Free movement of air in the soil and good drainage are essential for high crop yields. Tractors and heavy farm machinery are a major cause of soil compaction and soil pans, which restrict the movement of air in the soil and hinder drainage in arable land.
Subsoilers are used to break compacted ground to improve drainage. Most subsoilers are mounted on the three-point linkage and, depending on the size of tractor, have between one and five legs with shares at the foot. Normal working depth is from 400-500 mm. Power requirement is high: a typical three-leg subsoiler working at 500 mm will need a tractor of at least 80 kW (110 hp). Trailed subsoilers with up to nine legs are made for tracklayers and very large four-wheel drive tractors. Each share has a replaceable point and it is usually possible to fit new shins - the front vertical cutting edge of the leg - when they are worn. A pair of wings is often attached to the shares to increase the shattering effect on the subsoil.
Plate 23. A seedbed can be prepared in a single pass with this cultivator. The rear wheels are lowered for road transport.
Greater shattering of the subsoil is achieved with a vibrating subsoiler share. One type has a power take-off driven vibrator unit which agitates the shares as they pass through the soil. The agitation has a good shattering effect below the surface but does not heave the soil upwards to the same degree as a fixed subsoiling share.
Subsoiling is best carried out when the ground is dry and hard, for example after the pea or cereal harvest, to obtain the maximum shattering effect. It is usual to subsoil at right angles to the intended direction of ploughing. The spacing between passes of the subsoiler legs varies from 1-2 m.
Mole ploughs.Heavy land requires draining to reduce its water content to a level satisfactory for efficient plant growth. Heavy soils usually have a system of permanent drains using either perforated plastic or clay pipes which discharge into a ditch. Mole ploughs are used to form small tunnels (mole drains) in the soil at a depth of up to 950 mm (30 in) at an angle to the pipe drains. Water from the mole drains seeps into the pipes and runs along them into a ditch.
A simple check can be made to find if the subsoil is in the right condition for mole ploughing. Compact a tennis ball sized sample taken at moling depth by hand, then push a pencil through. If the hole remains intact without splitting the ball the soil is in an ideal condition for the mole plough.
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 very strong frame which slides along the ground when the machine is in work. A heavy leg, similar to a subsoiler leg, is attached to the frame and a circular section share with a larger diameter expander on a flexible link is bolted to the leg. The bullet shaped share forms a tunnel, about 75 mm diameter, in the soil and the expander presses the soil outwards to form a long-lasting drainage channel.
Chisel ploughs.Chisel ploughs are a type of tined cultivator with a heavy duty frame and a number of equally strong tines. Each tine has a replaceable share and most models have a shear bolt at the foot of the tine. When the tine hits an underground obstruction the shear bolt breaks, allowing the share to swing backwards and avoid unnecessary damage. Rigid tines are most common but some farmers prefer spring-loaded or heavy duty flexible tines.
Chisel ploughs have working widths of 2-5 m, they are tractor mounted and working depth is hydraulically controlled. The wider models usually have a wheel at each side to control working depth. The tines are arranged in two or three staggered rows and their number depends on power available, type of work and soil condition. Power requirement for chisel ploughing varies from 45-120 kW (60-100 hp).
Plate 24. Three-leg subsoiler.
Plate 25. Subsoiling.
Plate 26. This trailed mole plough requires a wheeled tractor of at least 150 kW (200 hp) or a 120 kW (160 hp) crawler.
Chisel ploughing is considered by some farmers to be an alternative to mouldboard ploughing, especially in arable areas when there is little surface rubbish to bury. The straw burning ban has seen a marked decline in chisel ploughing, but this implement is sometimes used to break stubble after harvest. The chisel plough, usually with three tines, is sometimes used as a subsoiler.