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Issue No. 22, Article 5/September 5, 2008

Making Informed Fertilizer Applications Through Accurate Soil Sampling

Soil tests are not perfect, so a soil-test value should be considered not as a single value, but as one within a range. These are some of the reasons why a soil test is imperfect: 1) it represents a one-point-in-time measurement, whereas the crop takes nutrients through an extended period of timeand typically under very different soil-water and temperature conditions than at the time of sampling; 2) the information generated typically comes from a sample in the plow layer, but crop roots extract nutrients below that layer; and 3) laboratory precision, which is typically within 5% to 10% of the true value. Despite these imperfections, soil testing is the most important guide to profitable application of P, K, and lime because it provides a framework to determine the fertility status of the field.

Of course a soil-test report can be only as accurate as the sample sent for analysis. In fact, the spatial variability of available nutrients in a field makes soil sampling the most common and greatest source of error in a soil test. To collect samples that will provide an accurate measurement of the fertility of an area, be sure to determine the sampling distribution, collect samples to the proper depth, collect samples from precisely the same areas of the field that were sampled in the past, and collect samples at the proper time.

Sampling Distribution

The number of soil samples taken from a field is a compromise between what should be done (information) and what can be done (cost). The most common mistake is taking too few samples to represent a field adequately. Shortcuts in sampling may produce unreliable results and lead to higher fertilizer costs, lower returns, or both. For whole-field uniform fertilizer applications, typically sampling at the rate of one composite (at least five soil cores taken from within a 10-foot radius around the sampling point) from each 2-1/2-acre area gives a general indication of the fertility of that field. For site-specific applications or for fields where large variations in test values over a short distance are suspected, it is better to collect one sample from each 1.1-acre area. For conservation tillage fields where fertilizers are applied in a band, there is no universally accepted method to collect soil samples. Typically it is suggested that if the location of the band is unknown, the number of samples to make a composite should be increased to 20 or 30 and the sampling position should be varied in relation to the row so the band does not bias test results. If the position of the band is known, then it is typically suggested to stay a few inches away from it or to take a large number of samples off the band for each sample taken in the fertilizer band. Another method of soil sampling a field is to collect samples from zones with common characteristics. This is a directed sampling approach (also known as smart sampling). This method integrates precision agriculture data (yield maps, crop canopy data, soil data, past management history information, etc.) and defines sampling zones with common characteristics that may be influencing crop productivity.

Depth of Sampling

A soil tube is the best implement for taking soil samples. An auger or a spade can also be used, as long as care is taken to collect an exact depth with a constant slice thickness. The proper sampling depth for pH, P, and K is 7 inchesthis is because the fertilizer recommendation system in Illinois is based on crop response to fertility levels in the top 7 inches of the soil. For fields in which conservation tillage systems have been used, accurate sampling depth is especially important, as these systems result in less thorough mixing of lime and fertilizers than a tillage system that includes a moldboard plow. This stratification has not adversely affected crop yield, but misleading soil-test results may be obtained if samples are not taken to the proper depth. Shallow samples will overestimate actual soil status, leading to underapplication of lime or fertilizers, while samples that are too deep or where some part of the top portion falls off during sampling will underestimate current soil status, causing overapplication of lime or fertilizers.

Sampling Location

The inherent variability of soils over even short distances (related to soil-forming factors) and management effects for which there is not a record (such as non-uniform distribution of fertilizer) make it important to collect soil samples from precisely the same points each time the field is tested to reduce the variation of soil-test values often observed between sampling times. Sample locations may be identified using a global positioning system (GPS) unit or by accurately measuring the sample points with a device such as a measuring wheel.

Time of Sampling

Sampling every 4 years is strongly suggested when soils are at optimum fertility. When maintenance levels are not being applied in cropping systems that remove large quantities of nutrients, such as hay or corn silage, soil tests should be done every 2 years. To improve the consistency of results, samples should be collected at the same time of year and, if possible, under similar soil-water conditions. Sampling done within a few months of lime or fertilizer treatment will be more variable than sampling done after a year.

Late summer and fall are the best seasons for collecting soil samples because K test results are most reliable during these times. The K soil test tends to be cyclic, with low test levels in late summer and early fall and high test levels in late January and early February. Phosphorus and pH levels are typically not seasonally affected in most soils in Illinois. In coarse-textured soils (sandy soils) with low buffer capacity, pH levels can increase as much as one unit under wet conditions.

This information has been adapted from the Illinois Agronomy Handbook.-Fabián G. Fernández

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