Introduction

The foundation of soil biological activity is soil organic matter. The source of that organic matter is plant residue, roots, and root exudates. Healthy soils that have been minimally disturbed and have sufficient additions of organic materials will maintain the soil organic matter, resulting in dark colors and lots of biological activity. Soils that experience regular disturbance or do not have sufficient additions of organic materials will lose soil organic matter over time, and as a result will exhibit lighter colors and less biological activity. In this lab you will be assessing the color of the topsoil as an indicator of high amounts of soil organic matter. You will also assess levels of biological activity as evidenced by direct observation of soil organisms and the presence of biopores that are created by these organisms.

Describing Soil Color

Dark top soil color is attributed to soil organic matter. As organic matter is lost from soil the topsoil will become less dark in color. In contrast, increases in organic matter result in darker soil color.

Assess the site for topsoil color by comparing colors determined using a Munsell soil color chart or the LandPKS app and the Official Series Description (OSD) or by comparing soil from the site to a nearby fencerow with the same soil type.

Soil Color Chart Method

Use a Munsell Soil Color Chart to determine the hue, value, and chroma of the surface horizon. Compare this color to that of the Official Series Description (OSD) for that specific soil series. Use the SoilWeb web app to identify the soil mapping unit for the location. Click on the map to bring up the soil mapping unit composition (Note: Further site investigation may be needed to confirm what soil series exists at that location). Click on the appropriate soil series name for the site. This opens an animated description of the soil profile. Click on the “Description” option towards the top-right of the web page. This will open the OSD for that soil series. Compare the color of the soil at the surface to the color of the surface horizon in the OSD. Record the results in the data sheet.

Field versus Fencerow Comparison Method

At several locations for field compare field topsoil to that of nearby fencerows or other undisturbed perennially-vegetated location. Ensure both soils are at similar moisture by moistening the sample before comparison as would be done for measuring soil color with a soil color chart. Do not use this method for irrigated fields where irrigation and fertility management of the field may lead to organic matter concentrations greater than the fencerow. Record the results in the data sheet.

Biological Diversity

Soil organisms can have a significant influence on soil properties and function, including pest suppression, nutrient cycling, aggregation, and water dynamics.

Assess biological diversity by observing soil organisms in the soil surface. Do this by taking a shovel full of soil and examining it for organisms. Look for meso- and micro-fauna such as earthworms, springtails, beetles, ants, millipedes, roundworms, etc. Also look for active nodules on the roots of legumes, or fungal hyphae that appear as thin, white threads or masses. The Soil Macrofauna Field Manual from the FAO (Ruiz et al. 2008) includes images of soil organisms you may possibly see. Page 10 of the Cropland In-Field Soil Health Assessment Guide also includes 12 photos of possible soil organisms (USDA-NRCS 2021).

Biopores

Large pores (macropores) in the soil that are created by earthworms or roots are called “biopores”. These biopores facilitate rapid movement of air and water, allow plants to access nutrients and water, and provide pathways for soil organisms to navigate the soil.

Assess the presence of biopores by taking a shovel full of soil and examine it for intact biopores. These biopores can appear darker than surrounding soil due to enrichment of organic matter. These pores may also connect to the soil surface. This indicator is easiest to assess in moist soils, but can also be assessed when the soil is dry.

References