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C.O.R.N
Newsletter
2004-36
October 19, 2004 -
October 26, 2004
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Sampling For Fall Soil Testing
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Soil Fertility Variation
The sources of variation in the field that affect the fertility of the soil can be grouped as natural variation and variation induced by human activity. The natural variation arises from the soil forming processes that cause accumulation or loss of nutrient in an area of the field. Examples of natural variation are variation in the geological formation from which soil was formed, erosion of the soil by wind and water and the kind of natural vegetation that was growing on the soil. Some examples of human activity that can cause variation of the nutrient concentrations are tillage, fertilizer rates and application methods, and the form of nutrient source added. A good sampling scheme attempts to provide accurate information about the variation of nutrient concentrations within the field of interest.
Sampling Methods
Remember that the sample is a representation of the portion of the field you are interested in and the test results are only as good as the sample taken. The best method for obtaining a representative soil sample will depend on several factors. Some of these factors are type and amount of fertilizer applied in the previous growing season, method used in applying the fertilizer, i.e. broadcast, banded, side-dressed, kind of tillage used, and exposure of the subsoil. These factors as well as those factors naturally expressed can allow classifying a field soil as uniform or non-uniform.
Random sampling - For fields considered uniform in regard to slope, soil type, management history, cropping sequences, and fertilization practices, the fertility variation is often small. Consequently, this type of variation allows for collecting 20 to 30 soil cores across 10 to 20 acres in a random manner. This approach often works well on small fields and especially where there is a good history of the nutrient concentrations from many previous soil tests.
Grid sampling - For non-uniform fields, where the fertility variation may be large, a systematic approach, such as dividing the field into a grid, and collecting sample cores within the grid is the best approach. The cores within each grid are usually mixed together to form a composite sample for the particular grid area. The grid sampling approach is especially useful if you have no previous knowledge of the soil’s fertility. For instance, if you are farming the ground for the first time, this approach will most accurately define the fertility variation. Grid sampling will involve a greater cost both in labor and testing than random sampling since usually more samples are taken than with the random approach. Once the nutrient concentration is known, it will not be necessary to use the grid sampling every time soil testing is done on that field in the future. Much has been written about the size of the grid to use. Many recommend a 2.5 acre grid, but this would largely depend on the field and the perceived uniformity of the field. The smaller the grid, relative to a larger grid, the more samples will be taken across the field area and the variation of nutrient concentrations across the field is measured with greater accuracy. However, this may not be practical in many cases of relative small and irregular fields.
Criteria that would favor using grid sampling are: (1) measure of non-mobile nutrients is the primary concern, (2) the soil test levels in the field vary from very high to very low with substantial acres both in the very high and very low categories, (3) history of manure use, (4) small fields merged into large fields, and (5) no history of the nutrient levels.
Zone sampling – Specific areas (zones) of a field that require sampling may be identified on a subjective or intuitive basis or from crop yield information obtained by yield monitors on harvesting equipment, or by other means. For example, soil showing differences in color across the field due to difference in organic matter could be sampled separately within a color zone. Soil sample cores are randomly collected separately from each zone. The soil cores from an individual zone are then mixed together to form a composite sample. The test results of the composite sample will provide average nutrient concentrations for that individual zone. Criteria favoring using zone sampling are: (1) cost of sampling and analysis is a major concern, (2) measure of mobile nutrients is the primary concern, (3) relatively low rates of fertilizer applied in the past, (4) no history of manure application, and (5) history of the nutrient level is known.
Depth of Sampling
For conventional tillage (plowing and disking), sampling the plow layer (0 – 8 inches) has proven adequate in acquiring a good sample. For pastures or shallow rooted crops it is best to take the sample to the rooting depth. For conservation tillage, such as chisel plowing, no-till, minimum tillage, and ridge tillage the sampling depth should be shallow at 4 inches. In addition, it is often recommended that two samples be taken for a no-till system. The first sample is taken at the 4-inch depth and the second sample taken at the 8-inch depth. The pH is measured on the shallow sample, and the pH plus plant available phosphorus and potassium are measured on the deep sample. It is important to determine the pH of the shallow depth in no-till systems because excessive acidity may alter herbicides less effective. The continued addition of fertilizer containing ammonia nitrogen over time in a no-till system will cause the upper soil layer to become more acid. The degree of this change in pH will depend on the kind of soil, with the change being much more rapid in sandy loam soils than in clay soils.
Time of Year to Sample and How Often to Test the Soil
Soils can be sampled and tested at any time during the year but it is best to sample in the same season every year. Sampling should be done before spring planting. Fall soil sampling is often recommended in that more time is available to plan a fertilizer program and to apply lime, if it is needed. Also, if the sample is taken soon after harvest, there is recent knowledge of the crop yield and any problem areas in the field. Another good reason for fall soil testing is that excessively wet springs prevent sampling that could have been done in the fall. Generally the pH decreases slightly during the growing season. In some soils, potassium levels may tend to be slightly higher in the spring than in the fall due to weathering of minerals over winter that release potassium.
Generally, it is sufficient to test the soil once every three years. However, for soils that are intensively cropped or are used for high value crops, it is important to test more frequently. Soil test records of each field should be kept over time regardless of how often the soil in each field is tested. This will allow the grower to get a feel for the variation of the nutrient levels from year to year.
Soil Sampling Tools and Handling the Soil Samples
Hand probes to vehicle-mounted hydraulic driven probes or augers are available. Large acreages and deep rock-free soils will allow a more mechanized approach than for small acreage and for rocky soils. It is important the sampling tools be kept clean and easily cleaned between each sample taken. The sampling probe should be constructed of stainless steel, especially if the soil is to be tested for micronutrients. It is important to use clean plastic buckets for collecting the soil sample cores to prevent contamination of the soil sample. Avoid sources of contamination such as cigarette or pipe ashes and dirty bench surfaces if the samples are spread out to dry. Be sure to accurately label each sample so they do not become mixed.
Additional information about grid sampling can be obtained from the North Central Regional publication Report 348 entitled “Soil Sampling for Variable Rate Fertilizer and Lime Application” through the University of Minnesota. It can be found on the Internet at:
http://www.extension.umn.edu/distribution/cropsystems/DC7647.html |
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Keith L. Smith, Director, Ohio State University Extension.
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