It looks like you need a different model equation. Contrary to the syntax of linear model fitting procedures, the parameters in proc nlin must be explicitly named and included in the model equation. You would need something like:
proc nlin data=work1;
parms xa=0.3 xb=10 xc=0.1 xf=7 xg=18 x0=0;
model salt_tons= a*xa*(b*xb + c*xc + f*xf + g*xg) + a*x0;
ods output ParameterEstimates=select;
run;
where xa, xb, xc, xf, xg, and x0 are the unknown parameters and a, b, c, f, and g are your observed variables.
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