found in the soil. Moderate increases in both nutrients enhance plant growth, and thus are desired for good crop growth. Maintaining aeration of the soil helps encourage bacteria to nitrify the soil rather than removing nitrates through denitrification in oxygen depleted soils. This is why farmers commonly till the soil prior to planting.

In addition, many farmers rotate their crops, planting legumes (pea family) every other year. The legumes contain nitrogen fixing bacteria in root nodules which increase the nitrate content of the soil.