Harrows.Harrows are the most important machines for seedbed preparation. They are still used in low input agriculture to bury the seedlings spread by hand or with disk broadcasters. In a similar way, they serve to incorporate fertilizer. They have to perform crumbling, loosening, mixing, leveling and to a certain extent stratifying the seedbed. At the same time, they are used to control weeds after the emergence of the actual crop. A further task of harrows is stubble cleaning; that is, cutting, loosening and mixing of the root zone in order to induce germination of fallen out grains and prepare the field for primary tillage.
The conventional harrow consists of tines grouped together in a frame and several of these frames grouped together for the complete harrow; various tines of different intensity (rigid or spring) and additional rollers for better crumbling and good leveling are available (Fig. 12). These elements are grouped in carrier frames for seedbed preparation. The tillage intensity is comparatively high. A harrow with a smaller working intensity is the chain harrow (Fig. 12), which adapts well to any soil profile or ridges, and which is used especially for weed control and for leveling dams and ridges.
Figure 12. Harrow, harrow elements with rigid and spring tines, roller with cutter bars and chain harrow (right below) with tines.
Spike Tooth Harrows.(Fig. 13) These consist of simple frames with tines bolted to them where the frame members cross. There is a variety of harrow sizes and weights from light harrows for final seedbed preparation to heavy harrows used in some areas for breaking down ploughed land. Spike tooth harrows will be found in many farmyards but their use has declined in recent years. Traditionally these harrows were used in sets of four or more, attached to a bar or pole with a wheel at each side, often towed behind a cultivator or roll.
The modern method is to suspend a number of harrows with chains on a three-point linkage mounted frame which folds for transport. Pressure harrows up to 8 m wide or more on a mounted frame with a closed hydraulic system to distribute the weight of the toolbar equally over the sections are a more recent development. By equalising the pressure, each harrow follows the ground contours more accurately and the individual harrow frames have a levelling and scrubbing effect. Small diameter crumbier rollers or open coil rollers can be attached to the rear of the implement to consolidate the surface.
It is important to keep harrow tines tight. Failure to do this will wear the holes in the frame.
Plate 13. Spike link chain harrow.
Chain Harrows.The links may be plain or spiked. Plain linked harrows are used by some farmers to spread droppings and molehills on pastures or to cover seed. Grassland can be aerated with a spiked link chain harrow.
Figure 14. Disk harrow (half in transport position and half in work position) (left) and rotary spade harrow (right).
Disc harrows.A machine with a very great importance is the disk harrow. It usually consists of several sets of disks on a horizontal axis, which are angled towards the direction of travel with an adjustable angle (Fig. 14.). This allows to control the intensity of the soil engagement of this machine. In order to avoid lateral forces and to avoid soil transport to one side, there usually are two (symmetrically) angled sets of disks grouped one after the other. In order to achieve central force application, disks of one row are angled in two opposite direction. Due to its scraping effect, the disk harrow is well-suited for stubble cleaning. The comparatively strong inverting effect allows tillage with disk harrows alone as its effect comes close to that of a disk plow. At the same time, it can also be used for secondary tillage to break clods, create a seedbed and level the soil after plowing.
A problem of this device can be that it may create a smear zone; furthermore, it is not pulled into the soil by the form of its implements, which means that it must be fairly heavy in order to sufficiently penetrate into the soil.
Disc harrows cut and consolidate the soil. Two or more sets of saucer-shaped discs are fixed to a frame which may be mounted or semi-mounted. It is more common for the larger models to be trailed with large diameter transport wheels raised and lowered with a hydraulic ram. Disc diameter varies from 300-750 mm or thereabouts and each set of discs is carried on ball or taper roller bearings. Some machines have plain discs while others have scallop edged discs, usually on the front gangs, and plain discs at the back. Plain discs have a better cutting and consolidating effect and scallop edged discs give better penetration in hard ground.
Working widths vary from 2.75 to 7m (9-23 ft) or more. Mounted disc harrows will be transported on the lift linkage and trailed models have a pair of pneumatic tyred wheels lowered by a ram for transport purposes. Wider disc harrows are folded hydraulically for road transport. Some models have three gangs wide with folding wing sections. The double tandem disc harrow (Fig. 14) has all four gangs rotated inwards with the angling rams to give a transport width of about 2.5 m. Another design (Fig. 15) has both gangs raised to the vertical position for road haulage.
Plate 15. Hydraulically folded disc harrows in the transport position.
Discs are used by some farmers in combination with other equipment, including tined implements or a roll to make a seedbed in a single pass.