Rhizobium bacteria convert atmospheric nitrogen into soluble form.

Explanation:
Nitrogen is an essential element for the growth and development of plants. However, plants cannot use the atmospheric nitrogen directly as it is in a gaseous form. Therefore, certain bacteria have the ability to convert atmospheric nitrogen into a soluble form that can be easily used by plants. One such group of bacteria is the Rhizobium.

Role of Rhizobium in nitrogen fixation:
Rhizobium bacteria are symbiotic in nature and live in the root nodules of leguminous plants. The bacteria provide plants with nitrogen by converting atmospheric nitrogen into a soluble form through a process called nitrogen fixation. The process of nitrogen fixation involves the following steps:

1. Rhizobium bacteria colonize the root nodules of leguminous plants.
2. The bacteria produce an enzyme called nitrogenase that converts atmospheric nitrogen into ammonia.
3. The ammonia is then converted into ammonium ions (NH4+) that can be easily used by plants.
4. The plants provide the bacteria with carbohydrates and other nutrients in exchange for the fixed nitrogen.

Importance of Rhizobium:
Rhizobium bacteria play a crucial role in agriculture as they contribute significantly to the nitrogen cycle. The fixed nitrogen provided by these bacteria reduces the dependency of farmers on synthetic fertilizers, which are expensive and have adverse effects on the environment. Additionally, the use of Rhizobium bacteria can improve soil fertility and increase crop yields.