Soil Quality and Soil Erosion

Raj Ratta, R. Lal, Raj Ratta, R. Lal

• Publisher: CRC Press
• Year: 2018
• eBook ISBN: 9781351415729
• Topic: Biological Sciences
• Subtopic: Biology
• Index: Biological Sciences

This work examines the issue of accelerated soil erosion, which has become an increasingly serious concern in the twentieth century. Aspects considered include on-site impact of erosion; application of soil science to problems of non-agricultural uses of soil, such as mineland restoration, urban uses and disposal of urban wastes; soil contamination and pollution by industrial activities; and athletic and recreational uses of soil. Soil Quality and Soil Erosion will be a useful text for soil scientists, agronomists, foresters, and environmental scientists as we enter the next century.

Section III

Soil Quality Management

6 Effects of Long-Term Cropping on Organic Matter Content of Soils: Implications for Soil Quality*

T.E. Fenton, J.R. Brown, and M.J. Mausbach

INTRODUCTION

The importance of soil organic matter (SOM) as a soil constituent and its contributions to plant growth have long been recognized. The amount of organic matter in a soil is dependent upon a complex set of interactions of physical, chemical, and biological processes. Decline in the organic matter content of soils when virgin soils are converted to cultivated soils has been shown with many long-term experiments. Most of the data show an initial rapid decline, then a period of a lower rate of decline, followed by a quasi-equilibrium under the same use and management. Changes in land use and/or management disturb the quasi-equilibrium, and the system undergoes adjustment to a new organic matter level dictated by the impact of residues returned to the soil. More complete data sets, including depth distribution and bulk density measurements, are needed to quantify changes in the total amount of organic matter in soils. SOM is an important component of the current concept of soil quality, the definition of which is still evolving. Additional parameters for characterizing SOM need to be developed and evaluated. The summation of soil properties present in soil profiles and their distribution on the landscape should also be included in the concept of soil quality.

Organic matter has long been recognized as an important factor in soil processes and crop growth. Waksman (1942) listed the following direct and indirect effects:

1.A source of nutrients for crops, especially N and P

2.Affects the physical condition of the soil, especially moisture-holding and buffering capacities

3.Supplies certain specific elements and compounds including minor elements and auxins

4.Favors the development of antagonistic organisms that serve to combat certain plant diseases

He went on to state that the fourth item was debatable.

Allison (1973) listed the important contributions of SOM as:

1.Provides the major natural source of inorganic nutrients and microbial energy

2.Acts as an ion-exchange material and a chelating agent to hold water and nutrients in available form

3.Promotes soil aggregation and root development

4.Improves water infiltration and water use efficiency

Composition of Soil Organic Matter

Schnitzer and Khan (1972) described SOM as a mixture of plant and animal residues in various stages of decomposition, substances synthesized microbiologically and/ or chemically from the breakdown of products, and the bodies of live and dead microorganisms and small animals and their decomposing remains. They also state that to simplify this complex system, SOM is usually subdivided into (1) nonhumic and (2) humic substances. The bulk of the SOM is composed of humic substances.

Campbell (1978) stated there are at least three and probably four components of SOM. In order of ease of decomposition they are fresh residues, biomass material, microbial metabolite and cell wall constituents adsorbed to clay colloids, and the old very stable humus.

Stevenson (1982) listed the following terms used to describe SOM fractions: organic residues, SOM, humus, soil biomass, humic substances, nonhumic substances, humin, humic acid, and fulvic acid.

Campbell (1978) proposed that SOM can be arbitrarily divided into relatively stable and active components. The former he more closely associated with physical stability of the soil, while the active fraction is associated with the ability of soil to cycle and supply available nutrients such as N. Stevenson (1956) showed evidence that long-term cultivation on the Morrow Plots had significantly increased the proportion of the basic AA-N nitrogen in the soil.

Depth Distribution

Much emphasis has been placed on the role of organic matter in surface soil, but less effort has been directed toward an understanding of the role of depth distribution of organic matter in soils. In addition, use of varying designations of surface soil and depths of study complicate interpreting the literature. Early work referred to “plow depth” (Epstein and Kohne, 1957). However, with the coming of conservation tillage systems, especially no-till, the term plow depth has little meaning. Hammer et al. (1995) and Conway-Nelson (1991) and others have renewed efforts to characterize organic matter with depth in various soils. Subsurface organic matter is important because its location affects nutrient and water fluxes and attenuation of chemicals within the soil profile. Roots and microbial activity are also important in the subsurface horizons. Therefore, long-term management of landscapes for improved quality of the visible surface soil may not be optimally successful without consideration of impact upon soil organic carbon in the entire solum.

Recently Recognized Functions

Rasmussen and Collins (1991) noted that SOM has recently received increased attention because of its potential to sequester carbon and its strong influence on the persistence and degradation of pesticides and organic waste in soils.

In a discussion of the importance of SOM, Gregorich et al. (1994) state: “It is the primary source of, and a temporary sink for, plant nutrients in agroecosystems and is important in maintaining soil tilth, aiding the infiltration of air and water, promoting water retention, reducing erosion and controlling the efficacy and fate of applied pesticides.”

Methods of Measurement

Quantitative methods used to determine SOM contents are variable. Therefore, it is recommended that organic carbon, which can be more accurately and precisely measured by a variety of procedures, be used as a measure of SOM (Nelson and Sommers, 1982). Organic carbon can then be converted to SOM content by use of an appropriate factor, usually by multiplying the organic carbon content by 1.724. Another major problem is that bulk density values have not been measured at many sites where SOM studies have been conducted. Therefore, knowing the total amount of SOM present is impossible. When bulk density data are available, the soil organic carbon (SOC) can be converted to a weight per unit area basis, for example kilograms per meter. Because there is disagreement concerning use of the conversion factor 1.724 for all horizons, the data comparisons should be made using SOC rather than SOM for soil profile comparisons.

The objective of our review is to evaluate the impact of crop and soil management on SOM content and distribution over time. Major variables are rotations, tillage, and fertility. Confounding factors are time and residue management.

LONG-TERM STUDIES

Treatments and Rotations

Jenny (1941) emphasized that man, through cultivation and other cultural treatments, acts as an independent variable or soil-forming factor. These activities result in changes in soil properties that can greatly affect the soil system. The most obvious changes related to cultivation are significant decreases in SOM contents and deterioration of soil structure.

Mitchell et al. (1991) presented an overview of long-term agronomic research. They emphasize that when the oldest research plots were established, land and labor were plentiful and the plots were large and unreplicated. Agricultural statistics did not exist. Early experiments concentrated on crop rotations and N-providing legumes. Over time, these basic experiments have led to other studies, and both have resulted in the development of widely accepted principles of soil and crop management. However, they go on to emphasize that many records of the long-term plots are only published in state agricultural experiment station bulletins and circulars.

Brown (1989) published brief descriptions of long-term field research sites. Information compiled included the site name and location, date of initiation, thrust, and a contact person for each site. The information was compiled from responses associated with planning for the Sanborn Field Centennial, held on June 27, 1989, and the Long-Term Research Symposium, sponsored by the American Society of Agronomy in 1989. Steiner and Herdt (1993) compiled a global directory of long-term agronomic experiments. The location, initiation date, purpose, soil and climatic information, experimental design, and management practices are described for these experiments.

Morrow Plots

The Morrow Plots, now considered the oldest continuous study in the United States, are at the University of Illinois. They were established in 1876 to study the effects of crop rotations and fertilization on yields. Crop sequences, in single replication, were continuous corn, corn–oats rotation, corn–oats–clover rotation, with and without lime, manure, and rock phosphate (Stauffer et al., 1940). Table 6.1 shows that continuous corn plot with no fertility treatment decreased in SOM content by 45.6% i...

Table of contents

• Cover Page
• Title Page
• Copyright Page
• Table of Contents
• Dedication
• Foreword
• Preface
• About the Editor
• Contributing Authors
• SECTION I: BASIC CONCEPTS
• SECTION II: TYPES OF SOIL QUALITY
• SECTION III: SOIL QUALITY MANAGEMENT
• SECTION IV: SOIL EROSION AND PRODUCTIVITY
• SECTION V: CONCLUSIONS
• Index