The heat extracted by heatexchangers from the garden soil is called by the amateur frequently "geothermal" energy. Which it isn't. Its mainly solar energy trapped for a while in soil. Geothermal energy in it's true meaning is the energy from the inner part of our planet, produced by nuclear energy.See this link: [broken link removed]
The garden soil heatexchanger is only economical efficient -if at all- when the temperature demand that is to be covered is rather low. Ideal would be a large surface like an UFH or a wall heating or -even better-the combination of the two.
Running radiators on low temperature is only sufficient when the energy demand of the building is low as well, for example in a modern low energy building.
Otherwise the backup energy (usually expensive peak demand electricity)needed to raise the low temperature derived from the soil heat exchanger would be very uneconomical for the consumer.