  The low-energy S-wave pi N and K N scatterings are studied by the K-matrix
approach within the meson exchange framework. The t-channel meson exchanges,
especially rho and sigma exchanges, are found to play a very important role in
these two processes. The t-channel rho exchange determines the isospin
structure of the scattering amplitudes, it gives attractive force in the low
isospin state but repulsive force in the high isospin state. The t-channel
sigma exchange gives a very large contribution in these two processes, while it
is negligible in meson-meson S-wave scatterings.
