The subsoiler does not go out of fashion

To ensure an optimal circulation of air and water, soil preparation (both traditional and minimum tillage methods) still avails itself of a periodic deep slicing, carried out with a subsoiler

Agricultural soil is a set of inorganic and organic elements which form a complex structure that is extremely variable in time and space. Therefore, there is no universal solution concerning the preparation of the so-called “seedbed.” Two-layer processing is a technique that became popular about 30 years ago, at the beginning of the “revolution” which, in recent decades, involved open field operations for preparing agricultural soil before sowing. In practice, it consisted of the combination of a deep soil slicing (minimum 50 cm) and classic plowing typically performed at 25-30 cm depth. The purpose of slicing was to break the plow sole that plowing inevitably generates: in this way, cutting and stirring up the soil in depth (without however mixing it and flipping it) promoted the circulation of air and especially the percolation of meteoric water in rainy periods, as well as its capillary rise in those of drought.

Since then, as they say, much water has passed under the bridge, and the developments in the field is oriented towards minimum processing techniques, for which the soil is disturbed as little as possible, and then only in the most superficial layer: thus the various solutions of zero, strip or minimum tillage. The need to optimize the circulation of air and water is however also essential in this case where it is appropriate to perform the slicing, albeit only periodically.

 

The subsoiler

The tilling of the layers of soil underlying the soil being worked is usually performed with an apparatus defined in numerous ways, namely ripper, slicer, loosener, subsoiler, etc., which stirs the soil in a more or less energetic way at depths exceeding 40 cm, without any mixing or flipping, in order to avoid bringing any inert components into the overlying fertile layer.

In versions expressly dedicated to deep processing (more than 55-60 cm and up to approximately 1 m), the subsoiler is essentially composed of a variable number of anchors (from 1 to 11), arranged, if appropriate, on different staggered rows, each consisting of a very sturdy vertical bar, equipped at its lower end (and sometimes even along its vertical length) with working elements adapted in each case to provide the process with the desired characteristics.

It is a machine made for a 3-point attachment, towed by the tractor, and thanks to its considerable weight it penetrates progressively into the ground, down to the desired depth. For its typical mode of operation, it requires a very high tensile strength, and should therefore be coupled with tractors of considerable tonnage (almost always also suitably ballasted) and at the top of the currently available power range, especially for subsoilers that work beyond the 70 cm deep and are equipped with many anchors. Consider also that the tractor is heavily engaged not only in terms of towing capacity, but also for power delivery, since a fast execution of the subsoiling enhances the breaking-up effect on the deep layers.

It is, in general, a conceptually simple device, but for its appropriate operating efficiency, especially in terms of reliability and durability, it requires careful design and a thorough selection of materials.

 

The working parts

The individual elements have various shapes, but are always curved with a progressive concavity facing the direction of movement, and they are made from sturdy steel plates of considerable thickness (even 30-40 mm), subjected to appropriate surface hardening treatments.

The terminal tool may differ depending on the desired break-up effect: the most common configuration provides a terminal tip (sometimes reversible, for an appropriate lengthening of the useful life), which can be complemented by a tungsten steel coating. The tip is often characterized by a more pronounced curvature compared with the supporting structure, and is normally fixed with bolts, for easy replacement in case of breakage or end of life due to wear. The tungsten coating can also be applied to the body of the element, also to increase its useful life in terms of wear.

In particularly rainy areas, and for soils subject to asphyxia, it is useful to create, at the end of the processed area, a channel that can serve as a drainage. To achieve this, at the lower end of the single anchor, an ogive of suitable diameter is hinged at the rear. As it advances, the tool takes a horizontal position, performing a continuous drilling, to collect and channel the water not held in overlying layers.

For greater effectiveness, a particularly deep subsoiling (up to 120 cm) can be combined with the laying of specific drainage pipes. If, however, the purpose of subsoiling is to maximize the movement of soil being processed, so-called “coulters” can be mounted on the individual anchors approximately in the middle of their vertical length. These sturdy fins, horizontal and transverse to the direction of movement, have the function of breaking up most heavily the upper part of the layer being processed.

 

The accessories

In addition to the deeper layers, subsoiling often also stirs and shatters coarsely the superficial layers. To even out the ground surface, the subsoiler can be paired in the rear with one or sometimes a pair of rollers, variously shaped. The most common type includes spikes, consisting of a heavy neutral rotor with a series of curved and tipped plates welded to it in staggered rows. Alternatively, especially in the presence of abundant crop residues, it can be combined with a roller with one or more rows of corrugated disks, which, in addition to evening it out, also ensures a partial burying of the superficial plant material. In any case, the positioning and stowing of the rollers is controlled hydraulically.

The working depth is normally adjusted by means of the tractor’s lifter, but this task may also be performed by mounting on the subsoiler frame a pair of metal wheels, adjustable in height, that rest on the ground, serving as support. In addition, side panels in sturdy sheet metal are usually also available for containing the created clods.

 

Safety devices

Especially in soils with an abundant skeleton, the slicing at depth inevitably involves the impact of the anchors with pebbles and stones of considerable size, with the danger of serious breakage and, above all, long periods of downtime. In this regard, all subsoilers are equipped with appropriate safety devices, aimed at “disengaging” the individual element when a much higher than usual resistance is encountered.

The most simple and economical solution is represented by the classical rupture bolt (usually about 20 mm diameter), which constitutes one of the anchor attachment points to the structure of the subsoiler. In practice, the bolt breaks (in this case, for tensile elongation and not for shear stress, as is the case for other similar uses), leaving the anchor free to rotate backward on the hinge bolt, thus overcoming the obstacle. It is, however, quite clear that in this case the full functionality of the machine is recovered only after mounting another bolt with identical characteristics as the previous one.

The alternative (defined as “non-stop”) is hydraulic: much more efficient, but more expensive without doubt. In practice, each anchor is fixed to the supporting frame of the subsoiler via a lever arm, which is operated by a hydraulic cylinder assisted in its movement by one or more nitrogen-charged accumulators. When the terminal tool encounters an obstacle, it automatically rotates backward and upward, while simultaneously the hydraulic cylinder piston rod retracts, and then quickly makes the opposite movement, bringing the anchor in the usual position, aided in doing so by the “buffer” action of the nitrogen accumulator.

Many subsoiler models are offered with the alternative of the two illustrated solutions.

 

Transport

In versions with a considerable number of anchors, the working width of the subsoiler sometimes exceeds by far the maximum allowable for road transport. Therefore, in these cases the machine is divided into sections that are moved hydraulically with a 180° rotation, to manage the field and transport configurations. In addition, to give the equipment the necessary strength and ability to penetrate conveniently in the soil, the greater working capacity models are characterized by a total weight which may exceed 4000 kg. In this case, the use of very high power and suitably ballasted tractors is required also to confer a sufficient level of stability and safety of the vehicle-operator for fast road transport.

 

Vibrating subsoilers

In this case, during processing the anchors are not fixed, but as they advance they also oscillate, thanks to a car driven by the PTO of the tractor, so as to perform a helical movement. This has the advantage of stirring up a greater amount of soil, while also requiring a smaller tractive strength. On the other hand, the tractor needs to provide power also at the PTO, so the use of tractors with considerable power is still recommended. It is a machine that is particularly suitable for subsoiling vineyards (and more generally specialized crops) and is often complemented by a smooth roller that evens out the limited swelling which this processing produces.