The trouble with planting trees to save the environment by sucking up carbon dioxide is that we tend to cut them down for other uses, in some instances releasing the carbon back into the atmosphere.
Carbon farming aims to trap carbon in the soil and in the vegetation on top of the soil by growing plants that will never be cut down. This means that the areas chosen for carbon farms are usually not suitable for anything else. It means that the local vegetation is probably the best choice because it will survive without any assistance or intervention.
Precisely because the soil is usually not very good, estimates of the amount of carbon to be trapped can be hopeful rather than realistic. It has been pointed out to the Federal Government that paying carbon farmers in advance for what they estimate will be their carbon harvest is not necessarily a good idea.