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Soil Testing Procedures for Calcareous Soils Soil Testing Procedures for Calcareous Soils Well-established soil testing methods on soils containing calcium carbonate or gypsum often underestimate fertilizer and soil amendment requirements. BY BRIAN WHITLARK magine a scenario where you have been consistently collecting soil data Table 1. Ifor tees, fairways, and greens for Laboratories may offer one or more of the following soil extracting more than fi ve years, using one lab solutions to determine exchangeable cations and estimated CEC. and one procedure. Does this sound Soil Testing Extractant Solution pH Notes familiar to you? If it does, great! Regular Mehlich 1 1.5 Should not be used to determine soil testing is an invaluable tool for cations on calcareous or developing and fi ne-tuning fertilizer gypsiferous soils and soil amendment programs. Receiving accurate and consistent Mehlich 3 2.5 Should not be used to determine values from the laboratory, coupled cations on calcareous or with personal observations, allows for gypsiferous soils intelligent decision-making in the fi eld. Modifi ed Morgan 4.8 Should not be used to determine But what if you were to discover that cations on calcareous or the data for all those years was wrong? gypsiferous soils Would you change to a more accurate Saturated Paste 7.0 Yields nutrients in the soil solution procedure? If you grow turf on calcar- eous soils (those containing calcium 1:5 soil-water extraction 7.0 Values must be corrected to carbonate) or gypsum, it is possible, correlate with saturated paste and even likely, that you should con- Ammonium Acetate 7.0 Most commonly used extraction sider changing soil testing procedures. method, but should not be used on However, in the absence of free calcareous or gypsiferous soils carbonates or gypsum, a change in Ammonium Chloride 5.5 Not commonly used due to soil testing methods is unneeded. The extra steps that account for the objective of this article is to alert golf dissolution of calcium carbonate course superintendents who manage turf on calcareous or gypsiferous soils Elevated pH >8.0 Most accurate extraction currently (containing gypsum in excess of 2%) Ammonium Acetate used for soils containing calcite or that nutrient data obtained from the gypsum most commonly used and accepted soil testing extractants may result in shaken with the soil, the extractant is IS THERE A PROBLEM WITH misguided fertilizer and soil fi ltered and analyzed for soil nutrient THE EXTRACTANT THAT I AM amendment programs. content. It is important to understand CURRENTLY USING? the nutrients removed from the soil The answer may be yes, especially if WHAT IS A SOIL TESTING represent those nutrients immediately you are growing turf on a calcareous EXTRACTANT? available and those that are slowly or gypsiferous soil. Unfortunately, a Nationwide, soil laboratories that test available for turf use. An entirely differ- “universal” soil testing extractant is golf tees, fairways, and putting greens ent test provides total nutrients in the not currently available, hence a wide use a variety of soil extractants to soil — those that are available and variety of solutions are used, including determine plant-available nutrients those that are unavailable or “structur- acid extractants such as Mehlich I, in the soil. You may wonder what ally bound” in the soil. A third test is Mehlich 3, and the Modifi ed Morgan, exactly a soil testing extractant is. An the water-based extraction (saturated a neutral pH ammonium acetate extractant is a solution of water and paste test), which measures only what extractant (pH 7.0), and the elevated various chemicals that, when mixed is in the soil-water solution or immedi- pH ammonium acetate solution (>pH with soil, extract or remove nutrients ately soluble at the time of testing and 8.0) (Table 1). The problem is this: from the soil that are: 1) contained in grossly underestimates the nutrient when testing a soil that contains cal- the soil solution, 2) easily dissolved in supplying power of the soil. These cium carbonate, magnesium carbonate, water, and 3) held on soil exchange three extracting methods are often or gypsum, the acid and neutral pH sites and available for plant use. Once confused. extractants overestimate calcium (Ca) Green Section Record Vol. 49 (9) TGIF Record Number 176852 March 4, 2011 Page 1 HOW WILL THE WRONG EXTRACTANT AFFECT MY FERTILIZER PROGRAM? When testing a calcareous soil, acid and neutral extractants will overesti- mate ECEC, exchangeable Ca, and related base saturation percentage and cation ratios. Although research does not support the use of cation ratios and percentages as a basis to develop fertilizer programs, those turf managers who still use this method will fi nd the data they have been using is incorrect if their soils contain calcium carbonate and the soil testing extractant is acidic or has a neutral pH. HOW WILL THE WRONG EXTRACTANT AFFECT MY SODIUM REDUCTION PROGRAM? Mike Rothenberg, a soil consultant for Brookside Laboratories, encourages When acid is placed on a soil containing appreciable calcium carbonate, the soil turf managers in the Southwest to use fi zzes in the form of carbon dioxide gas bubbles. the elevated pH ammonium acetate extractant. Mr. Rothenberg stresses the importance of testing the soil to and to a lesser extent magnesium is quite accurate and results in a value confi rm the presence of calcite prior to (Mg). For example, when the pH- similar to the actual CEC. The actual recommending a change in soil testing neutral ammonium acetate extractant CEC is measured via a complex, time- procedure. Over the last few years, Mr. is used on a calcareous soil, it may consuming procedure and is typically Rothenberg noticed that when super- overestimate Ca by as much as 60% only performed upon request. Testing intendents changed to the correct (3). In another example, the pH 7.0 a calcareous soil with a neutral or acid extractant, they were always surprised ammonium acetate procedure may extractant will overestimate Ca and Mg, to see the Ca levels decreased and the yield up to 40% more calcium than is and when the cations are summed, the sodium percentage increased. Many actually available for plant use and estimated CEC is much higher than realize they need to modify their Na exchange for sodium (Na) in the soil the actual CEC. It is important to note reduction program to address the (2). Overestimating Ca levels in the that such an error does not occur in correct values. Let’s take a look at a soil not only affects Ca, but will cause the absence of Ca or Mg carbonates few examples where turf managers in inaccurate estimates of CEC, base or gypsum. Largely dependent on the Arizona have realized the benefi ts of saturation levels, and cation ratios, and calcium carbonate levels in the soil, accurate soil test data. will underestimate the exchangeable the ECEC may be overestimated by sodium percentage (ESP). a substantial margin. The more the calcium carbonate, the greater the HOW WILL THE WRONG difference will be. EXTRACTANT AFFECT THE It is important to know whether the SOIL CATION EXCHANGE soil has a high or low CEC when deter- CAPACITY? mining fertilizer frequency and rates. A Overestimating Ca and Mg will result low-CEC soil will require more frequent, in erroneously high estimated CEC light fertilizer applications to sustain values (ECEC). You may be curious healthy turf and to avoid unnecessary what “estimated CEC” means. Soil nutrient losses due to over-application testing laboratories often calculate and subsequent leaching. The CEC the CEC by summing the four basic is also commonly used to determine Different soil testing methods are cations: Ca, Mg, K, and Na. Such a gypsum, sulfur, and lime requirements used to determine the total nutrients in calculation assumes the cations and to calculate amendments to the soil, the available nutrients, and occupy 100% of the soil exchange remediate sodium affected (sodic) those in the soil solution. These three sites. In alkaline soils, this assumption soils. extracting methods are often confused. Green Section Record Vol. 49 (9) March 4, 2011 Page 2 Calcium extracted with ammonium acetate pH 7.0 and 8.5 Exchangeable Sodium Percentage using ammonium acetate extractant at pH 7.0 and 8.5 6000 0.300 ESP (pH 7.0) ESP (pH 8.5) 5000 Avg. 0.238 4,596 ppm Avg. 18% pH 8.5 4000 0.175 Ca pH 7.0 Ca pH 8.5 Avg. 13% pH 7.0 Calcium in parts per million Text Avg. Exchangeable Sodium Percentage (ESP) 3000 2,845 0.113 ppm 2000 0.050 123456789101112131415161718192021222324252627282930 123456789101112131415161718192021222324252627282930 Figure 1. Calcium values extracted with ammonium acetate Figure 2. Exchangeable sodium percentage computed using at pH 7.0 (red line) and pH 8.5 (green line) from 30 soil exchangeable cation values from 30 different soil samples samples collected from a calcareous soil fairway in collected from a calcareous soil fairway in Goodyear, Ariz., Goodyear, Ariz. with ammonium acetate at pH 7.0 (red line) and at pH 8.5 (green line). GOODYEAR, ARIZ., fact, the turf was so thin, the members pore space, yet not so much that any GOLF COURSE FAIRWAY were not allowed to drive on the fair- water rests on top of the soil. The In a study completed in 2008 (3), 30 way. Soil testing using the Mehlich 3 soil-water paste is poured into a funnel soil samples were collected from a extractant revealed Ca levels equaled lined with fi lter paper and placed over calcareous soil fairway in Goodyear, about 7,400 ppm, Na levels at 1,700 a vacuum to suck the soil solution from Ariz. The soils were analyzed with ppm, and an ESP of 14%.
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