Spectrophotometers Refine Munsell Chart Identification in Soil Drainage Surveys


Posted on March 16, 2017

On the moon, even the dirt is deadly. Astronauts returning from the Apollo missions were quick to point out the long list of problems the Lunar regolith caused. In the absence of any atmosphere, the lunar surfacehas been bombarded1 by meteorites large and small for millennia, resulting in an extremely finely ground particulate material. The vacuum also exposes the soil directly to the solar wind and unfiltered UV rays, which supercharge the fine particulate with incredible static electric properties. Astronauts reported that2 the fine, abrasive particulate gummed up their spacesuit joints, wore through layers of kevlar on their boots, swirled around inside the lander and irritated their eyes, nostrils, and lungs, and could not be removed with wet wipes due to their static properties. Long before any construction or other usages can be attempted on the Moon, the dirt has to be taken into consideration. It may seem terrestrial in comparison, but the same is true of land use projects back on Earth.

Soil Surveys Are Essential to Land Use Management 

Dirt matters. Soil surveys are an essential component of land use management, necessary for construction, agriculture, and conservation, as well as for archaeological and geological research. This need is driven by the drastic differences between soil types3 found across the globe, and over far more minute distances. Variations occur in texture, compaction, pH, elemental composition, relative amounts of organic material, and the ability to retain water, all of which can significantly impact the potential usages of the land. Soil scientists and engineers know that in order for project managers to choose the correct construction or agricultural techniques, they need to have an accurate diagnosis of their dirt. 

Soil drainage is of particular concern. Poorly drained soil occurs4 as a result of a high water table, or when water from rainfall or nearby areas overloads a soil’s downwards drainage, lateral drainage, or evapotranspiration capacity. In order to accurately assess the drainage capabilities of a particular soil, which is important for planning appropriate residential, commercial, or industrial construction methods as well as potential crops, soil scientists and engineers conducting land use surveys turn to color as an indicator. As misidentifying the color of a soil may lead to improper usage, it’s important to get an as accurate reading as possible.

Assessment of Soil Color Determines Drainage Capabilities 

Drainage affects the color of soil by its relationship to the oxidization of iron. Well-drained soils redden as their iron rusts; poorly drained soils appear more gray as water inhibits oxidation of iron. By assessing the relative redness of soil—after taking into account the soil’s age and the color of the parent material and any organic matter—the drainage potential of a soil can be accurately measured. 

The current method of soil color assessment relies on human eyesight to match the color of soil samples with printed swatches5 arranged according to the Munsell scale6, which varies on three axes: hue, value, and chroma. The scale accounts for all known terrestrial soil colors and provides a simple way for scientists and engineers to assess soil color in the field. However, according to the United State Department of Agriculture7

The visual impression of color from the standard color chips is accurate only under standard conditions of light intensity and quality. Color determination may be inaccurate early in the morning or late in the evening. When the sun is low in the sky or the atmosphere is smoky, the light reaching the sample and the light reflected is redder. Even though the same kind of light reaches the color standard and the sample, the 15 FSA Wetland Identification Procedures (Foundations to Sound Decision Making) reading of sample color at these times is commonly one or more intervals of hue redder than at midday. Colors also appear different in the subdued light of a cloudy day than in bright sunlight. The intensity of incidental light is especially critical when matching soil to chips of low chroma and low value. 

Due to variations in illumination conditions and the subjectivity of human eyesight, false readings and misidentification can occur, potentially causing construction issues or poor crop yields.

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