The nonlinear ac-field dependence of dielectric and piezoelectric coef
ficients in soft PZT piezoceramics is studied at f=100 Hz. We demonstr
ate that two different thresholds for the onset of nonlinearity exist
at E(c2)approximate to 1 V/cm and E(c1)approximate to 100 V/cm, respec
tively. At weak fields, the ac-field dependence of the effective diele
ctric coefficients obeys a scaling law x=x(0)+A[(E-E-c)/E-c](phi) with
the effective coefficient phi=1.5+/-0.1. Above the large held thresho
ld, both dielectric and piezoelectric coefficients exhibit a non-analy
tic scaling behavior. The nonlinearity is here anisotropic which is re
flected by exponents phi=1.0+/-0.1 and phi=1.2+/-0.1 for fields parall
el and perpendicular to the poling direction of the ceramic, respectiv
ely. The nonlinear domain wall response at fields well below the elect
rical depoling held is apparently dominated by the depinning of domain
walls rather than by the anharmonicity of the potential energy of the
wall.