The soil testing program starts with the collection of a soil sample from a field. The first basic principle of soil testing is that a field can be sampled in such a way that chemical analysis of the soil sample will accurately reflect the field’s true nutrient status. This does not mean that all of the samples must, or will, show the same test results, but rather that the results must reflect true variations within the field. Remember that the soil test recommendations for lime and fertilizer can never be more accurate than the accuracy of soil sampling.
Note: A separate chapter in the Fertilizer Guide is devoted to soil sampling.
Nutrients are only taken up by roots when dissolved in water. Insoluble nutrients are not immediately useful for plant nutrition.
Extraction and Chemical Analysis
Once the soil samples have been collected and prepared, the level of available nutrients in each sample must be determined. Many chemical methods have been suggested and are being used for the measurement of essential plant nutrients. The criteria for chemical extracting and analysis of plant nutrients are that those processes must be rapid, accurate and reliable.
Most chemical extracting methods allow the extracting solution, which may consist of water, alkali, weak or strong acid or combinations of these chemicals, to react with the soil sample in a relatively short time. The sample is then filtered and the solution analyzed for the available nutrients.
Soil Test Parameters
In addition to extracting solutions, several other parameters of each soil test are important in determining the final number that is printed on a soil report for any one soil sample. These parameters include:
Ratio of soil to extractant
Shaking time, action and speed
Method of expressing the results (e.g., lb/acre, ppm, index systems)
"Cut-off" levels for high test results
Overall techniques used in the lab
The extractants containing the dissolved plant nutrients are analyzed to determine the concentration of the plant nutrient(s). Results are usually reported as parts per million (ppm), or pounds per acre (lb/acre). For most nutrients, ppm may be converted to lb/acre by multiplying by two (40 ppm of potassium = 80 lb/acre). For nitrate, sulfate and chloride, essentially all the nutrient forms present in the soil are extracted, and depth increments, other than the standard 6- to 7-inch surface layer, are sampled. For these measurements, ppm is converted to lb/acre by the following formula: lb/acre = ppm x 0.3 x depth increment in inches. For example, a 10 ppm nitrate N test on a soil sample taken to a 24 inch depth would convert to 72 lb/acre (10 ppm x 0.3 x 24 inches). In this case, 72 lb/acre of nitrate nitrogen were present in the top 24 inches of the soil sampled. Extracting available plant nutrients helps give an educated estimate as to the amounts of plant nutrients that will be available to a particular crop during the growing season. The amount of plant nutrients extracted will depend on the strength of the extracting solution and various other parameters. Soil test values are a relative number and should be interpreted as low, medium or high for a particular nutrient.
Calibration and Interpretation
Perhaps the greatest challenge in soil testing is calibration of the tests. It is essential that the results of soil tests be calibrated against crop responses from applications of the plant nutrients in question. This information is obtained from field and greenhouse fertility experiments conducted over a wide range of soils. Yield responses from rates of applied nutrients can then be related to the quantity of available nutrients in the soil.
The results of long-term soil test calibration studies on different soil types are then utilized to establish recommended amounts of plant nutrients to apply to a particular crop at a given soil test level. For instance, if the soil test P level is in the range of 0–10 ppm (which is low), the P recommendation for a 150 bu/acre corn crop may be 100 lb/acre of P₂O₅; whereas, if the soil test P level is above 40 ppm (very high), the recommendation may be 0 to 20 lb/acre.
In this example to the right, more than 85 percent of the fields testing very low in a particular plant nutrient may give a profitable yield response to the added nutrient. At the very high level, there is only a 15 percent probability of a profitable yield increase to the added nutrient. These values are arbitrary, but they illustrate the idea of expectation of response.
The tools of site-specific precision management now allow growers to manage more homogeneous areas within fields. Some of those areas have much higher yield potentials than the database with which most of today’s soil tests were calibrated. This lack of calibration for high-yielding areas is one of the factors driving interest in using yield monitors and global positioning satellites to conduct strip trials to determine the adequacy of existing soil fertility programs. New precision ag tools have the ability to develop algorithms that allow for management of multiple site-specific zones within individual fields. This means a balanced crop nutrition prescription can be delivered to each square foot of every field.
When interpreting soil test results, several things should be kept in mind:
The chances of getting a profitable response to fertilization are much greater on a soil that tests low in a given nutrient than on one that tests high.
This does not rule out the possibility of a profitable response from nutrient application at a high level of fertility or lack of a profitable response on soils of low fertility.
Soil tests are better at predicting the probability of a profitable response to nutrient application than predicting the actual quantity of nutrient that will be needed in any one year.
Research in the United States and Europe shows that in any one season, a soil testing low in a nutrient often will not yield as well as a soil testing at an optimum level, no matter how much fertilizer is applied that year.
Interpretation of soil test results and recommendations often becomes a matter of how to improve the fertility status of soils testing less than optimum. How much will be needed to change the soil from low to medium or high in that element? What will be the most economical level at which to maintain the nutrient status of the soil?
With top-level management practices, yields increase and the probability of a response at any given soil test likewise increases.
Wise use of soil testing incorporates a long-term approach to fertility management, in which site-specific soil test target levels are established for each field and nutrient management plans developed to reach and maintain the target levels.
The goal of soil testing is to help farmers achieve economical optimum yields while protecting the environment. The basic philosophy of soil test fertilizer recommendations is:
Base them on soil test results;
Recommend that lower-testing soils be built up to higher test levels by adding extra fertilizer;
Apply maintenance amounts of plant nutrients to higher-testing soils to keep them there and to keep productivity high; and
Do not apply specific nutrients to soils testing very high in these nutrients.
Individualized fertilizer recommendations use site- and grower-specific information, rather than laboratory-generated recommendations based on assumptions and generalizations. Computer programs are available that help personalize recommendations by considering the following:
Soil Test Calibration Relevancy
How appropriate is the calibration used in the standard recommendation for the field in question? Unusual soil types, a different climate, no-till or ridge-till culture, crop variety, cropping history and field variability are examples of factors that could cause differences.
Yield potential determines the economic value of each percentage change in relative yield and may influence the shape of the calibration curve.
Band placement often reduces lost yield as sub-optimal soil test levels are built to optimum levels because the short-term recovery of applied fertilizer by crop plants is improved. Some recommendation systems reduce the rate recommended when banding is used, compared to broadcast. However, rate studies have shown the optimum rate when banding is sometimes equal to or greater than the optimum broadcast rate. It is wise to build soil test levels to optimum regardless of placement method used.
Farmer Financial Circumstances
The financial objective of farmers, like other investors with limited capital, is to maximize the return on the last dollar invested after considering all possible investment alternatives and their associated risks. Therefore, cash flow influences fertility management decisions.
Uniform and Balanced Nutrient Distribution
Balance recommendations to ensure each nutrient is used efficiently.
Land Tenure (Period of Time the Grower Will Farm the Field)
Soil test phosphate and potassium are capital investments, and buildup costs should be amortized over the expected time of ownership or operation. The longer the period of time benefits will be accrued from buildup, the lower the cost of buildup becomes and the higher the optimum soil test level becomes. Landowners and operators, as well as the environment, benefit from the development of agreements in which the costs and returns of soil test buildup are equitably shared. Such agreements can help avoid the loss of productivity and accelerated erosion typical of run-down farms having impoverished soil fertility.
Soil Test Buffer Potential
Soil test buffer potential is the quantity of fertilizer required to change the soil test level, and is usually expressed as pounds of P₂O₅ or K₂O required per ppm of soil test level change. Some low-pH and some high-pH soils fix applied phosphate readily, and increasing soil test phosphate is more costly, decreasing the optimum soil test level. Soil test phosphate and potassium levels are usually easier to change in sandier soils than on medium or fine-textured soils, except with very sandy soils, where potassium leaching becomes significant.
Recommendations When Levels Are High
Once soil tests are interpreted, possible approaches to a nutrient management plan may include the following:
Sufficiency: Add necessary rates of deficient nutrients so yields are not limited in present crop.
Build-Maintenance: Add enough of needed nutrient(s) to supply present crop need, and gradually increase soil supply to non-limiting level. Replace crop harvest–removed nutrients to keep plant nutrient levels at non-limiting levels.
If soil tests high in a plant nutrient, applying more of that nutrient is not recommended, at least for the current crop. This is especially true if there is an abundance of the nutrient present to the extent that there is almost no chance of response even if the nutrient was not applied for several years. However, some laboratories assign the value high to a level that points to little or no response to applications of that nutrient that year.
Failure to apply any of these nutrients will result in soil test depletion. Also, under some conditions, crops will respond profitably to a nutrient even with a high test. For example, on early-planted corn, the addition of N, P and K as a row application may produce response on soils testing high.
Fertilizer application when soils test in the high range is influenced substantially by the factors discussed in the section on individualization of recommendations. Maintenance in the high soil test category will be appropriate for some growers and sites but not for others.
Adapted from "The Efficient Fertilizer Use Manual",
Soil Testing chapter by Dr. Paul Fixen