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The amount of crop residue that remains on or near the soil surface is an important factor in controlling erosion from cultivated fields. Crop residue at or near the surface protects the soil by: (1) absorbing the erosive force of wind and raindrops, (2) slowing runoff water, (3) providing channels for water to enter the soil, (4) anchoring soil together and (5) insulting the soil against heat loss thus reducing the presence of impermeable frost layers.
Residue at the surface, however, can pose problems because of diseases, interception of herbicides and interference with seeding, harvesting or other equipment. An effective reduced tillage system maintains adequate ground cover to reduce soil erosion, yet attempts to minimize the problems associated with excessive residue. Maintaining surface residue above an adequate, but below an excessive amount is achieved through the selection and operation of appropriate tillage implements. Goals of a conservation tillage system are to bury excessive amounts of residue, reduce the size of residue and redistribute it evenly over the field. In general, 30 percent cover is the national standard set for reduced and minimum tillage systems, but more may be required depending upon soil, slope and rainfall conditions (see' 'How Much Surface Residue is Enough," Spring 1987 issue of this newsletter).
The Soil Conservation Service, Cooperative Extension or local Conservation District can assist producers in determining adequate levels of residue cover for particular fields and crop rotations. Once this target level is determined, growers need a rapid, simple method to access residue amounts in the field. A relatively quick and very simple technique that has been used for several years is the "point or line transect" method.
The Line Transect Method
The line transect method has evolved over the past several years and is the preferred method of measuring residue cover. Although the method is quick, simple and reliable, there is sometimes confusion or misunderstanding about the procedure and/or the concepts on which it is based, To use the method effectively, the concepts and procedure should be clearly understood. Let's review each of these.
The percent ground cover is the proportion of the soil surface that is covered by crop residue and living plant tissue. Measuring ground cover requires a method or technique to compare the proportion of covered soil to the amount of unprotected soil or to the total area. This can be accomplished through (1) visual estimates, (2) photographic means or (3) field sampling,
Visual methods are quick, but not usually very reliable. Photographic techniques are more credible but require considerably more time and effort. Photographic slides of the ground surface are taken and the ground cover measured by projecting the image onto a grid to determine area. More recently computers have been used to analyze these images. A major disadvantage of this method is the long delay in getting an answer.
In general, field sampling to determine residue cover involves selection of a set of sampling locations and then evaluation of cover at these locations. The line transect this type of method. Ground cover is evaluated at points along a line. This procedure is valid provided that a sufficient number of points are observed and that they are separated by a reasonable distance. For convenience, 50 or 100 points separated by 6-inch, 12-inch or l-meter intervals are commonly used. Points do not necessarily have to be of uniform separation. Random intervals are equally valid, although less convenient. The line transect is simply a convention to observe a number of points.
Ground cover is determined by counting the number of points that intersect residue. Points must be finite in comparison to the size of residue. In other words, a point should be no larger than the smallest dimension of residue, which is capable of protecting the soil, For practical purposes, this is about 1/8 inch or less. In principle the line transect is a method of systematically selecting a number of points, about 1/8 inch in width, and determining the percentage of time these points are covered by residue.
Using the method involves laying a line and inspecting the selected points along the line. This can be accomplished with a variety of tools. Some devices that are used are tape measures, knotted ropes or beaded strings. Other similar tools are also suitable. Strings or ropes usually have 50 or 100 knots or beads spaced at 6- or 12-inch intervals. Tapes can be read at 6- or 12-inch intervals. A disadvantage of using a tape is that it must be placed flat with the numeral side up. This is particularly difficult when done alone or on windy days.
Lay the tape, rope or string at an angle to the rows or last tillage direction. Do not attempt to stretch it too tightly but allow it to loosely follow the surface. Walk carefully along the line and view each bead, knot or tick mark using the same reference point and viewing position. A reference point could be the center of each knot or bead or the edge of each tick mark. Count the points that intercept residue.
Fig. 1 shows reading residue along a tape. The reference points in the figure are the tick marks above the numerals. At position 21 there is no residue, hence this point is not counted. At position 22 residue intersects the tape at the reference mark, and the point is counted. At position 23 two pieces of residue intersect at the reference mark, and the point is counted once and only once.
Remember, your intention is to observe how often pointsare covered, but not by how much. (Numerous points with thick residue cover are an indication that residue could be more evenly spread to achieve a higher percent cover.) The number of points that intersect residue divided by the total points along the line times 100 is the percent cover. If 100 points are used, each point is 1 percent. If 50 points are used, each point is 2 percent.
Residue cover is seldom uniform, therefore it is advisable to take several line transects to get a value that is representative. Five measurements are probably sufficient for most purposes. Choose locations that are typical for the field. Avoid turn areas, field edges and other atypical locations unless ground cover in these areas is of interest.
Growers and conservationists need a quick, reliable method to evaluate ground cover in minimum till and notill systems. The line transect satisfies these requirements. The procedure is a systematic way of selecting a set of points at which ground cover observations are taken. The method provides reliable results when the procedure is carefully and correctly performed. Representative ground cover estimates for a field can probably be achieved with a minimum of five line transects.
us: Hans Kok, (208)885-5971
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