21.4: Sources of Confusion About Soil Tests

People may be easily confused about the details of soil tests, especially if they have seen results from more than one soil testing laboratory. There are a number of reasons for this, including:

• laboratories use a variety of procedures;
• labs report results differently; and
• different approaches are used to make recommendations based on soil test results.

Varied Laboratory Procedures

One of the complications with using soil tests to help determine nutrient needs is that testing labs across the world use a wide range of procedures. The main difference among labs is the solutions they use to extract the soil nutrients. Some use one solution for all nutrients, while others will use one solution to extract potassium, magnesium and calcium; another for phosphorus; and yet another for micronutrients. The various extracting solutions have different chemical compositions, so the amount of a particular nutrient that lab A extracts may be different from the amount extracted by lab B. Labs frequently have a good reason for using a particular solution, however. For example, the Olsen test for phosphorus (see Table 21.1) is more accurate for high-pH soils in arid and semiarid regions than the various acid-extracting solutions commonly used in more humid regions. Whatever procedure the lab uses, soil test levels must be calibrated with the crop’s response to added nutrients. For example, do yields increase when you add phosphorus to a soil that tested low in P? In general, university and state labs in a given region use the same or similar procedures that have been calibrated for local soils and climate.

Reporting Soil Test Levels Differently

Soil testing reports are unfortunately not standardized and labs may report their results in different ways. Some use parts per million (10,000 ppm = 1%); some use pounds per acre (usually by using parts per two million, which is twice the ppm level, because 1 acre of soil to 6 inch depth weighs approximately two million pounds) or kilograms per hectare; and some use an index (for example, all nutrients are expressed on a scale of 1–100). Some report Ca, Mg and K in milliequivalents (me) per 100 grams. In addition, some labs report phosphorus and potassium in the elemental form, while others use the oxide forms: P₂O₅ and K₂O. Most testing labs report results as both a number and a category such as low, medium, optimum, high and very high. This is perhaps a more appropriate way to report the results as the relationship between soil test levels and yield response is affected by soil variability and seasonal growing conditions, and these broader categories provide a more realistic sense of the probability that fertilizer applications will increase yields. Most labs consider high to be above the amount needed (the optimum), but some labs use optimum and high interchangeably. (High, and even very high, does not mean that the nutrient is present in toxic amounts; these categories only indicate that there is a very slim chance of getting a yield increase if that nutrient is applied. With regard to P, very high indicates the potential for greater amounts lost in runoff waters, causing environmental problems in surface waters.) If the significance of the various categories is not clear on your report, be sure to ask. Labs should be able to furnish you with the probability of getting a response to added fertilizer for each soil test category.

Different Recommendation Systems

Figure 21.1. Percent of maximum yield obtained with different amounts of fertilizer K applied to a soil with a very low soil test.

Even when labs use the same procedures, as is the case in most of the Midwest, different approaches to making recommendations lead to different amounts of recommended fertilizer. Three different systems are used to make fertilizer recommendations based on soil tests: 1) the sufficiency level system, 2) the buildup and maintenance system, and 3) the basic cation saturation ratio system (only used for Ca, Mg and K).

The sufficiency level system suggests that there is a point, the sufficiency or critical soil test value, above which there is little likelihood of crop response to an added nutrient. Its goal is not to produce the highest yield every year but rather to produce the highest average return over time from using fertilizers. Experiments that relate yield increases with added fertilizer to soil test levels provide much of the evidence supporting this approach. When applying fertilizers when soil tests indicate a need (see Figure 21.1 for K applications to a soil with a very low K test), yields increase up to a maximum yield, with no further increases as more fertilizer is added beyond this so-called agronomic optimum rate. Farmers should be aiming not for maximum yield but for the maximum economic yields and the economic optimum rate, which are slightly below the highest possible yields obtained with the agronomic optimum rate. With a higher testing soil than shown in Figure 21.1, let’s say low instead of very low K, there would be less of a yield increase from added K and smaller amounts of fertilizer would be recommended.

The buildup and maintenance system calls for building up soils to high levels of fertility and then keeping them there by applying enough fertilizer to replace nutrients removed in harvested crops. As levels are built up, this approach frequently recommends more fertilizer than the sufficiency system. It is used mainly for phosphorus, potassium and magnesium recommendations; it can also be used for calcium when high-value vegetables are being grown on low-CEC soils. However, there may be a justification for using the buildup and maintenance approach for phosphorus and potassium—in addition to using it for calcium—on high-value crops because 1) the extra costs are such a small percent of total costs and 2) when weather is suboptimal (cool and damp, for example), this approach may occasionally produce a higher yield that would more than cover the extra expense of the fertilizer. Farmers may also want to build up their fertility levels during years of good prices to have a buffer against economic headwinds in future years. If you use this approach, you should pay attention to levels of phosphorus: adding more P when levels are already optimum can pose an environmental risk.

The basic cation saturation ratio system (BCSR—also called the base ratio system), a method for estimating calcium, magnesium and potassium needs, is based on the belief that crops yield best when calcium, magnesium and potassium—usually the dominant cations on the CEC—are in a particular balance. It grew out of work in the 1940s and 1950s by Firman E. Bear and coworkers in New Jersey, and later by William A. Albrecht in Missouri.

This system has become accepted by many farmers despite a lack of modern research supporting it (see “The Basic Cation Saturation Ratio System” at the end of this chapter). Few university testing laboratories use this system, but a number of private labs do continue to use it. It calls for calcium to occupy about 60–80% of the CEC, magnesium to be 10–20%, and potassium 2–5%. This is based on the notion that if the percent saturation of the CEC is good, there will be enough of each of these nutrients to support optimum crop growth. When using the BCSR, it is important to recognize its practical as well as theoretical flaws. For one, even when the ratios of the nutrients are within the recommended crop guidelines, there may be such a low CEC (such as in a sandy soil that is very low in organic matter) that the amounts present are insufficient for crops. If the soil has a CEC of only 2 milliequivalents per 100 grams of soil, for example, it can have a “perfect” balance of Ca (70%), Mg (12.5%) and K (3.5%) but contain only 560 pounds of Ca, 60 pounds of Mg and 53 pounds of K per acre to a depth of 6 inches. Thus, while these elements are in a supposedly good ratio to one another, there isn’t enough of any of them. The main problem with this soil is a low CEC; the remedy would be to add a lot of organic matter over a period of years and, if the pH is low, it should be limed.

The opposite situation also needs attention. When there is a high CEC and satisfactory pH for the crops being grown, even though there is plenty of a particular nutrient, the cation ratio system may call for adding more. This can be a problem with soils that are naturally high in magnesium, because the recommendations may call for high amounts of calcium and potassium to be added when none are really needed, thus wasting the farmer’s time and money.

Research indicates that plants do well over a broad range of cation ratios, as long as there are sufficient supplies of potassium, calcium and magnesium. But still, the ratios sometimes matter for a different reason. For example, liming sometimes results in decreased potassium availability and this would be apparent with the BSCR system, but the sufficiency system would also call for adding potassium, because of the low potassium levels in these limed soils. Also, when magnesium occupies more than 50% of the CEC in soils with low organic matter and low aggregate stability, using gypsum (calcium sulfate) may help restore aggregation because of the extra calcium as well as the higher level of dissolved salts. However, this does not relate to crop nutrition, but results from the higher charge density of Ca promoting better aggregation.

Want To Learn More About Bscr?

The preponderance of research indicates that there is no “ideal” ratio of cations held on the CEC with which farmers should try to bring their soils into conformity. It also indicates that the percent base saturation has no practical usefulness for farmers. If you would like to delve further into this issue, there is a more detailed discussion of BSCR and how it perpetuates a misunderstanding of both CEC and base saturation in the appendix at the end of this chapter.

Plant Tissue Tests

Soil tests are the most common means of assessing fertility needs of crops, but plant tissue tests are especially useful for nutrient management of perennial crops, such as apples, blueberries, peaches, citrus and vineyards. For most annuals, including agronomic and vegetable crops, tissue testing, though not widely used, can help diagnose problems. However, because a large amount of needed fertilizers (aside from nitrogen) can’t usually be delivered to the crop during the season, tissue nutrient tests are best used in combination with soil tests. The small sampling window available for most annuals and an inability to effectively fertilize them once they are well established, except for nitrogen during early growth stages, limit the usefulness of tissue analysis for annual crops. However, leaf petiole nitrate tests are sometimes done on potatoes and sugar beets to help fine-tune in-season nitrogen fertilization. Petiole nitrate is also helpful for nitrogen management of cotton and for managing irrigated vegetables, especially during the transition from vegetative to reproductive growth. With irrigated crops, particularly when the drip system is used, fertilizer can be effectively delivered to the rooting zone during crop growth.

Note

To estimate the percentages of the various cations on the CEC, the amounts need to be expressed in terms of quantity of charge. Some labs give concentration by both weight (parts per million, ppm) and charge (milliequivalents per 100 grams, me/100g). If you want to convert from ppm to me/100g, you can do it as follows:

(Ca in ppm)/200 = Ca in me/100g

(Mg in ppm)/120 = Mg in me/100g

(K in ppm)/390 = K in me/100g

As discussed in Chapter 20, adding up the amount of charge due to calcium, magnesium and potassium gives a very good estimate of the CEC for most soils above pH 5.5.

What Should You Do?

After reading the discussion above you may be somewhat confused by the different procedures and ways of expressing results, as well as by the different recommendation approaches. It is bewildering. Our general suggestions for how to deal with these complex issues are as follows:

1. Send your soil samples to a lab that uses tests that have been evaluated for the soils and crops of your state or region. Continue using the same lab or another that uses the same system.
2. If you’re growing low value-per-acre crops (wheat, corn, soybeans, etc.), be sure that the recommendation system used is based on the sufficiency approach. This system usually results in lower fertilizer rates and higher economic returns for low-value crops. (It is not easy to find out what system a lab uses. Be persistent, and you will get to a person who can answer your question.)
3. Dividing a sample in two and sending it to two labs may result in confusion. You will probably get different recommendations, and it won’t be easy to figure out which is better for you, unless you are willing to do a comparison of the recommendations. In most cases you are better off staying with the same trusted lab and learning how to fine-tune the recommendations for your farm. If you are willing to experiment, however, you can send duplicate samples to two different labs, with one going to your state-testing laboratory. In general, the recommendations from state labs call for less, but enough, fertilizer. If you are growing crops over a large acreage, set up a demonstration or experiment in one field by applying the fertilizer recommended by each lab over long strips and see if there is any yield difference. A yield monitor for grain crops would be very useful for this purpose. If you’ve never set up a field experiment before, you should ask your Extension agent for help. You might also find SARE’s publication How to Conduct Research on Your Farm or Ranch of use (download or order in print at www.sare.org/research).
4. Keep a record of the soil tests for each field, so that you can track changes over the years (Figure 21.2). (But, again, make sure you use the same lab to keep results comparable). If records show a buildup of nutrients to high levels, reduce nutrient applications. If you’re drawing nutrient levels down too low, start applying fertilizers or off-farm organic nutrient sources. In some rotations, such as the corn-corn-four years of hay shown at the bottom of Figure 21.2, it makes sense to build up nutrient levels during the corn phase and draw them down during the hay phase.

Recommendation System Comparison

Most university testing laboratories use the sufficiency level system, but some make potassium or magnesium recommendations by modifying the sufficiency system to take into account the portion of the CEC occupied by the nutrient.

The buildup and maintenance system is used by some state university labs and many commercial labs. An extensive evaluation of different approaches to fertilizer recommendations for agronomic crops in Nebraska found that the sufficiency level system resulted in using less fertilizer and gave higher economic returns than the buildup and maintenance system.

Studies in Kentucky, Ohio and Wisconsin have indicated that the sufficiency system is superior to both the buildup and maintenance and cation ratio systems.