Subsoiling and chiselling in conservation agriculture
The deep tillage of soil is a periodic practice, to be carried out selectively, depending on the specific requirements. Sustainable management of agricultural productions involves the use of various equipment, such as subsoilers, chisels and cultivators, aimed at restoring the most suitable physical structure of different agricultural soil horizons
Since agricultural soil is a mixture of inorganic and organic components forming a complex structure extremely variable in time and space, it is now well established that there is no universal solution for what concerns the preparation of the so-called "seedbed." Starting with the classic combination of plowing + harrowing, the evolution in the subject is increasingly moving towards minimum tillage techniques, for which action is taken only on the topmost layer and not always on the entire surface. Hence the various solutions: minimum tillage, strip tillage, no-tillage, or sod seeding when sowing directly on turf. This is a key step in the implementation of so-called "conservation agriculture," which aims to minimize the alteration of soil composition, structure and natural biodiversity, safeguarding it from degradation and erosion.
The main difference with plowing is that the latter has the main purpose of inverting soil layers, burying deep into the surface soil rich in plant residues (and any additional inputs, such as organic matrices like manure, compost, and the solid fraction of digestate), in order to promote their decomposition and humification. Conversely, minimum tillage equipment tends to remix the layer on which it acts (with a maximum depth usually not exceeding 15 cm), distributing plant material, if any, more or less evenly throughout the entire intervention horizon, including the surface.
Moreover, the constant passage of machinery in the field, which is increasingly powerful and high-performance, but also progressively heavier, has a negative effect on the soil structure - compaction - which causes a reduction in the empty spaces within which air and water circulate and prevents the expansion of the plants' root system. Periodic tillage is aimed precisely at restoring an optimal soil structure, but playfully, one intervenes only on the topsoil. Still, nothing is done to improve the situation in the layer below, the "subsoil," which is, in any case, explored by the roots of cultivated plants and which thus undergoes progressive degradation. In addition, the typical action of some implements, such as the plow and the hoe, leads to the formation of the so-called "soil pan," which is a layer of a few centimeters strongly compacted, placed immediately below the tilled horizon, which is an additional obstacle to the normal development of the root system (see box).
Soil cracking
To solve the described problems, it is necessary to intervene at depth, trying to restore even then a physical structure of the soil that allows proper air and water circulation.
The purpose of soil cracking is, therefore, precisely to break up the sole that is generated by the plow and hoe: by cracking and loosening the soil in depth (without, however, remixing it and turning its horizons over), the aeration and, particularly, the percolation of meteoric water in rainy periods, as well as its capillary rise in drought periods, is promoted. This type of tillage becomes even more important if one decides to adopt a minimum tillage technique, both because the deep layers are no longer disturbed but are still subject to progressive compaction due to machinery traffic and because in the transition from the traditional to the minimum tillage technique it is necessary to break up the sole that has been created over time by repeated plow intervention. This is an operation that does not require annual periodicity but can be performed every 3-5 years or even less frequently whenever a reduction in crop productivity related to a degradation of the physical state of the soil is noticed.
Subsoiling or chiseling?
One of the three pillars on which conservation agriculture is based is the reduction of agrarian soil tillage, aimed at recreating the biological balances necessary for the development of viable, fertile agricultural ecosystems capable of generating environmental benefits. Hence, minimum tillage techniques are implemented, which, however, require deep periodic interventions. Tillage of the soil layers below the tilled soil is usually carried out with different equipment, which in the Italian language are defined in numerous ways (i.e., subsoiler, chisel, dissolver, decomposer, disassembler etc.), but traceable in good substance to two types, which in English are the chisel and the ripper or subsoiler. Bearing in mind that the literature in the field is by no means in agreement in defining the various implements since there are many models available with intermediate characteristics, the real difference lies in the disruption characteristics: the chisel coarsely dislodges and crushes the worked layer, thus creating a certain elevation above the compacted soil, while the subsoiler aims to make the deep fissure only, with no apparent surface disturbance, or almost. A further important difference lies in the depth of tillage, which for the chisel is usually no more than 40-50 centimeters, while with the subsoiler, it can sometimes exceed one meter.
The subsoiler
This is a conceptually simple piece of equipment, but one that requires careful design and choice of materials. Basically, it is composed of a variable number of anchors (from 1 to 11), where appropriate, arranged in different staggered ranks, each consisting of a very strong vertical bar equipped at its lower end (and sometimes also along its vertical development) with tools suitable for carrying out from time to time the work with the desired characteristics. The subsoiler is an operating machine brought to the three-point hitch, which, thanks to its conformation and its considerable weight, progressively penetrates the soil to the desired depth. Because of its typical mode of operation, it requires a very high tractive force and must, therefore, be coupled to tractors of considerable tonnage (almost always also suitably ballasted) and, of top-of-the-line power, especially for those subsoilers equipped with anchors, which work over 70 cm in depth. It should, in fact, be considered that the tractor is severely engaged not only in terms of pulling capacity but also in the delivery of power since the fast execution of the subsoiling improves the disrupting effect of deep layers. The anchors have various conformations, either straight or curved, with the concavity facing in the direction of advance, and are made of strong-shaped plates of steel of considerable thickness (even 30-40 mm), subjected to appropriate surface hardening treatments. The end implement can differ according to the desired disruptive effect: the most common configuration involves an end ferrule (sometimes with reversible mounting, for appropriate life extension), which a tungsten steel coating can complement. The ferrule is characterized by a more pronounced curvature than the supporting structure and is normally bolted to facilitate its replacement in case of breakage or end of life due to wear. Tungsten coatings can also be applied to the element body to increase its wear life.
The chisel
The depth of tillage, in this case, is lower than that of the subsoiler, typically being between 30 and 50 cm. The main difference, however, lies in the mode of soil disruption produced by the working tools, which, in this case, take on a typical arched shape with an interchangeable end tool because of wear and tear issues. Chisels also have the working organs arranged in several rows in a staggered manner, but with shorter distances than the anchors of the subsoilers and in greater numbers (up to 20 or more in the most productive models). The tractor to be coupled to the implement must again be of high power to ensure that high forward speed, which increases the effectiveness of breaking up the tilled soil. In any case, the tractive force required is also important, such that appropriate ballasting of the tractor is also recommended here. For these reasons, the chisel exerts an effective stirring action on the tilled layer (and any crop residues present on the surface), such that it is considered the staple of cultivators, i.e., the equipment of choice for tillage in conservation agriculture (see section below). To improve tillage efficiency, each working organ of the chisel is often equipped with a horizontal triangular blade placed at the lower end of the arc, sometimes supplemented by a second similar blade set in the middle of the curved development.
The accessories
With the chisel, the surface layer is broken up and coarsely crushed; soil surface leveling can be achieved with one or a pair of rear rollers, variously shaped. The most common type is the spike rollers, consisting of a heavy idler rotor on which a series of strong, pointed, curved plates are welded in staggered rows. Alternatively, especially in the presence of abundant crop residues, a roller can be combined with one or more rows of wavy disks, which, in addition to leveling, also ensure partial burial of the more superficial plant material. The working depth is normally adjusted through the tractor lift, but this task can also be met by mounting a pair of height-adjustable metal wheels on the equipment frame, which rests on the ground, acting as a contrast.
Safety devices
In soils with plenty of skeleton, deep discing inevitably involves the impact of working elements with stones and stones of considerable size, with the danger of serious breakage, and particularly long periods of machine downtime. For this reason, chisels and subsoilers provide appropriate safety devices aimed at freeing the individual element from a much higher-than-usual load. The simplest and most economical solution is the classic break bolt (typically 20 mm in diameter), which is one of the anchor attachment points to the implement frame. The bolt breaks under tension, leaving the anchor free to rotate backward on the other hinge bolt, thus overcoming the obstacle. The full functionality of the machine is recovered by putting another bolt with identical characteristics to the previous one back in place. Reversible mechanical or hydraulic (referred to as "non-stop") technologies, which are more efficient but also more expensive, are also available on the market. In the former case, each working organ is hinged to the subsoiler's supporting frame by means of a strong spring, spiral or leaf spring, which is capable of bearing a load of several hundred kilograms. Alternatively, a hydraulic cylinder assisted in its movement by one or more nitrogen accumulators is used. When the end implement encounters an obstacle, the automatically reversible device allows a backward and upward rotation and then quickly performs the opposite movement, returning the tool to its usual working position when the overload is overcome.
The cultivator
It is the equipment at the base of tillage in conservation agriculture. It has the task of preparing the seedbed in a single pass, stirring up the tilled layer instead of performing the reversal of horizons implemented by ploughing. When contextual action is needed in the deep layer, often the subsoiler (to operate a periodic discing of the deep layers when required), or more often the chisel (for effective disruptive action), represent the most important element of the cultivator, which is then complemented with a very wide range of other working organs (discs, rollers, hoes of various conformation) dedicated to the stirring and breaking up of the surface layer.
The soil pan
This is a well-known problem in tillage due to the establishment of a compact layer, a few centimeters thick, just below the horizon within which the implement is deployed. It is caused by the typical ways in which the working organs disrupt and/or break up the soil: e.g., the plowshare of the plow, which crawls with a certain amount of pressure on the bottom of the furrow created, or the curved hoes of a tiller, which with their rotary motion, combined with the low forward speed, contribute to forming a compact layer. The phenomenon is exacerbated if, over the years, the soil is worked repeatedly at the same depth and is aggravated on soils predisposed to clogging. The tillage sole hinders water drainage and air circulation, depressing plant development, both due to hypoxia of the root system and the inability to explore horizons below the tilled one.
The drainage cone and the wings
In extremely rainy areas and for soils subject to asphyxiation, it is useful to intervene with a subsoiler that can create drainage cones at the bottom of the tillage. For this purpose, at the lower end of each anchor, a drainage cone of 60-100 mm diameter is hinged in the rear position, which, during the advance of the implement, is arranged horizontally, making a continuous perforation suitable for collecting and channeling water not retained in the layers above. For greater effectiveness, extremely deep subsoiling (up to 120 cm) can be combined with the deposition of specific drainage pipes. If one of the purposes of subsoiling should also be to increase the disruption of the soil affected by the operation, so-called "wings," i.e., strong fins with a horizontal orientation and transverse to the advance, can be mounted on the individual anchors at about half of their vertical development, which have the function of dislodging and disrupting more of the upper part of the layer affected by the work.
