Parahydrogen-induced polarization (PHIP) can deliver nuclear spin hyperpolarization to suitable substrate molecules in the form of proton singlet order. However, many applications require that heteronuclear spins in the molecule (e.g., C or 15 N) be hyperpolarized. In this work we present a simple and convenient experimental approach to optimize the transfer of polarization from protons to C spins in multinuclear substrates with the natural isotope abundance. We used isotropic mixing in arbitrary heteronuclear spin systems in an ultralow field (ULF) to optimize the magnetic field sweep profile that provides the maximum polarization of C. To support our approach, we first conduct a set of experiments to elucidate the coherent nature of polarization transfer at ULF. We test the maximum achievable C polarization in various experimental protocols, including continuous hydrogenation at ultra-low magnetic field, coherent PHIP transfer induced by sudden field variations, and adiabatic transfer via magnetic field sweeps. Such direct comparisons of different methods highlights the efficiency and robustness of adiabatic transfer compared to other methods. Next, we demonstrate a facile route to optimize adiabatic sweep experiments by calculating the optimal profile of the field variation. More specifically, we suggest a twostep procedure. In a first step, we simulate and/or experimentally detect the field dependence of C polarization. Experimental detection in this case includes catalytic hydrogenation of the substrate molecule with pH2 in a range of ultra-low fields. This indirectly indicates the conditions of avoided level crossings, or level anti-crossings (LACs), even for complex heteronuclear spin systems. In a second step, using a graphical inversion of the data by interchanging the axes, we obtain an experimentally optimized magnetic field profile for sweeping the field from zero to a few microTesla. The field profile that was thus experimentally determined is compatible with the constant adiabaticity approach proposed in the earlier work : the slope of the field profile is small and the magnetic field changes slowly in the vicinity of the maxima of the field dependence, and the field profile is steep at the edges far from the LAC conditions. To preserve the polarization, the field sweep has to be short in comparison with nuclear relaxation times, and the total duration of the field sweep can be tuned by linearly stretching the profile along the time axis for better efficiency of polarization transfer.