This was one of the main procedures promoted by the US Soil Conservation Service (the current
Natural Resources Conservation Service) during the 1930s. The
US Department of Agriculture established the Soil Conservation Service in 1935 during the
Dust Bowl when it became apparent that soil erosion was a huge problem along with
desertification. The extent of the problem was such that the 1934 "Yearbook of Agriculture" noted that
Approximately 35 million acres [142,000 km2] of formerly cultivated land have essentially been destroyed for crop production. . . . 100 million acres [405,000 km2] now in crops have lost all or most of the topsoil; 125 million acres [506,000 km2] of land now in crops are rapidly losing topsoil. This can lead to large-scale
desertification, permanently transforming a formerly productive landscape into an arid one that becomes increasingly intensive and expensive to farm. The Soil Conservation Service worked with state governments and universities with established agriculture programs, such as the
University of Nebraska, to promote the method to farmers. By 1938, the introduction of new agricultural techniques, such as contour plowing, had reduced soil loss by 65% despite the continuation of the drought. Demonstrations showed that contour farming, under ideal conditions, will increase yields of row crops by up to 50%, with increases of between 5 and 10% being common. Importantly, the technique also significantly reduces soil erosion and fertilizer loss, making farming less energy and resource-intensive under most circumstances. Reducing fertilizer loss saves the farmer time and money and decreases the risk of harming regional freshwater systems. Soil erosion caused by heavy rain can encourage the development of rills and gullies which carry excess nutrients into freshwater systems through the process of
eutrophication Contour plowing is also promoted in countries with rainfall patterns similar to those in the United States, such as western
Canada and
Australia. The practice is effective only on slopes with between 2% and 10% gradient and when rainfall does not exceed a certain amount within a certain period. On steeper slopes and areas with greater rainfall, a procedure known as
strip cropping is used with contour farming to provide additional protection. Contour farming is most effective when used with other soil conservation methods such as
terrace farming, and the use of
cover crops. The proper combination of such farming methods can be determined by various climatic and soil conditions of that given area. Farming sites are often classified into five levels: insensitive, mild, moderate, high, and extreme, depending on the region's soil sensitivity. Contour farming is applied in certain European countries such as Belgium, Italy, Greece, Romania, Slovenia, and Spain in areas with higher than 10% slope.
Modifications P. A. Yeomans'
Keyline design system is critical of traditional contour plowing techniques and improves the system through observing normal landforms and
topography. At one end of a contour, the slope of the land will always be steeper than at the other. Thus, when plowing parallel runs paralleling any contour, the plow furrows soon deviate from a true contour. Rainwater in these furrows will flow sideways along the falling "contour" line. This can often concentrate water to exacerbate erosion instead of reducing it. Yeomans was the first to appreciate the significance of this phenomenon. Keyline cultivation utilizes this "off contour" drift in cultivating furrows to control the movement of rainwater for the benefit of the land. German agronomist Dr. Philipp Gerhardt has developed the related
Seepage Line System, which has been taken up in parts of Germany. ==See also==