Overstory #157 - Soil Erosion and Water Infiltration
Soil erosion is caused by rainwater that does not infiltrate into the soil, but instead runs off the soil surface. It is essential that the interconnected processes of soil erosion and water infiltration be well understood by farmers, extension workers, and scientists.
The consequence of conventional cultivation methods can be the gradual loss of soil and fertility until the land becomes unproductive. Erosion, as well as intensive weathering under hot, humid conditions, has brought about widespread, poor, badly eroded, and infertile soils throughout the tropics and subtropics (Ochse, et al., 1961). The same process has also taken place in temperate climates (e.g. the United States and Russia). Eroded, unproductive and abandoned lands, as well as advanced signs of desertification are a silent testimony to this phenomenon worldwide.
Besides making agricultural soil unproductive, erosion and runoff result in the deposition of soil particles in unwanted areas (sedimentation of roads, creeks, rivers, lakes, dams, etc.) with all its negative consequences for traffic, electric power generation, sources of drinking water, leisure areas, etc. Correcting these collateral effects costs huge sums, usually from public sources. Efficient erosion control is therefore very advantageous from the ecological and social perspectives, besides being highly significant from an economic point of view. It helps maintain the productive potential and fertility of soils for future generations, and is an effective means to ensure continued economic viability of farmland and therefore gainful employment in rural areas.
Soil erosion by runoff water is often accepted as an unavoidable consequence of agriculture on sloping land, but this is not necessarily so. According to Lal (1982), erosion on cultivated land is a symptom of land misuse for a particular environment. In other words, it is not slope or rainfall intensity that are responsible for erosion and its negative consequences, but rather farming methods--inappropriate farming practices cause erosion. Through the utilization of site specific and adapted farming systems and management practices, farmers can effectively control erosion, reduce runoff, and increase water infiltration on their land.
FIGURE 1: swcc.cn/waswc/articles/f-1.jpg swcc.cn/waswc/articles/f-1.1.jpg The impact of raindrops on a bare soil surface. Pictures showing raindrop impact on a bare soil surface and information explaining the mechanisms of water infiltration go back to the 1940s. When it rains, drops up to 6 mm (0.24 inch) in diameter bombard the soil surface at impact velocities of up to 32 km/hr (20 mph). The force throws soil particles and water in all directions up to a distance of 1 m (3.3 feet). (Pictures made by USDA in the 1940s).
THE EROSION PROCESS
Despite scientific and empirical evidence explaining erosive processes, many people still think that the soil has to be loosened by tillage to increase water infiltration and reduce runoff.
Runoff and erosion start with the impact of raindrops on bare soil. Soil splash seen on fence posts or walls next to bare soil is evidence of the impact of large raindrops (Harrold, 1972). Meyer and Mannering (1967) reported that raindrops annually deliver impact energy equivalent to 50 tons of TNT to a hectare (20 t/ac) of land. The impact of falling raindrops breaks (disaggregates) the soil into very fine particles, which clogs soil pores and creates a surface seal that impedes rapid water infiltration (Figure 2). The use of tillage systems leaves the soil bare allowing rain to pulverize it excessively, creating conditions where it is carried away by heavy rains. Later, the surface sealing dries, resulting in crusting that may hinder or impede the germination and emergence of crop seeds.
Due to surface sealing, only a small portion of rainwater can infiltrate the soil; most of it runs off over the soil surface and causes erosion damage. Research conducted in Brazil (Roth, 1985) shows that the percentage of soil covered with plant residues is the most important factor that influences water infiltration into the soil. While virtually all water from a simulated rainfall of 60 mm/hour infiltrated when the soil was 100% covered with plant residues, 75% to 80% of rainwater left the plots as runoff when the soil was bare (Figure 4). Researchers in many parts of the world have obtained similar results.
When the soil is covered with living plants or plant residues, the plant biomass absorbs the energy of falling raindrops. Rainwater flows gently downward where it infiltrates into soil that is porous and undisturbed. By protecting the soil surface from impact, soil cover impedes the clogging of soil pores (Figures 2, 3, 4 and 5). Soils highly susceptible to crusting do not have this problem once no-tillage and permanent cover systems are used. Soil crusting only develops under bare soil conditions.
It is therefore essential to maintain soil cover with plants or plant residues year-round. Any attempt to control runoff and erosion by tillage to maintain a loosened and uncovered soil surface or by burying plant residues will eventually lead to failure. A no-tillage system with a cover of crop residues or green manure cover crops is the most efficient method for preventing and controlling erosion, and should be the "best practice" technology promoted worldwide.
FIGURE 5: swcc.cn/waswc/articles/f-5.jpg Rainfall simulator demonstration performed at the No-till on the Plains Winter Conference in Salina, Kansas, in 2001. From left to right: (1) 100% soil cover, little runoff and no sediments. (2) 30% soil cover, more runoff and some sediments. (3) Bare soil, no cover, resulting in a huge amount of runoff and the dark color of water shows also a lot of sediments. (4) Pasture with 100% soil cover and undisturbed soil, even less runoff than (1).
Not tilling the soil, crop rotations combined with the use of cover crops, and not burning plant residues ensure permanent, year-round soil cover. Conservation agriculture, using these no-tillage methods, offers the most effective strategy and affordable methods available today to control soil erosion and, in this way, achieve sustainable agriculture. The long-term gains from widespread conversion to no-tillage could be greater than from any other innovation in developing countries (Warren, 1981).
While most of the numerous advantages of the no-tillage system come from the permanent cover of the soil with plant residues, there are several additional advantages from no-till. Tillage destroys the vertical pore system created by roots, earthworms and other soil fauna, destroys soil structure, accelerates organic matter mineralization (depletion) and reduces aggregate stability. Fields under no-till systems for many years are expected to further increase water infiltration as the vertical pore system builds up and organic matter increases. In this way, no-till with abundant soil cover allows for both the natural rebuilding of soil structure and porosity, as well as protecting the soil from damaging raindrop impact.
In addition to increasing water infiltration and controlling erosion, soil cover has a major impact in reducing soil temperature, reducing evaporation, increasing available water for plants, enhancing the life and biological activity of the soil, contributing to the reduction of soil compaction and soil crusting. These all have positive effects on the chemical, physical and biological properties of the soil and leads to higher productivity and long-term agricultural sustainability.
Derpsch, R., Roth, C.H., Sidiras, N. and Köpke, U. 1991. Controle da erosão no Paraná, Brasil: Sistemas de cobertura do solo, plantio direto e preparo conservacionista do solo. Sonderpublikation der GTZ, No. 245 Deutsche Gesellschaft für Technische Zusammenarbeit (GTZ) GmbH, Eschborn, TZ-Verlagsgesellschaft mbH, Rossdorf, 272 pp.
Harrold, L.L. 1972. Soil erosion by water as affected by reduced tillage systems. Proceedings No-tillage Systems Symp. Feb. 21-22, 1972, Ohio State University, 21-29.
Lal, R. 1982. Management of clay soils for erosion control. Tropical Agric., 59(2), 133-138.
Merrill, S.D., Krupinsky, J.M. and Tanaka, D.L. 2002. Soil coverage by residue in diverse crop sequences under no-till. USDA-ARS. Poster presented at the 2002 Annual Meeting of ASA-CSSA-SSSA, November 10-14, Indianapolis, IN.
Meyer L.L. and Mannering, J.V. 1967. Tillage and land modification for water erosion control. Amer. Soc. Agric. Eng. Tillage for Greater Crop Production Conference. Proc. Dec. 11-12, 1967, 58-62.
Ochse, J.J., Soule Jr., M.J, Dijkman, M.J. and Wehlburg, N.C. 1961. Tropical and Subtropical Agriculture, Vol. 1. The Macmillan Company, New York, London, 760 pp.
Roth, C.H. 1985. Infiltrabilität von Latossolo-Roxo-Böden in Nordparaná, Brasilien, in Feldversuchen zur Erosionskontrolle mit verschiedenen Bodenbearbeitungs-systemen und Rotationen. Göttinger Bodenkundliche Berichte, 83, 1-104.
Warren, C.F. 1981. Technology Transfer in No-tillage Crop Production in Third World Agriculture. Proc. Symp. August 6-7, 1981, Monrovia, Liberia. West African and International Weed Science Societies. International Plant Protection Center, Oregon State University, Corvallis, OR 97331 USA. IPCC Document 46-B-83. 25-31.
This article was adapted with the gracious permission of the author from:
Derpsch, R. nd. "Understanding the Process of Soil Erosion and Water Infiltration." url: swcc.cn/waswc/articles/Rolf's%20article%202%20DS%20040614.htm
The author thankfully acknowledges the revision, inputs and improvement of the original manuscript by Matt Hagny, Board of Directors, No-till on the Plains, Salina, Kansas, USA, as well as the revision by Melissa McDonald, Asuncion, Paraguay.
ABOUT THE AUTHOR
Rolf Derpsch studied agronomy at the Universidad de Chile in Santiago and the Instituto Superior de Agricultura Adolfo Matthei in Osorno, Chile, and obtained a M.Sc. degree from the University of Reading, UK. He worked for the German Agency for Technical Cooperation (GTZ) as Team Leader for the Soil Conservation Project at the Research Institute of Paraná, IAPAR, in Londrina, Brazil and Senior Advisor to the MAG - GTZ Soil Conservation Project. Since September 2001 he has worked as a freelance consultant in no-tillage and conservation tillage, development and diffusion of sustainable agricultural production systems with permanent soil cover, green manure cover crops and crop rotations, and development of production systems for small farmers. He has worked in Paraguay, Brazil, Argentina, Bolivia, Chile, Honduras, Colombia, Cuba, Somalia and Germany. He was among the first to research no-tillage technology in Brazil and Latin America beginning in 1971. Rolf can be reached at Rolf Derpsch, No-tillage Consultant, CC 13223, Shopping del Sol, Asunción, Paraguay; Tel/Fax: 00 595 – 21 - 609717; E-mail: firstname.lastname@example.org; Web: .rolf-derpsch.com/.
- The World Association of Soil and Water Conservation (WASWC) is an international non-government organization of professionals and informed laypersons dedicated to promoting the sustained use of the earth's soil and water resources. There quarterly newsletter is now published in English, Spanish, French, Chinese and Portuguese: waswc.org and Photo Websites: community.webshots.com/user/waswc and community.webshots.com/user/waswc1
- The US Department of Agriculture's Natural Resources Conservation Service provides helps people conserve, maintain, and improve natural resources and the environment: nrcs.usda.gov/
- The Soil and Water Conservation Society fosters the science and the art of soil, water and related natural resource management to achieve sustainability: swcs.org/
- CONTOUR is a newsletter dedicated to the exchange of information on soil and water conservation in South East Asia: asocon.org/main.htm
- International Soil Reference and Information Centre provides soils information and promotes sustainable use of the land: lime.isric.nl/
- Cornell University Worldwide Portal to Information on Soil Health has an excellent list of links: mulch.mannlib.cornell.edu/newslett.html
- Management of Organic Inputs in Soils of the Tropics (MOIST) covers green manures and cover crop research and exchange. This site also maintains the high quality Mulch-L discussion forum on green manures/cover crops: ppathw3.cals.cornell.edu/mba_project/moist/home2.html
- Centro Internacional de Información Sobre Cultivos de Cobertura (CIDICCO) has green manure/cover crop information for small farmers: cidicco.hn/
- UC SAREP Cover Crop Resource Page has data on thousands of species used for organic matter production and erosion control: sarep.ucdavis.edu/ccrop/
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