Introduction to Soil Chemistry

P
soil color 15
B
P = pores
B = bridges
Figure 1.9. Two soil peds showing a pore with restricted openings on the left and a pore with only
one opening on the right. In between are bridges holding the two peds together.
pendently from each other. This condition is called single-grained, like beach
sand, and occurs in coarse sandy soil. The second condition is where there are
no lines of weakness between units and the soil is said to be massive; that is,
it acts as one massive block. Soil becomes massive if it is worked when it is
too wet, forming hard clods that, when dry, form smaller irregular clods when
broken. Neither of these conditions is common in soil, although the massive
condition occurs in the soil parent material or rigolith before soil formation
begins.
1.3. SOIL COLOR
In Figures 1.1–1.5 the horizon descriptions give a color in both words and in
Munsell color book designations. The Munsell system describes color in terms
of hue: the basic color, value (lightness), and chroma or the purity of the color.
The hue is the first number followed by a color designation. In Figure 1.1 the
A horizon is 10YR (where YR indicates a yellow-red color); in Figure 1.2
the Btg1 horizon has a color designated as 2.5Y (where Y indicates yellow).
In the Munsell system the value for white is assigned the number 10 and black,
the number 0. The chroma or purity increases from left (1) to right (8).
However, not all colors in the Munsell system are used to describe soil colors.
The typical Munsell color chart for soils covers only those colors described
as being yellow or red or some combination of the two. This is not to say that
P