Building Soils for Better Crops, Third Edition

Sources of Confusion About Soil Tests

SARE Outreach
Fred Magdoff and Harold van Es | 2010 | 294 pages
PDF (6.8 MB)

This title is temporarily out of print. We expect to publish an updated edition in the spring/summer of 2021.

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 the following:

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

Varied Lab Procedures

One of the complications with using soil tests to help determine nutrient needs is that testing labs across the country 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 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

Different 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); and some use an index (for example, all nutrients are expressed on a scale of 1 to 100). 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. However, although most labs consider high to be above the amount needed (the optimum), some labs use optimum and high interchangeably. 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 with different K soil test levels.

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 level provide much of the evidence supporting this approach. As the soil test level increases from optimum (or medium) to high, yields without adding fertilizer are close to the maximum obtained by adding more fertilizer (figure 21.1). Of course, farmers should be aiming for the maximum economic yields, which are slightly below the highest possible yields, as indicated in figure 21.1.

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. This approach usually 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. 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. This system was developed out of work by Firman E. Bear in New Jersey and William A. Albrecht in Missouri and has become accepted by many farmers despite a lack of modern research supporting the system (see “The Basic Cation Saturation Ratio System”). Few university testing laboratories use this system, but a number of private labs use it because many “alternative” and organic farmers believe that it is valuable. This system 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 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 is 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 moderately high in magnesium, because the recommendations may call for high amounts of calcium and potassium to be added when none are really needed—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. However, the ratios are sometimes out of balance. For example, 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. As mentioned previously, liming very acidic soils sometimes results in decreased potassium availability, and this would be apparent when using the cation ratio system. The sufficiency system would also call for adding potassium, because of the low potassium levels in these limed soils.

The sufficiency-level approach is used by most fertility recommendation systems for potassium, magnesium, and calcium, as well as phosphorus and nitrogen (where N tests are available). It generally calls for lower application rates for potassium, magnesium, and calcium and is more consistent with the scientific data than the cation ratio system. The cation ratio system can be used to reduce the chance of nutrient deficiencies, if interpreted with care and common sense—not ignoring the total amounts present and paying attention to the implications of a soil’s pH. Using this system, however, will usually mean applying more nutrients than suggested by the sufficiency system—with a low probability of actually getting a higher yield or better crop quality.

Labs sometimes use a combination of these systems, something like a hybrid approach. Some laboratories that use the sufficiency system will have a target for magnesium but then suggest adding more if the potassium level is high. Others may suggest that higher potassium levels are needed as the soil CEC increases. These are really hybrids of the sufficiency and cation ratio systems. At least one state university lab uses the sufficiency system for potassium and a cation ratio system for calcium and magnesium. Also, some labs assume that soils will not be tested annually. The recommendation that they give is, therefore, produced by the sufficiency system (what is needed for the crop) with a certain amount added for maintenance. This is done to be sure there is enough fertility in the following year.

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 (ppm) and charge (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.

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, citrus and peach orchards, and vineyards. For most annuals, including agronomic and vegetable crops, tissue testing, though not widely used, can help diagnose problems. The small sampling window available for most annuals and an inability to effectively fertilize them once they are well established, except for N during early growth stages, limit the usefulness of tissue analysis for annual crops. However, leaf petiole nitrate tests are sometimes done on potato and sugar beets to help fine-tune in-season N fertilization. Petiole nitrate is also helpful for N management of cotton and for help 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.

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.

What Should You Do?

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

Figure 21.2 Soil test phosphorus and potassium trends under different fertility management regimes. Modified from The Penn State Agronomy Guide [2007-2008]
  1. Send your soil samples to a lab that uses tests 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 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 brochure How to Conduct Research on Your Farm or Ranch of use.
  4. Keep a record of the soil tests for each field, so that you can track changes over the years (figure 21.2). 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.