Certified Crop Advisor Practice Exam

Which condition is most favorable for symbiotic nitrogen fixation?

1. High soil pH

2. High soil nitrate

3. Low soil nitrate

4. High temperature

The correct answer is: Low soil nitrate

Symbiotic nitrogen fixation, primarily carried out by certain bacteria in partnership with leguminous plants, thrives under specific soil conditions. The most favorable condition, which is indicated in this case, is low soil nitrate levels. When soil nitrate levels are high, plants experience a sufficient supply of nitrogen, which diminishes their reliance on symbiotic nitrogen fixation. This can lead to a reduced activity of the nitrogen-fixing bacteria because the presence of alternative nitrogen sources may inhibit the root nodules’ development and the overall relationship. Thus, low soil nitrate levels encourage the plants to engage more actively with their symbiotic partners to access nitrogen, stimulating the process of fixation. High soil pH, while a relevant factor in soil chemistry, does not specifically promote the efficient functioning of nitrogen-fixing bacteria. Similarly, high temperature can affect microbial activity either positively or negatively, but it is not a direct indicator of favorable conditions for nitrogen fixation. Therefore, the choice of low soil nitrate as the most favorable condition highlights the importance of maintaining soil health and promoting beneficial microbial activities in agriculture.