Rhizobium is a soil bacterium that is capable of fixing atmospheric nitrogen into ammonia, which can be used by plants. Nitrogen fixation by Rhizobium can be increased in the presence of certain elements.

Potassium
Potassium is an essential nutrient for plants and Rhizobium. It helps in the formation of nodules on legume roots, which are the sites of nitrogen fixation. Potassium deficiency can decrease the number and size of nodules, leading to reduced nitrogen fixation. Therefore, the presence of potassium can increase nitrogen fixation by Rhizobium.

Nitrogen
Although it may seem counterintuitive, the presence of nitrogen can actually increase nitrogen fixation by Rhizobium. This is because nitrogen is needed for the growth and reproduction of Rhizobium cells. When there is a low concentration of nitrogen in the soil, Rhizobium cells may not be able to grow and reproduce as quickly, which can limit the amount of nitrogen fixation. Therefore, the addition of nitrogen can actually stimulate Rhizobium growth and increase nitrogen fixation.

Phosphorus
Phosphorus is another essential nutrient for plants and Rhizobium. It is involved in various processes, including energy transfer and DNA synthesis. Phosphorus deficiency can decrease the number and size of nodules on legume roots, leading to reduced nitrogen fixation. Therefore, the presence of phosphorus can increase nitrogen fixation by Rhizobium.

None
While the presence of certain elements can increase nitrogen fixation by Rhizobium, it is important to note that Rhizobium is still able to fix nitrogen in the absence of these elements. However, the rate of nitrogen fixation may be lower, which can limit plant growth and productivity.

In conclusion, the presence of potassium, nitrogen, and phosphorus can increase nitrogen fixation by Rhizobium. However, Rhizobium is still able to fix nitrogen in the absence of these elements.