Evaluation of subsoil compaction status and effects in different agricultural land management systems in temperate and tropical climates

Project summary

Abstract From BSU Master Thesis:

Subsoil compaction is one of the major causes of land degradation worldwide. Over the years, susceptibility of agricultural fields to subsoil compaction has increased due to increasing weight, tire size and inflation pressure of agricultural machinery. Unlike other forms of land degradation such as erosion and salinization, subsoil compaction is usually difficult to ameliorate largely because it is invisible. However, experimental and field data to assess and quantify subsoil quality of trafficked agricultural soils under temperate and tropical conditions are scarce. This study investigated subsoil compaction status and quality of sandy loam soil in Denmark and tropical soils from Ghana using soil profile evaluation based on a numerical visual evaluation of subsoil structure (SubVESS) method and laboratory measurements.

In Denmark, the study of the effect of subsoil compaction were evaluated from a four year (2010-2013) compaction trial in Flakkebjerg [FK] with a slurry tanker weighing 8 Mg wheel load equipped with 3.1 bars inflation pressure tires while in Ghana, the study was conducted on five selected agricultural fields in the Volta and Ashanti Regions where farm machinery had been used for at least 10 years. In all the sites, visual evaluation was done from ~30-100 cm depth. In addition, soil cores were sampled for laboratory analysis. For the FK site, soil cores were sampled from the upper subsoil (~30-40 cm) in the vertical and horizontal direction, but only in the vertical direction for the Ghanaian soils at 30, 50 and 70 cm depth. On the field, the status of subsoil quality was determined by evaluating soil profiles for mottling, soil strength, porosity, rooting, and aggregate size and shape using scores from a subsoil structural quality flowchart. In the laboratory, measurements were made to determine soil water content, air permeability (ka), air-filled porosity (εa) and gas diffusivity (Dp/Do) at selected water potentials. In addition, the following were also determined: tortuosity (τ), effective pore diameter (dB), the number of air-filled pores per transect of soil (nB), pore organization (PO) and blocked air-filled porosity (εb). For the samples from FK, a factor of anisotropy (FA) was quantified to determine the ratio of a given property of soil in the horizontal direction to that in the vertical direction. Anisotropy was considered to prevail when FA for the soil property significantly differed from 1. The vertical stress exerted by machinery was estimated using the Terramechanical model (Terranimo model).

The visual assessment showed that the compaction treatment in FK resulted in soil quality deformation down to ~75 cm depth. This was confirmed by the laboratory measured data which showed that the compacted soils had reduced pore characteristics and aeration than the non-compacted reference soils. The soils in FK also displayed anisotropy even though it was only significant for ka and PO at -30 hPa. The visual evaluation in Ghana indicated that the effect of traffic on soil compaction was minimal and was mostly limited to the upper-subsoil. This was confirmed by the laboratory measurements of pore characteristics and aeration which showed that in general, there were no major differences between the trafficked and counterpart reference soils. The soils studied in Ghana displayed variability between the trafficked and reference fields. On the whole, the tropical soils generally had low aeration at the selected matric potentials (-30, -100 and -300 hPa). The Terranimo model revealed that the wheel load and inflation pressure of the machine used in the FK site exerted enormous vertical stress on the sandy loam soil than the normal agricultural machinery used on the fields studied in Ghana which justified the severe compaction effects in the former.

The study illustrated that laboratory measurements of soil pore characteristics and aeration could be combined with visual assessment data to determine the status of subsoil compaction in temperate and tropical soils. Nevertheless, the SubVESS method could be used in tandem with tools for determining soil moisture content prior to visual evaluation of soil profiles to give more robust results for comparison between and among fields.