For heavy clay fields and paddocks that stay waterlogged through the winter. Mole ploughing pulls a network of underground channels through the clay, so surface water can drain away to a ditch or existing drain, at a fraction of the disruption of laying pipes.
A mole plough has a single deep leg with a steel cylinder, the "bullet" or "mole", on the bottom, followed by a slightly larger expander. Drawn through the clay by a powerful tractor, it forms an unlined round channel typically around 45 to 60 cm down. The leg also cracks the soil above, letting water run down into the channel.
The channels are pulled across the slope at a spacing of a few metres, so they fall towards a ditch or a permanent pipe drain with gravel backfill. That outfall is essential: a mole channel with nowhere to empty just fills up.
Mole channels are not permanent. In a suitable clay they can last several years before collapsing, after which the field can be mole ploughed again.
Mole ploughing needs the right soil at the right moisture. The subsoil at mole depth should be moist and plastic enough to form a smooth, firm channel, while the surface is dry enough for the tractor to grip and for the leg to crack the soil above. In many areas that means late spring or early summer.
Too dry, and the channel crumbles; too wet, and the walls smear and the tractor loses traction. A dry spell after the channels are pulled helps them set firm.