Biological Nitrogen Fixation by Local and Improved Genotypes of Cowpea in Burkina Faso (West Africa): Total Nitrogen Accumulated can be used for Quick Estimation

Figure 2

Relationship between Ndfa-ID and Ndfa-TND, as well as the partitioning of genotypes into two categories by utilizing a graphical method (Cate and Nelson, 1971). The two perpendicular lines (vertical and horizontal) separate the genotypes into two categories by maximizing the number of points in two quadrants or categories. By maximizing the number of points in two categories, the point where the vertical line intersects the X-axis (27 kg Ndfa-TND) divides the genotypes into categories A and B (high and low Ndfa-TND). Ndfa-TND and Ndfa-ID: nitrogen derived from the atmosphere with total N difference and isotopic dilution methods, respectively.