Introduction

Soil compaction can be caused by machinery or vehicle traffic, hoof traffic, or tillage. Soil compaction results in the loss of pore space, which limits aeration and water movement. Compaction can also inhibit or hinder root growth or interfere with soil organism habitat and nutrient cycling. Compaction can be indicated by increased soil penetration resistance as well as irregular root growth.

Penetration Resistance

Penetration resistance should be measured at field capacity if possible, due to the increase in soil strength as soil dries. Penetration resistance should be measured at 8-10 locations at the site, or 8-10 locations for each soil type or management system.

Wire Flag Method

Hold a wire flag near the flag and insert the wire to a depth of 30 cm (12 in) in an area known to not be compacted, such as a nearby fence row. Repeat this within the assessment area and determine if greater force was required to insert the wire. Record your observations in the data sheet.

Penetrometer Method

Penetrometers (Figure 6.1) are hand-powered instruments that measure the force exerted on a cone at the tip of a rod which is inserted into the soil.

Assess soil compaction by applying consistent, downward force at a rate of 2.5 cm (1 in) per second while observing readings on the pressure gauge. Cones should be used as follows: 0.5 in cones for silty, clayey, or loamy soils; 0.75 in cones for sandy soils. Note the depth of restrictive layers and resistance pressure. A layer of soil is considered restrictive to roots when penetration resistance exceeds 104 N/cm² (1035 kPa, 150 PSI) in the top 15 cm (6 in) of soil and 207 N/cm² (2070 kPa, 300 PSI) at 15-45 cm (6-18 in) of soil.  Using Table 6.1, determine the compaction rating and whether subsoiling or an alternative practice is recommended based on the percent of observations where penetration resistance would be root limiting in the top 45 cm. Record your the results.

Table 1. Compaction rating determination based on observations of root-restrictive layers.

Table adapted from (Murdock et al. 1995) and (Duiker 2002).

Plant Roots

Plants are the primary producers of terrestrial ecosystems, and plant roots are the primary means of carbon additions into the soil. This is done via root exudates from living plants, or decomposition of plant roots once a plant dies or sheds roots. These exudates and root-derived carbon feed soil microbes. The roots also promote soil aggregation and create biopores, thus influencing soil porosity, aeration, water movement, nutrient cycling, and soil organism habitat.

To assess plant roots as a soil health indicator observe rooting patterns in the top 20 cm (8 in) or deeper depending on the plants present and soil conditions. Roots should be abundant and be deeply rooted, and not be impeded by compacted layers as indicated by kinked or irregular root growth or roots growing horizontally above a compacted layer. Healthy roots should have root hairs, whereas a lack of root hairs may indicate anaerobic soil conditions. Roots should be intermingled with soil aggregates that stick to the roots, indicating roots and root hairs are supporting soil structure. Some plant species may form rhizosheaths, which can indicate beneficial soil microbe colonization. Record your observations in the data sheet.

References

Duiker, S. W. 2002. Diagnosing Soil Compaction Using a Penetrometer (Soil Compaction Tester). Pennsylvania State University. https://extension.psu.edu/diagnosing-soil-compaction-using-a-penetrometer-soil-compaction-tester.