SUBSOILING IN FORESTRY - Shank Designs available for Subsoiling with Savannah Subsoil Plows

SUBSOILING IN FORESTRY:

Over the years, there's been a lot of discussion about subsoiling for the forestry or agricultural industries. With current widespread interest in subsoiling for forest establishment, it is probably relevant to discuss some of the terminology and design techniques used in this industry.

"Subsoiling" is an agricultural term used to describe "fracturing" of the lower strata of soil (the subsoil). Fracturing is done mainly to break up the soil and leave it in place. Then, in due course, a seedling is planted which receives dual benefits from the subsoiling treatment. The roots are able to grow through the fractured soil more easily, and moisture from rainfall is able to move down and be held in the soil for a longer period. Subsoiling is usually a continuous operation performed over large areas of relatively low return so the cost of subsoiling is extremely relevant and can be a constraint when deciding on the viability of a project. There is often an economic limit to the amount of money that can be spent to improve the rooting ability and the moisture retention capability of that soil.

Tine Design for Forestry Subsoiling

At Savannah, we've developed many designs for subsoiling tines over the past ten years. It has been an ongoing exercise in research and development, as this is a field where not a lot of previous work had been done. Documentation on subsoiling mainly relates to purely agricultural situations. Some extensive trials done in England in the 1970s showed that the most efficient method of fracturing the soil was to have two smaller and shallower tines forward and to the side of the main tine, which had a wing fitted. These smaller tines, which had a lower draft requirement, fractured the top ground layer, allowing the wing on the main tine to be more effective in the fracture of the lower subsoil. This configuration is inappropriate for a forest site where there is sometimes significant debris.

Subsoiling in a forest site is completely different to agricultural applications, due to the surface debris and root matter below the surface. It is completely unacceptable for an operator to have to spend time backing up to clear debris, or if the tine pulls stumps completely out of the ground. Savannah subsoil plows now rarely uproot stumps. You will see us marketing 'parabolic' tines, but these are definitely not for sites with debris - only for agricultural or farmland soil fracture. The reverse of a parabolic tine is our 'swept' tine, used on a lot of our subsoil plows where they are working through significant amounts of debris. In the USA, almost all sites are considered 'second rotation' sites, as they have already had at least one rotation of trees.

Optional shank designs available for subsoiling.

Optional shank designs available for subsoiling:
(a) 'Swept' ripper shank used for ripping through stumps and logging debris with the big Magnum Subsoil Plows.
(b) General-purpose vertical shank, as used in Tomahawk 446 Subsoil Plow.
(c) Semi-parabolic shank, as used in new Tomahawk 446 Subsoil Plow.
(d) 'Parabolic' ripper shank, designed to rip down to over one metre deep behind Cat Challenger Tractors. Features hard-faced shank wearplate.

Coulters and Tines: A Combination Tailored for Forestry

To cut through surface debris and root material in front of the tine, Savannah plows use a 48" diameter high-tensile steel coulter. This coulter is mounted on an extra-large set of tapered roller bearings capable of taking the weight of the back of the dozer, should the coulter blade encounter an immovable obstruction, such as surface rock. The large diameter allows the coulter to be set at a working depth of 12 inches, which is sufficient to cut through material that might wrap around or get caught on the vertical front edge of the tine. It acts like a giant pastry cutter. On most stumps, the coulter will cut deep enough into the stump to allow the tine to pass through this sliced area. The tine will continue to fracture the soil around and under the stump without pulling the stump out of the ground - a perfect result.

We probably still don't have the perfect tine design, but we have learned that different designs can dramatically improve efficiency. The combination of the coulter and the swept-back tine as normally supplied on our 410 and 420 subsoiling plows, is extremely effective in heavy slash and root mat. If root material gets under the coulter blade, the swept-back leading edge of the tine will force the debris deeper into the soil, where there is likely to be more resistance, allowing the tine to break through the obstructing root matter.

'Swept' ripper shank and coulter on 350 trailing subsoil plow.

Wings and Efficient Soil Fracture

The use of a ripper point with "wings" to assist in uplifting and fracturing the lower strata is well known. Not as well known is the fact that efficiency is improved if the wing is set back from the leading edge of the point. The wing assists in soil fracture by directing an upward force as it moves through the subsoil layer. This force is used to fracture the soil to the upper ground level. The force (power) required depends on the operating depth and the area of the top surface of the wing. The power available at the drawbar of the tractor limits the working depth and the size of the wing that can be used and this will have an effect on the amount of soil fracture. You will hear the term "soil fracture volume - which refers to the amount of fractured soil which is available to the seedling. There is no hard and fast rule for the size of the wing, as soil conditions, horsepower availability and client requirements, will all have a bearing on what is used. It is generally considered that unless you need to get down very deep to break up a specific compacted layer, 45-60cm (18-24") is sufficient depth for subsoiling. If the extra horsepower is available, a wider wing could be used to increase soil fracture volume.

Magnum 410 Subsoil Plow fitted to a 300hp dozer

The wings on a tine take the brunt of wear and tear when subsoiling, so Savannah subsoiling tines now have a replaceable wing, positioned in a slot cut through the tine. This system allows the operator to replace the worn wing simply by cutting or grinding the small weld on the keylock bar and, after knocking the keylock out, sliding the worn wing out and a new one in. The new keylock is fitted and tack welded into place. This is done independently of replacing the point. In the USA, Savannah wings are made from Astralloy, one of the most durable materials available. New Savannah 'parabolic' ripper tynes now also have a replaceable shank plate.

Better Designs for Lower Horsepower Machines

Traditionally, D7 or D8 size tractors have been the norm in subsoiling heavy clay or "Piedmont" areas. Smaller landowners, however, seek similar results with lower horsepower tractors, providing a challenge for equipment manufacturers like Savannah Equipment. In the past, the tendency has been to modify heavy-duty plows to pull behind less powerful tractors, but this has been done at the expense of excessive wear and tear on the tractor.

Current design developments are producing machines like the Savannah 445B or 446 subsoil plows, which have been developed specifically for tractors in the Cat D6 or John Deere 850 range and can produce very efficient subsoiling results for the amount of power available.

Our philosophy is that if we get a strong, healthy seedling established, the roots will develop aggressively, quickly expanding through the loosened or fractured soil. It is not economic to mechanically fracture every inch of the field to ensure perfect conditions for root development. Neither do we feel it is necessary, because healthy, aggressively growing roots will seek out the old root canals of the previously harvested group of trees, and develop in their place.

It is important not to glaze the sides of the fractured zone with the passage of the steel tine, as this inhibits root development from the lower density fractured soil through into the higher density of the unfractured zone. Using the upward thrust of the wing to burst the soil in the direction of the surface ensures that the sides of the fracture zone have a roughened interface with the non-fractured subsoil strata.

Trial Plots

The benefits of subsoiling, and the economic viability of doing so, need to be considered on a site by site basis. One way of ensuring that subsoiling is appropriate for a given site is a continuing policy of control plots placed throughout different soil types. The minimum control plot should consist of three rows left without the subsoiling treatment. The growth in the centre row is the only row that is measured as it will not be affected by the roots finding their way into an adjacent subsoiled row.

Plots should be located using GPS and recorded, as it is often difficult to find the trial plots after a few years, especially if the establishment forester has moved on to another company, or is living in the Bahamas!