# Sandy Loam Soil Shear Strength Parameters and Its Colour
## 5 Spectral Behaviour of the Tested Sandy Loam Soil at Different Moisture Contents (Soil Colour)  
The description of the soil colour has been used by soil scientists in the field to classify the soil and in mapping [19; 20].  
The pedologists relied on the [PERSON] soil colour charts (1975) for more than 60 years in describing the normal range of soil colours, which is a qualitative colour estimate method [21; 22]. This method relies on determining the colour visually and then comparing it to standard chips that are arranged systematically based on their [PERSON]'s notations.  
Many scientists replaced this method by fast instrumental technologies and methods to rely on in specifying the soil colour, such as UV-VIS spectrophotometry (ultraviolet visible), leading to a precise and quantitative approach in specifying the colour (quality control) [12; 20; 23]. The spectrophotometer removed some of [PERSON]'s method limitations by using standard values and removing the human 'judgement' (observer viewing angle and the lighting conditions) [15; 20; 22; 24; 25; 26].  
The colour of the tested soil was measured using a spectrophotometer (CM-700d; shown in Figure 7). The spectrophotometer is a modern technology used for measuring the colour of a body (material); the working principle of the used type is by sending light waves in the visible band, spectrum (400-700 nm), and, based on the reflected wavelength, the colour of the soil is identified. The colour of the soil is described by the reflected wavelength, which is a digital value (quantitative). It is difficult to specify the colour of the soil based on the vision as in Munsell charts, since it is a qualitative method. Despite the errors that might result from the traditional Munsell charts method, such as human errors (vision), using this method in the case of moist soil becomes complicated. In most soil types, the increase in the moisture content of the soil makes the soil darker, but, even though not being able to see the change in the colour by the eye's vision, the spectrophotometer is able to decode the colour of the soil to a digital value.  
Figure 6: Sandy loam soil internal friction angle at different moisture contents.  
The sandy loam soil was moistened to be at different moisture contents. Each soil moisture value was measured using the moisture analyser (gravimetric measurement). At each moisture content value, three soil colour records were taken by the spectrophotometer, and the average of these records was taken into account. The reflected wavelength results of the soil at the same moisture content are not precisely the same (but close results), and the reason behind the difference is that the soil is a particle material. There are factors that influence the reflectance, such as the pores and the density. The reflected wavelength results of the sandy loam soil at different moisture contents in the visible band (spectrum, 400-700 nm) are shown in Figure 8.  
Figure 8: Sandy loam soil colour reflected wavelength at different moisture contents.  
Figure 7: Spectrophotometer CM-700d.  
The soil reflectance increases with the increase in the moisture content from 0.9 to 1.88%, indicating a slight brightening in the colour. Beyond the moisture 1.88 (or a value close to it), with the increase in the moisture amount, the reflected wavelength decreases, showing that, with the increase in the moisture, the soil colour becomes darker. The soil reflectance decreases with the increase in the moisture content till reaching 6.74% moisture value, and that can be recognised clearly in Figure 8 (at 700 nm). At values above 6.74% moisture, the reflectance points (clear at 700 nm) start to overlap in a reflectance range.