D5 Soil and Root Analysis

Topic

The physical and chemical properties of the soil determine the availability of water, nutrients, and potential contaminants to the root at the root-soil interface, and a set of tools allows for the separate characterization of each of these soil properties.

The first physical property characterized is soil texture—that is, the proportion of particles of various sizes making up the soil; soil texture largely determines the behavior of other physical soil properties related to water.

The water retention curve is derived from soil texture; it shows the amount of water retained in the soil at various levels of soil water potential. A finer-textured soil retains more water at a given soil water potential than a coarser-textured soil, meaning that each soil's water retention curve depends directly on its texture.

In addition to the water retention curve, soil texture determines saturated hydraulic conductivity, which quantifies the speed at which water moves through the soil when it is fully saturated. Coarser-textured soil allows water to pass through with higher saturated hydraulic conductivity than finer-textured soil, which retains water more strongly and allows it to pass more slowly.

Alongside physical soil properties, chemical analysis of the soil solution characterizes the soil's chemical properties: the concentration of each nutrient dissolved in the soil solution—available for root uptake at the root-soil interface—and the concentration of soil contaminants dissolved in that same solution. This enables the quantification of soil contamination using the same tool used to quantify nutrient availability. In addition to the properties of the soil itself, the root-soil interface is characterized by directly observing the fine roots occupying it using *in situ* root observation techniques: minirhizotrons are transparent tubes installed in the soil that allow fine roots to be photographed where they grow, while a soil laser sensor—known as soil LiDAR—scans the soil volume to reconstruct the arrangement of fine roots without the need for excavation. By repeating these observations over time, both techniques enable the monitoring of fine root dynamics—specifically their growth, mortality, and turnover rates.

Taken together, soil texture, the water retention curve, saturated hydraulic conductivity, chemical analysis of the soil solution, and *in situ* root observation techniques make it possible to assess—based on the soil's physical and chemical properties—the water and nutrient availability encountered by roots at the root-soil interface, to quantify soil contamination, and to monitor fine root dynamics over time.

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