We loosen the soil to eliminate its harmful compaction. Over time and with continuous use, our soil becomes compacted and silted, resulting in water stagnation in spring, inability to absorb heavy rains, surface runoff, and rapid drying in summer. Plants suffer on such soil, drowning in water in spring, drying out in summer, with roots unable to grow downward and generally lacking air. There are a few exceptions where a penetrometer can be inserted 50 cm into the soil at any time, but most soils in our country need to be loosened with a medium-deep subsoiler.
Characteristics of cracking
Let’s see how loosening works! Essentially, the subsoiler loosens by cracking. The tip of the subsoiler presses against the soil in front of it and tries to move it upward, causing it to split. The soil cracks in a semicircular arc in front of the tip, depending on the soil properties. On heavy, dry soil, these cracks can be felt on a subsoiler with few blades, as they intermittently stop the tractor.
Variation of crack angle with blade spacing and soil cohesion
What follows from this? Firstly, wings placed on the side of the spine are not very useful, as they operate in an already loosened section, but we’ll return to this. Wings are placed below, where they widen the section cracked by the tip. The heavier the soil, the flatter the crack angle, hence the less useful the wings are. This poses a good dilemma: what is the shallowest peak between the work of two blades?
Winged and wingless subsoiler heads
Based on this, in drier summers and on heavier fields, we can use wider blade spacing or omit the wings, while in less extreme cases, wings and denser spacing are needed. So why don’t we use densely spaced wingless subsoilers? Simple, the spine plate has resistance, so it’s more efficient to widen the working head.
Critical working depth and distribution of depth of cut
The next theoretical concept is the critical working depth. This limits the depth of loosening, so you can’t loosen at any depth by force. In other words, it can be done, but it’s not loosening. So, if we imagine our loosening head lifting the soil forward and upward, the deeper we work, the more mass it lifts and moves, the greater the pressure on the loosening head. The steel is still there, but after a while the load is so great that the soil near the head gives way and tends to bypass the head somehow, practically compacting. In this way, we do exactly the opposite of what the goal would have been. This can occur by increasing the depth, i.e. the load, or by decreasing the bearing capacity of the soil. That’s why we don’t loosen wet soil, because we would just be compacting a canal below.
This aligns with the fact that the deeper we go, the harder it is to pull the loosener. So here comes the next concept, the distribution of depth of cut. Or, how to loosen deeply if we don’t have a big tractor. Gradually. It’s not hard to imagine that it’s easier to work on raw stubble than on disked stubble, or even plowed, grubbed land. At first, it seems like nonsense because we trample the previous work, and there will be some loss due to the loose surface, but it’s still better than struggling and tearing the machine at a depth of 30 cm because we don’t have a bigger tractor. With depth of cut distribution, we only need to add the part to be deepened under the previous operation, so the goal can be achieved gradually. Oh, and there’s one more significant advantage. The surface will be nicer, and ideally, no wild soil will come to the surface.
Soil elevation rate with straight and curved shanks
And here we can turn to the shape of the shank. We can encounter two different trends: one is the curved shank, the other is the straight one. The curved shank is considered more traditional, like the old IH, Jimpa, or Vibrolaz. Its spread is due to its lower draft resistance. This is because the soil cracked by the tip can gradually roll on the shank arriving at a flat angle, while the straight shank cuts it. However, this results in significant differences in work quality. The work of curved blade subsoilers is much more uneven, with much greater surface elevation. They turn out huge clods, which are then tried to be broken up with rough spike rollers. This is not a problem if we plan the primary tillage with a powered tool, which will manage it, but a grubber might just roll these clods under the surface. An interesting aspect of this is that switching from a curved shank subsoiler to a straight one, the resistance will be surprisingly high on the first pass because it will go into a depth that has not been loosened before. The previously measured loosened depth will not correspond to the unloosened one now.
Here I would turn back to the wings attached to the side of the shank plate. Those wings, as well as the spikes placed at the front of the shank, acknowledge the above. However, their additional resistance may be more than that of the straight shank.
The first BUSA subsoilers also had forward-reaching, curved blades. We developed the straight-blade version when, working on our own fields, the subsoiler brought the subsoil to the surface. The clay could slide up on the relatively long head and wide shank.
Currently, we manufacture our subsoilers in two-row or V arrangements with 3, 5, or possibly 7 working elements. The working elements are shear bolt protected, and their spacing can be adjusted from 520 mm to 700 mm, thus achieving appropriate loosening of the medium-deep layers of the soil. In its standard configuration, the implement comes equipped with winged subsoiler tools. The tips of the tools are made of hardened steel or, as the latest development, carbide inserts. The working depth can be adjusted with the tractor’s hitch system, a limiting wheel, but most commonly with the closing roller element. The working depth can be adjusted between 300 mm and 500 mm. Each of our subsoilers can be ordered with a closing element, which can optionally be connected to the implement with a hydraulically adjustable three-point hitch. It is also possible to equip different profile rollers and disc sets.