# Sandy Loam Soil Shear Strength Parameters and Its Colour
## 6 Sandy Loam Soil Colour Reflectance at 700 nm  
For simplifying reading the above curve (reflectance of the spectrophotometer), the reflectance results at 700 nm (points are dispersed) were plotted as a function of the moisture content values (each at its moisture content; shown in Figure 9).  
Analysing the curve in Figure 9 shows that the moisture values used in the measurement are divided into two ranges, the first section ranging from 0.9% to approximately 7.41%. Fitting a curve into the plotted points (for an accurate curve with a good regression analysis, measurement repetitions are required) will end with a useful equation for calculating the moisture based on the colour reflected wavelength at 700 nm. In this range, the reflectance is changing in a clear curve path without having overlapping in the results, the case that makes calculating the moisture possible, and, thus, ending with a single moisture result in the mentioned range upon substituting a reflectance into the curve equation (equation is not given here; measurement repetitions and equation validation are required before confirming an equation).  
Even though the moisture result will not be precisely similar to the moisture value calculated (gravimetric or volumetric calculation), it should be of a value close to the precise value. The main reason behind the difference in the moisture values (colour equation calculation and gravimetric/volumetric calculation) is the reflected wavelength. Upon repeating the soil colour measurement at the same moisture content, the resulting reflected wavelength will not be precisely the same, since the material that is being studied is a particulate material. The density of the soil has influence on the wave reflectance; also, the technical usage of the spectrophotometer (the front measuring part of the instrument  
Figure 9: Colour reflectance of the tested sandy loam soil (700 nm) at different moisture contents.  
should be normal to the soil surface and its pressing level should be accounted, since pressing leads to soil compaction) has influence on the reflectance result.  
The second moisture section is ranging from a moisture value approximately beyond 7.41% till the maximum moisture value, which is 17.65% (shown in Figures 8 and 9). In this range, the reflectance is approximately constant and, thus, a horizontal straight-line fit into the points. Having the soil at any moisture value in the mentioned range, and upon measuring its colour, the colour reflectance will be the same (constant value on the horizontal line). The same reflectance indicates that either (1) beyond the moisture approximately equal to 7.41%, the soil colour is changing no more; thus, the soil absorbed an amount of water and the additional amount of water in the soil is water film occupying the space between the particles and leading to the same reflectance (water influencing the measurement).  
The other reason is (2) the used spectrophotometer measurement ability. The used spectrophotometer measures the colour based on sending light waves in the visible band (spectrum 400-700), and maybe, by sending waves in this band, the spectrophotometer is unable to receive precise reflectance at high moisture values; thus, the usage of a spectrophotometer that sends waves in other ranges (such as infrared) might end up with good results at this stage.