Contour plowing orcontour farming is thefarming practice ofplowing and/or planting across a slope following its elevationcontour lines. These contour line furrows create a water break, reducing the formation of rills and gullies during heavy precipitation and allowing more time for the water to settle into the soil.[1] In contour plowing, the ruts made by the plow run perpendicular rather than parallel to the slopes, generally furrows that curve around the land and are level. This method is also known for preventing tillage erosion.[2]Tillage erosion is the soil movement and erosion by tilling a given plot of land.[3] A similar practice iscontour bunding where stones are placed around the contours of slopes. Contour plowing has been proven to reduce fertilizer loss, power, time consumption, and wear on machines, as well as to increase crop yields and reduce soil erosion.
Soil erosion prevention practices such as this can drastically decrease negative effects associated with soil erosion, such as reduced crop productivity, worsened water quality, lower effective reservoir water levels, flooding, andhabitat destruction.[4] Contour farming is considered an active form ofsustainable agriculture.[5]
ThePhoenicians first developed the practice of contour farming and spread it throughout theMediterranean. However, the Romans preferred cultivation in straight furrows and this practice became standard.[6]
This was one of the main procedures promoted by the US Soil Conservation Service (the currentNatural Resources Conservation Service) during the 1930s. TheUS Department of Agriculture established the Soil Conservation Service in 1935 during theDust Bowl when it became apparent that soil erosion was a huge problem along withdesertification.
The extent of the problem was such that the 1934 "Yearbook of Agriculture" noted thatApproximately 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-scaledesertification, permanently transforming a formerly productive landscape into an arid one that becomes increasingly intensive and expensive to farm.[7]
The Soil Conservation Service worked with state governments and universities with established agriculture programs, such as theUniversity 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.[8] 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 ofeutrophication[9]
Contour plowing is also promoted in countries with rainfall patterns similar to those in the United States, such as westernCanada andAustralia.
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 asstrip cropping is used with contour farming to provide additional protection.[10] Contour farming is most effective when used with other soil conservation methods such asterrace farming, and the use ofcover crops.[11] 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.[12] Contour farming is applied in certain European countries such as Belgium, Italy, Greece, Romania, Slovenia, and Spain in areas with higher than 10% slope.[13]
P. A. Yeomans'Keyline design system is critical of traditional contour plowing techniques and improves the system through observing normal landforms andtopography. 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.
Phoenicians ... practiced some of the earliest forms of contour farming ...(and)... helped spread contour farming throughout the Mediterranean ... however, the Romans ... preferred straight furrows. Over a period, societies who embraced irrigation farming adopted this method of plowing and planting.
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