[en] For selectively excited calcium K X-rays in rice saplings, the relations α=N_KR(1+Aγ+Bβ/(1+α)) and 1/N_Ca=a/β+b+cβ are found to support the enhancement of potassium K X-rays and N_Ca calcium K X-ray counts during potassium and calcium determinations, respectively. Here α, β and γ are weight fractions of potassium, calcium and the matrix substrate, respectively, N_KR is the relative potassium K X-ray counts of the sample to that of pure potassium and A, B, a, b and c are constants. Tests of these relations, using eight known mixtures of CaCO₃, KNO₃ and Na₂B₄O₇10H₂O (borax), have further supported the findings