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Demonstration of Subsoiling to Improve Soil Compaction and Infiltration Rates on Pasture

Project Site: Mackay Ross' Farm - Cleardale

Research Program Manager: Dr. Akim Omokanye From: Peace Country Beef & Forage Association 2018 Annual Report


In the Peace Country, soil compaction is one of the main problems for hay and pasture production and is mostly likely linked to the intensification of production and our soil type, which is referred to as ‘gumbo’. ‘Gumbo’ soils are characterized by a tough, impermeable hardpan that may vary from 2-12 inches or more below the soil surface (Lickacz, 1993). This hardpan severely restricts root and water penetration of the subsoil. Also, soil compaction resulting from cattle trampling in pastures could reduce soil respiration by reducing pore space and limiting oxygen diffusion. Subsoiling can break compacted soil layers without disturbing plant life, topsoil or surface residue. When properly done, subsoiling loosens the soil, allowing roots to penetrate deeper into the profile, which increases water infiltration and improves conditions conducive to biological activity. It is therefore the starting point for the alleviation of compacted soils. However, subsoiling is a complex and expensive operation which must be well planned and executed in order to achieve the desired results. The objective of this study was to conduct an assessment on the suitability of different types of subsoilers (in combination with or without rolling) for reducing soil compaction, increasing soil infiltration, and improving soil moisture.

Methods:

The project site was in Cleardale at Mackay Ross’ farm. The paddock was initially seeded to creeping red fescue. Alsike clover was later broadcast (12 years later, 2011) onto the paddock.

A demonstration strip design was used. We used 2 types of subsoilers - a Sumo (GLS-Grassland) subsoiler and an Agrowplow (Model AP91). The subsoiling treatments consisted of the following:

1. Sumo alone – subsoiling to a depth of 12’’

2. Sumo + rolling - subsoiling to a depth of 12’’ followed by rolling

3. Agrowplow alone - subsoiling to a depth of 12’’

4. Agrowplow + rolling - subsoiling to a depth of 12’’ followed by rolling

5. Control (check)

All the treatments were implemented on October 9, 2015. The PCBFA Annual Reports for 2016 and 2017 have progress reports on this project 2-3 years after the treatments were implemented.

In 2018, we collected the following measurements later in the year (September 21, 2018):

1. Soil compaction with a digital penetrometer at 1” intervals to a depth of 12”

2. Soil moisture (gravimetric method). A direct measurement of soil water content from 0-6 inches soil depth was done with gravimetric method of soil water content determination.

3. Infiltration rates. The water infiltration rate was measured with single-ring infiltrometers. The ring infiltrometers (aluminized coated) were about 6 inches in diameter and height. Using the sledge hammer and block of wood, the rings were hammered 2 inches into the soil.

Results

Soil moisture

After about 4 years of subsoiling and rolling, the moisture content recorded was generally higher for subsoiled strips (21.1-24.5%) than check (Table 1). Rolling after subsoiling seemed to further increase the amount of water in the soil, compared to the unrolled subsoiled strips.



Infiltration

Infiltration rate is a measure of how fast water enters the soil, and is typically expressed in inches per hour. The highest infiltration rate was from Agrowplow–Rolled, followed by Sumo-Unrolled, and then Agrowplow-Unrolled. For some reason, which is difficult to explain, Sumo-Rolled subsoiler recorded lower infiltration rates than the other subsoiling treatments. The check had the lowest infiltration rate and was 3.4-14.9 inches/hr slower than the Agrowplow and Sumo suboiler treatments (Table 1).

The downward movement of water within the soil is called percolation, permeability, or hydraulic conductivity. Permeability is generally rated from very rapid to very slow (Table 2). It is clear from Table 1 that the subsoiling treatments improved the downward movement of water within the soil (with all treatments being classified as rapid to very rapid) compared to check, which had moderately rapid permeability.



Compaction

Soil compaction is measured by soil penetrometers in PSI (pounds per square inch). Penetrometers measure soil strength and their movement through the soil, which is related to the soil’s resistance to root penetration. Plant roots, however, grow around obstacles and can exert tremendous local pressure on soil pores so that penetrometers can only provide a relative root resistance value. Readings greater than 300 PSI indicate potential soil compaction.

Unlike in 2017, where check had higher soil compaction readings than the subsoiling treatments (see PCBFA Annual Report 2017), in 2018, 4 years after the subsoiling was done, the treatments generally did not seem to differ much from check in reducing soil compaction (Figure 1).



Conclusion

Four years after subsoiling, both soil moisture content and infiltration rates still appeared to be better with subsoiling than check, but not in further reducing soil compaction on pastures. It is important to note that before 2018, all subsoiling treatments had significant effects on reducing soil compaction, which was monitored yearly. Again, the site has been grazed at least twice a year.

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