in the right condition for mole ploughing. If the hole stays intact without splitting the ball, the soil is in ideal condition for the mole plough. Heavy land requires draining to reduce its water content to a level efficient for plant growth. Heavy soils usually have a system of permanent drains, using perforated plastic or clay pipes that discharge into a ditch. The small tunnels (mole drains) that mole ploughs form lie at a depth of up to at an angle to the pipe drains. Water from the mole drains seeps into the pipes and runs along them into a ditch. Mole ploughs are usually trailed and pulled by a crawler tractor, but lighter models for use on the three-point linkage of powerful four-wheel drive tractors are also made. A mole plough has a strong frame that slides along the ground when the machine is at work. A heavy leg, similar to a sub-soiler leg, is attached to the frame and a circular section with a larger diameter expander on a flexible link is bolted to the leg. The bullet-shaped share forms a tunnel in the soil about diameter and the expander presses the soil outwards to form a long-lasting drainage channel. Para-plough The para-plough, or paraplow, loosens compacted soil layers 3 to 4 dm (12 to 16 inches) deep while maintaining high surface residue levels. It is primary tillage implement for deep ploughing without inversion. Spade plough The spade plough is designed to cut the soil and turn it on its side, minimising damage to earthworms, soil microorganism and fungi. This increases the sustainability and long-term fertility of the soil. Switch plough Using a bar with square shares mounted perpendicularly and a pivot point to change the bar's angle, the switch plough allows ploughing in either direction. It is best in previously-worked soils, as the ploughshares are designed more to turn the soil over than for deep tillage. At the headland, the operator pivots the bar (and so the ploughshares) to turn the soil to the opposite side of the direction of travel. Switch ploughs are usually lighter than roll-over ploughs, requiring less horsepower to operate. Effects of mould-board ploughing Mould-board ploughing in cold and temperate climates, down to , aerates the soil by loosening it. It incorporates crop residues, solid manures, limestone and commercial fertilisers alongside oxygen, so reducing nitrogen losses by denitrification, accelerating mineralisation and raising short-term