Please see the image. Since plant mass would be related to how much nitrogen it receives from the Rhizobium, something is more effective in some strains (3, 4) interaction with the plants than in others (2, 5). This could have to do with how well the plants' roots are colonized, how well the bacterium fixes nitrogen, or how well the exchange of nutrients with the plant occurs.
Work Step by Step
You need to see that the only variation in the experiment has to do with the strain and that the growth of the plants should be related to the receipt of nitrogen from the bacterium. The variation should then be due to one or another factor in the fixing of nitrogen and its being turned over to the plant.