Growing Food: A Guide to Food Production

Nitrogen-fixing bacteria, which are found in nodules on the roots of leguminous

plants. They take in Nitrogen gas from the air in the soil and pass it on to the plant,

De-nitrifying bacteria, which break down humus and reduce nitrate ions to Nitrogen

gas, which returns to the atmosphere. Because this reduction requires anaerobic

conditions, these bacteria are most active in waterlogged soils.

Bacteria are most active, and multiply most rapidly, when the soil is warm, moist and
aerated; their activity is reduced as the soil dries out during the dry season, or if the soil
becomes cooler or less well aerated.
Fixation of nitrogen also occurs during lightning flashes; the huge energy release
breaks apart the nitrogen molecules and allows them to react with oxygen in the air to
form nitrogen oxides. These oxides dissolve in raindrops and fall to the ground, where
they form nitrites and nitrates. Fixation of nitrogen by leguminous plants is discussed in
1Fe, page 54.

B Arid Regions

Plants are said to suffer from water stress, or moisture stress, when their growth is
slower than normal due to insufficient water in the soil. An indication of the
interrelationship between the soil, plants, and water is shown in Figure 5.

Figure 5. Growth of a cereal crop in relation to moisture availability

Rainfall

Evaporation

Run-off

Transpiration

Rainfall pattern

Soil level

Soil water

Deep drainage

Time (months)

Plant cycle

Sowing EstablishmentFloweringMaturation Harvest

8 TONY WINCH

2.

3.

discussed in 1Fe. Inoculation/Nitrogen Fixation, page 54.

1Ba. Plant / Soil / Water Relationships