In this paper, we obtain a characterization of the positive Borel measures
on a homogeneous space X, for which the trace inequality
(integral (x)(I-Phi*f(x))(q) d mu (x))(1/q) less than or equal to C(integra
l (x) f(x)(p) d sigma (x))(1/p)
holds for any f greater than or equal to 0. Here I-Phi is a general potenti
al type kernel, and 1 less than or equal to q <p < +infinity. The character
ization is given in terms of a generalized Wolff's potential.