The accuracy of a fertilizer recommendation depends on how well the soil sample on which the recommendation was based represents the area on which the recommendation will be used. The physical and chemical characteristics of soil in an area can vary considerably from place to place because of natural factors and the management to which the area has been subjected. Natural variation arises from soil-forming processes (such as mineral weathering and erosion) that lead to accumulations or losses of nutrients at different sites. Management factors might include tillage and fertilization practices, crop selection and irrigation. It may be necessary to take many samples from a given area (at random or in a systematic manner) to assess its fertility accurately.
Four variables are generally considered when taking soil samples:
Proper consideration of these variables ensures that the sample accurately reflects the fertility of the area in question and allows for the best possible fertilizer recommendations.
Sample distribution usually depends on the degree of variability in a given area. In relatively uniform areas smaller than 25 acres, a composite sample of 20 to 30 cores taken in a random or zigzag manner is usually sufficient. Larger areas are usually subdivided into smaller ones. Non-uniform areas should be subdivided on the basis of obvious differences such as slope position or soil type.
Banding fertilizer creates zones of very high fertility in soils because the fertilizer is mixed with only a small portion of the soil. Samples taken in the band can greatly overestimate the overall fertility of a site. Because the position of fertilizer bands is rarely known with certainty, one should take more random samples than usual in fields with fertilizer bands and vary sampling position with respect to row location to ensure that the bands do not bias test results.
For non-uniform sites, a systematic sampling approach is best. Sampling in a grid pattern can give an idea of variability in a field and fertilizer application can be adjusted according to the distribution of soil test results within the grid. The grid spacing can vary from as little as 30 feet to several hundred feet. Often the grid spacing is some multiple of fertilizer applicator width. Grid geometry can be adjusted to account for characteristics of the site in question. For example, a rectangular grid may be more useful than a square grid when fertilizer applications have been primarily in one direction. Eight to 10 cores are usually taken and combined for analysis at each sampling point in the grid.
Soil samples used for nutrient recommendations should be taken at the same depth that is used in the research generating the recommendations, normally 0 to 8 inches. A major exception involves sampling sites subjected to little or no inversion tillage, including those in established forages, no-till and ridges. In such cases, additional samples should be taken at a shallower depth (0 to 4 inches) to assess acidification of the soil surface and make appropriate lime recommendations. Surface soil pH may greatly affect herbicide activity and/or carry-over problems. Occasionally sampling the soil profile in 4-inch increments also may be useful for assessing the degree of nutrient stratification in fields managed with conservation tillage, but no recommendations are being made at this time based on the results of such samples.
Sampling after harvest in the fall or before planting in the spring is recommended. Fall sampling is preferred if lime applications are anticipated. Sampling during the growing season may give erroneous results due to effects of crop uptake and other processes. In-season sampling should be used only to test soils for nitrate as a guide to sidedressing additional N. Recommendations for sampling soils for nitrate are not consistent across Indiana, Michigan and Ohio, so those interested in such tests should use in-state recommendations.
Sampling should occur at the same time of the year each time a particular field is sampled. This allows better tracking of trends in soil test values over time, which may be as important as the test values themselves.
Most sites should be sampled every three to four years. On sites where rapid changes in fertility (particularly decreases) are expected or when high-value crops are involved, shorter sampling intervals (1 to 2 years) are recommended. Regardless of the sampling interval, records of changes in soil test values over time should be kept for each site tested.
After the sample has been collected, contamination must be avoided. Common sources of contamination include dirty sampling tools, storage vessels and surfaces on which soils are spread to dry. Ashes from tobacco products can cause considerable contamination of soil samples. Soils should be shipped to the testing laboratory only in containers approved by the lab.
Individual cores should be mixed thoroughly to form a composite sample. Moist cores should be crushed into aggregates approximately 1/8 to 1/4 inch across for optimum mixing. If the mixed sample is to be dried, the drying should be done at temperatures no greater than 120 degrees F (50 degrees C). After drying, a sub-sample of appropriate size should be taken from the composite mixture and sent to the testing laboratory for analysis.
Several tests are available to measure the availability of individual nutrients in the soil. The recommendations made here are based on research conducted using very specific tests, which are identified for each nutrient. Producers and consultants should always be certain their fertilizer recommendations are based on research using the same procedures used to generate their soil test results.
The specific procedures used to test soils in Indiana, Michigan and Ohio are described in NCR Publication 221, 1988, Recommended Chemical Soil Test Procedures for the North Central Region, written by the USDA-sanctioned North Central Regional Committee on Soil Testing and Plant Analysis (NCR-13) and published by the North Dakota Agricultural Experiment Station. Other procedures may yield results incompatible with the recommendations given here.
All soil test data in this publication are reported as parts per million (ppm) rather than pounds per acre (lb/acre). The change to ppm is being made because it more truly represents what is measured in the soil. Soil test values are an index of availability and do not reflect the total amount of available nutrients in soil. The use of lb/acre in the past has also led to some confusion about soil testing and the resulting fertilizer recommendations. Most commercial soil test laboratories are currently reporting soil test values in terms of ppm. To convert ppm to lb/acre multiply ppm by 2.
Jay W. Johnson
Extension Agronomist, Natural Resources
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