3.1: Introduction
- Page ID
- 25107
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)The depletion of the soil humus supply is apt to be a fundamental cause of lowered crop yields.
—J.L. Hills, C.H. Jones and C. Cutler, 1908
The amount of organic matter in any particular soil is the result of a wide variety of environmental, soil and agronomic influences. Some of these, such as climate and soil texture, are naturally occurring. Hans Jenny carried out pioneering work on the effect of natural influences on soil organic matter levels in the United States more than 70 years ago. But agricultural practices also influence soil organic matter levels. Tillage, crop rotation and manuring practices all can have profound effects on the amount of soil organic matter.
The amount of organic matter in a soil is the result of all the additions and losses of organic materials that have occurred over the years (Figure 3.1). In this chapter, we will look at why different soils have varying levels of organic matter. While we will be looking mainly at the total amount of organic matter, keep in mind that all three “types” of organic matter—the living, dead and very dead—serve critical roles, and the amount of each may be affected differently by natural factors and agricultural practices.
Anything that adds large amounts of organic residues to a soil may increase organic matter. On the other hand, anything that causes soil organic matter to decompose more rapidly or to be lost through erosion may deplete organic matter.
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Organic matter is protected in soils by:
- Formation of strong chemical bonds between organic matter and clay particles (and fine silt)
- Being inside small aggregates (physically protected)
- Conversion into stable substances such as humic materials that are resistant to biological decomposition
Large aggregates are made up of many smaller ones that are held together by sticky substances from roots, bacterial colonies and fungal hyphae. Organic matter in large aggregates—but outside of the small aggregates that make up the larger ones—and freely occurring particulate organic matter (the “dead”) are available for soil organisms to use. However, poor aeration resulting from restricted drainage because of a dense subsurface layer, compaction or being at the bottom of a slope or wetland area may cause a low rate of use of the organic matter. So the organic matter needs to be in a favorable chemical form and physical location for organisms to use it; plus, the environmental conditions in the soil—adequate moisture and aeration—need to be sufficient for most soil organisms to use the residues and thrive.
If additions are greater than losses, organic matter increases, which happens naturally when soils are formed over many years. When additions are less than losses, there is a depletion of soil organic matter, which generally happens when soils are put into crop production. When the system is in balance and additions equal losses, the quantity of soil organic matter doesn’t change over the years.