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Interpretations

Interpretations
C. Owen Plank
Associate Professor, University of Georgia

Since the inception of soil testing, several philosophies have been proposed for interpreting the analytical results. Today the two basic philosophies utilized by soil testing laboratories to interpret the results are (1) sufficiency level of available nutrient (SLAN) concept and (2) the nutrient maintenance concept. Although both concepts recognize that there is a fraction of a given nutrient measured by soil tests that is available to plants, they differ widely in the way these fractions are interpreted and expressed on soil test reports. Furthermore, the methodologies used in obtaining the interpretative guidelines differ markedly. Some laboratories use combinations of the two concepts in interpreting and reporting the recommendations. Consequently, if the client is not well versed with the two concepts, the reports and recommendations can be confusing.

Sufficiency Level of Available Nutrient (SLAN) Concept

 The sufficiency level of available nutrient (SLAN) approach is based on the concept that as the soil test level increases crop response to application of a given nutrient diminishes. When the soil nutrient level is sufficiently low that there is a high probability (80-100%) of getting a response from application of that nutrient most laboratories classify the soil level as “low”. In terms of yield or growth, a low level generally corresponds to achieving 80% or less of the expected maximum if no supplemental fertilizer is added.  However, as the level increases and the probability of getting a response to application of the nutrient diminishes to approximately 50%, the soil level would be classified as “medium”. At this level, expected growth would be between 80 to 100% of the expected maximum. As the soil level continues to increase, the point at which there is little or no crop response to further additions of the nutrient is referred to as the “critical level”. For most laboratories this coincides with the low end of the “high” category. At this level plants are capable of attaining 100% of the expected growth without application of the nutrient. “Very high” and/or “excessive” categories are used by many laboratories and imply that further additions of the nutrient could lead to nutrient imbalances and/or reduced growth.  Once the categories (low, medium, high, etc.) have been established, calibration studies are conducted to determine the amount of fertilizer required at each category level to meet plant needs and to raise the soil nutrient status if the category is “low”.

 As can be surmised, the SLAN concept is based on extensive field research and this has been one of the major deterrents for some laboratories to adopt this approach for interpreting all soil test results. Nevertheless, it is still the only approach used for interpreting soil test data for phosphorus (P), sulfur (S), and micronutrients.  Historically, most correlation and calibration work has been conducted on agronomic, forage, and horticulture crops. Until recently, turfgrass category ratings have been derived from other closely related crops and adjusted over the years by experienced university turfgrass scientists.

 Soil test laboratories that do not have much experience with turfgrass samples tend to overestimate phosphorus needs and underestimate potassium needs (Christians, 1992). Laboratories that routinely test turfgrass samples and that are supported by active turfgrass research programs adjust their calibrations and recommendations as new information becomes available.

Maintenance Level Concept

 The maintenance level concept is based on the theory that for most crops there is an “ideal cation saturation percentage or ratio” of basic cations (potassium, calcium, magnesium) on the soil cation exchange complex. The concept evolved from research by Dr. Firman Bear and co-workers in New Jersey in 1945 from which they proposed an “ideal” soil should have exchangeable cation levels of 65% Ca, 10% Mg, 5% K, and 20% H (Haby, et al., 1990). Using these levels, the “ideal” ratios suggested were Ca/Mg of 6.5:1, Ca/K of 13:1, and Mg/K of 2:1. Based on this research, the term “basic cation saturation ratio” (BCSR) had its beginning. These saturation values and ratios have been used extensively by some laboratories while others have modified them for soils in their region. For example, Graham(1959) modified the saturation levels for Missouri soils and proposed saturation ranges of 65 to 85% Ca, 6 to 12% Mg, and 2 to 5% for K. Some soil test laboratories may interpret results based on either of these two concepts (basic cation saturation percentage or basic cation saturation ratio) or a combination of the two.

 The maintenance level concept differs from the SLAN concept in that the main focus is on “fertilizing the soil” rather than “fertilizing the crop”. Optimum soil test levels are selected using either basic cation saturation percentages (BCSP) or ratios of the basic cation saturation percentages (BCSR) and sufficient nutrients are applied annually to attain these levels. Once optimum levels have been reached, maintenance applications equal to the amount of nutrient removed by the crop are applied to maintain the optimum soil test level.

 It is important to note that the manner in which the BCSR is determined can impact the interpretation of results considerably. Many of the reported ratios in the literature are based on the ratio of nutrients as a percent of the exchangeable bases. However, attempts are made to determine the amounts of fertilizer K, Ca, and Mg to apply from ratios of these nutrients in the soil sample analyzed without determining the CEC. If ratios are calculated in this manner ratios such as those reported by Vitosh et al. ( 1995) cannot be used for interpretative purposes. This can be illustrated by assuming that a coarse texture soil was extracted and found to contain 20 ppm exchangeable acidity (H+), 500 ppm Ca2+, 46 ppm Mg2+, 59 ppm K+. The following table shows the effect that method of calculation has on the resulting ratios and how they could influence the interpretation of results.


Element

  Soil Test  Results
 ppm

Soil Test Results, meq/100g

Ratios

BCSR†

BCR‡

Suggested BCSR Ratios††

H+

20

2

 

 

 

 

Ca2+

500

2.5

Ca/Mg

6.5:1

11:1

6.5:1

Mg2+

46

0.38

Mg/K

2.5:1

0.78:1

2:1

K+

59

0.15

Ca/K

16.7:1

8.4:1

13:1

CEC

 

5.03

 

 

 

 

  †Calculated based on percent cation saturation of CEC.
   ‡Calculated based on concentration of elements in soil; CEC not considered.
   ††Calculated from Bear et al. (1945) “ideal” saturation CEC percentage.

 As can be seen from these data there is no resemblance in the ratios calculated by the two methods. So it is apparent that fertilizer recommendations based on the BCR method and using the BCSR guidelines would be grossly inaccurate.

 To measure cation percentages on the cation exchange sites of an acidic to neutral soil, the soil sample is saturated with a cation such as ammonium (NH4+) using neutral ammonium acetate. The NH4+ ions are then displaced with another cation such as Ca2+ or Ba2+ and the amount of NH4+ displaced is determined which gives a measure of the cation exchange capacity (CEC). The amount of cations (Ca, Mg, K and Na) contained in the ammonium acetate leachate is determined using appropriate analytical procedures and the cation percentages are calculated based on the portion of the CEC they occupy. This is a time-consuming procedure and does not lend itself to most soil testing operations. Consequently, most laboratories determine the amount of cations contained in extracting solutions such as Mehlich 1, Mehlich 3, or ammonium acetate and the amount of exchangeable acidity using buffer solutions such as the SMP, Adam-Evans, or the Mehlich buffer. Summing the cations in the extracting solution and the exchangeable acidity in the buffer solution gives a reasonably good estimate of the CEC. The cation percentages can then be easily calculated. The numbers may differ slightly from those obtained with the ammonium acetate method but does not affect the percentage calculation appreciably.

 The maintenance level concept (BCSR) has gained increased use in turf, especially from commercial laboratories. One of the primary advantages for the laboratories using this approach is that extensive research databases do not have to be developed in order to make recommendations. Thus recommendations may be made  a) to fertilize to bring a particular cation up to a certain percentage on the CEC sites, b) to adjust a particular ratio, or  c) to raise the percent base saturation to 80% or other designated value. This approach generally results in higher recommendations for Ca, Mg, and K as compared with recommendations based on the SLAN approach.

 A number of scientists have questioned the usefulness or validity of the BCSR approach as noted by Haby et al. (1990) in a review of soil test approaches, who states "Numerous experiments over the past 40 years ... have demonstrated that the use of the BCSR approach alone for making fertilizer recommendations is both scientifically and economically questionable". These observations are primarily due to:

• Scientists have found that wide variations in percent CEC saturation for each cation and BCSR ratios occur and do not correlate well with plant response. Little evidence can be found for "ideal" cation ratios or a percent base saturation level of 80% as being "ideal".
• Raising the base saturation percentage of low exchange capacity soils to 80%, results in excessively high soil pH.
• Adjusting the percent K saturation on the CEC to 2 to 5% would not provide sufficient K to bentgrass on USGA specification greens with a CEC of 4 to 5 meq/100g . Therefore, to make meaningful recommendations as to the quantity of fertilizer to apply, plant needs have to be established for the growing season using the SLAN approach. Even using the SLAN approach, with USGA spec greens small and frequent applications of potassium must be made to prevent excessive leaching losses.
• Tendency to overestimate the need for expensive fertility level modifications (Christian, 1993).
Until recently very little correlation work has been conducted with micronutrients on turfgrasses. In some instances this has lead to misinterpretations because interpretative guidelines for agronomic or horticultural crops have been used to interpret data for turf. According to Carrow (1995) turfgrasses are very efficient in extracting micronutrients from the soil.  Therefore, using agronomic or horticultural interpretative guidelines to evaluate soil test data for turfgrasses would result in overestimating the micronutrient needs.

Confusion arises when a laboratory categorizes a micronutrient as low (only Fe and Mn have been proven to be sometimes deficient on some turfgrass soils) or excessively high.  When a micronutrient is classified as low recommendations are made.  When recommendations are made one would expect a growth or visual response.  This rarely occurs with most micronutrients.  Likewise, when a micronutrient is classified as very or excessively high it is often understood by a grower as a potential danger or as being toxic.  Turfgrasses can tolerate rather high levels of micronutrients and toxicities are very rare.

 

 

In This Section

Introduction
Sampling
Plant Analysis
Extraction
Interpretation
Recommendations
Soil pH
Reference

 

   

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