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Unlocking the Mysteries:. Interpreting aSoil Nutrient Test for Sand-Based GreensReading and interpreting a soil nutrient test requires both knowledgeof the testing methods and value of the information.
by JAMES E. SKORULSKI
SOIL NUTRIENT tests and theirrecommendations can be confus-ing and perhaps even intimidat-ing. The confusion often arises fromthe methodology used to complete thetests, the terminology used in the testreports, and differing interpretationphilosophies employed by the labora-tories. Do not get discouraged. With alittle work, you can better understandhow tests are conducted, how themethodology used in the laboratorycan affect the test results, and whatinformation is most pertinent inmanaging fertilizer inputs for greens.
Extraction MethodsMuch of the confusion and mystery
surrounding soil nutrient tests arisesfrom the multitude of extractingagents used by laboratories to deter-mine concentrations of plant-availablephosphorus (P), calcium (Ca), mag-nesium (Mg), potassium (K), micro-nutrients, and calculate total CEC(cation exchange capacity) of the root-zone mix. The two most commonextracting agents used for P are theacidic Bray I, used when soil pH isbelow 7.2,and the alkaline Olsen, usedwhen soil pH is higher than 7.2. TheMorgan, Mehlich I, and Mehlich IIIare acidic extracting agents that alsoare used. The acidtc agents will dis-solve higher quantities of P fromcalcareous sands than may actually beavailable to the plant.
Neutral ammonium acetate (pH 7)is used by most soil laboratories todetermine K, Ca, Mg, and Na cationconcentrations on exchange sites andin soil solution. Laboratories located inthe central and western states, wheremost soils are calcareous, may useammonium bicarbonate or sodium bi-carbonate agents. A number of labora-tories choose to use more acidicammonium acetate (pH
based on research in agriculturalforage crops. Laboratories developnutrient target values and subsequentfertilizer recommendations from theSLAN or BCSR interpretation aloneor from a combination of the twomethods.
The arguments as to which methodis more effective are being debated byscientists and superintendents alike.The SLAN method is the moreproven, traditional approach that willprovide an accurate assessment ofnutrient and fertilizer needs in puttinggreens. Percent base saturation andthe ratios between Ca, Mg, and K willadjust closely to the "ideal" when pHproblems are corrected and fertilizerapplications are made to eliminatespecific cation deficiencies.
The BCSR information can be ahelpful tool to avoid any gross imbal-ances between cations and to trackthe effects of your fertility programs onthe soils over time. It is also helpful fortracking Na levels on the exchangesites in salt-affected soils. However, itis not advised to become overly con-cerned with trying to meet the "idealratio," especially if pH is in a desirablerange. Such efforts may result inunnecessary fertilizer applications andlead to nutrient deficiencies and anundesirable pH.
So what should you look for inregard to target values for the l?, Ca,Mg, and K in sand-based greens?Target ranges developed through theSLAN approach will be effective forP and the cations. Request thosevalues along with subsequent limingand fertilizer recommendations fromthe laboratory conducting the test.Remember, however, that limitedexchange sites in a sand-based systemwill not make it possible to meet thesufficiency target values for potassium,and more frequent and light appli-cations will be required to meet theturf's needs. Calcium deficiencies arevery rare in the field. Adjusting soil pHto optimal levels should provide all theCa required by the turf. Mg defi-ciencies are more likely to occur insystems built with calcium carbonatetype sands. Strive to meet the Mgtarget values generated from SLANinterpretations and monitor BCSRdata to avoid a deficiency.
Total salinity and Na saturation arealso concerns in saline, sodic, orsaline-sodic soils or when waterquality issues exist. Total dissolvedsalts (TDS) and electrical conductivity(EC) are measures of soil salinity. Note
Table 2"Ideal" Base Saturation
Ca 65-85/0Mg 10-20%K 2-7/0Na 0-5%H 0-5/0
Ratios: Ca/Mg < 6.5:1CalK < 13:1MglK < 2:1
Some laboratories base target nutrientranges on the calculated CEC value.
Available NutrientsSoil nutrient tests most often pro-
vide information on available l?, Ca,Mg, K, and Na. Information on nitratenitrogen can be requested, but isusually not provided because its levelschange so rapidly. Information fornitrate- Nand P will likely becomemore critical for environmental moni-toring and "best management pro-grams." Requests can be made to testfor the availability of S, Fe, Mn, Cu,Zn, Mo, and B as well. The infor-mation is provided in pounds per acreor parts per million (multiply ppm by2 too convert to pounds per acre). Thetest report mayor may not providespecific target ranges for nutrients, inaddition to fertilizer recommendations.
Lime and fertilizer recommenda-tions are based on the soil pH andsufficiency level of available nutrients(SLAN), which is the traditionalmeans of predicting the total quantityof plant-available nutrients. Manylaboratories report calculated percentsaturation of Ca, Mg, K, and Na onexchange sites. This is not the totalquantity of cations that are availablein the soil solution. Laboratories usingthe base cation saturation ratio(BCSR) approach develop recommen-dations by ranking the calculatedpercent cation saturation with an"ideal base saturation" (see Table 2),
Table 1Nutrient Holding Capacity' of Soil 'Based on CEC
Potassium Magnesium CalciumCEC
Cation Exchange CapacityThe cation exchange capacity (CEC)
or total exchange capacity is usuallyprovided on a test report. It reflectsthe potential ability of the sand or soilto exchange cations (Ca, Mg, K, Na,and H). The number provided on thereport estimates the negative chargesavailable to bind with cations. It isdetermined by saturating a samplewith an exchanging or extractingagent, and is measured in milli-equivalents per 100 grams of soil. CECvalues will vary depending on theagent used to complete the test. TheCEC of a sand rootzone is termed verylow or low, usually ranging from 1 to10 meqll00g soil (often 2-5), ascompared to a loam soil that mayrange from 15 to 28 meq/l00g soil.CEC generally increases in new sand-based greens as the organic mattercontent and pH increase. Peat, com-post, small quantities of soil, or someinorganic amendments are oftenadded to sands used in green con-struction to increase water retentionand CEC.
A "very low" or "low" CEC valuemeans there are fewer negativelycharged exchange sites to bind withpositively charged cations. Sand mixeswill also have a lower buffering andnutrient-holding capacity (see Table 1)and fertility programs become morecomplex. Such systems require morelight and frequent applications of Nand K to minimize leaching potential.
larger quantities of lime to raise thepH. Applications of calcitic limestone(25-30% Ca) are recommended toraise pH if calcium is considereddeficient, whereas dolomitic limestone(20-250/0 Ca, 10-15% Mg) will berecommended when Mg is deficient.Sands with a pH>7.2 (alkaline andoften calcareous) can also be tested forfree calcium carbonate (free lime).That information is used for develop-ing recommendations for acidificationor reclamation programs for sodic andsaline soils.
10 USGA GREEN SECTION RECORD
Iron deficiencies are the most common micronutrient concern in alkaline rootzonemixes, especially when P and Mn concentrations are high. The deficiency is easilycorrected with an iron sulfate or chelated iron product.
that TDS (ppm) = EC (mmhos/cm) x640. Exchangeable sodium percentage(ESP) and sodium adsorption ratio(SAR) are measures used to determinethe potential for sodium to influencesoil structure and permeability. Re-quest total salinity, ES~ SAR calcu-lations, and concentrations of toxicions (B, Na, Cl, and S04) if you aredealing with salt-affected soils. A morethorough discussion of this topic canbe found in Salt-Affected TurfgrassSites (see references below).
MicronutrientsThe practicality of recommenda-
tions for micronutrients is also ques-tionable in most cases. Micronutrientsare usually extracted in the laboratorywith the chelating agent DTPA.Mehlich I and Mehlich III extractionmay also be used to extract certainmicronutrients. There have been noactual micronutrient deficienciesreported for turf in the field with theexception of Fe and Mn, which can bedeficient in certain parts of thecountry. Soil tests may also report thata micronutrient is excessively high,which can raise unnecessary concernsin the field, where such toxicities arevery rare. A tissue test can be con-ducted if a micronutrient deficiencyor toxicity is suspected in the field.Micronutrient deficiency or toxicityproblems will be more of a concern atextreme soil pH levels.
ConclusionThe soil nutrient test is a very useful
tool for managing fertility programsfor your greens. It can also bemisleading if extracting agents areunknowingly changed or the testresults are not interpreted correctly. Itis wise to choose one soil-testinglaboratory that uses extractants thatare appropriate for your soil type, andto use that laboratory consistently tobetter correlate the test values withturfgrass response and performanceunder your conditions.
Remember that the test only pro-vides a "snapshot" of the nutrientstatus of the sand rootzone and thatconcentrations of specific nutrientswill fluctuate rapidly, especially whenCEC is low. Therefore, use the tests asa basic roadmap for your fertilitypractices and concentrate on the mostimportant information of soil pH andliming