Submitted to: American Journal of Alternative Agriculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/15/2001
Publication Date: 2/1/2002
Citation: KARLEN, D.L. SOIL-TEST DYNAMICS THROUGHOUT A FIVE-YEAR "THOMPSON FARM" ROTATION. AMERICAN JOURNAL OF ALTERNATIVE AGRICULTURE. 2002. V. 17(1). P. 9-17. Interpretive Summary: This study of yearly changes in soil-test values showed phosphorus (P) and potassium (K) stratification (layering) reappeared within two years of moldboard plowing. This did not appear to be a yield-limiting problem based on the sufficiency-level soil-test interpretation philosophy. The animal manure/municipal sludge applications in 3 of every 5 years of the crop rotation have raised soil P and K concentrations to levels where crop response to additional fertilizer would not be expected. We also compared different soil-test methods known as Bray P1, ammonium acetate (NH40Ac) exchangeable, and Mehlich III. Laboratories often use Mehlich III because P, K, calcium (Ca), magnesium (Mg), sodium (Na) and zinc (Zn) can all be measured using just one extraction. The other methods are more specific with Bray P1 being used just for P and NH40Ac being used to only measure exchangeable cations (K, Ca, Mg, and Na). Our results for P and K showed good agreement between the soil-test methods for this relatively "high" fertility soil. Values for the surface 15-cm (6-inch) depth gave the same soil-test rating (i.e. medium, high, or very high) and thus the same fertilizer recommendation. An advantage for using the Mehlich III method is that it can provide producers with more soil-test data at very little additional cost. A disadvantage demonstrated in our study was that the Mehlich III method extracted more Ca than the NH40Ac. This may be a problem if cation concentration data are to be evaluated using a cation ratio soil-test interpretation philosophy. Our study provides producers with real-farm data showing how rapidly P and K stratification can occur. It also compares common soil-test methods that they may be offered to farmers and discusses three interpretation philosophies.
Technical Abstract: Soil-testing is an effective guide for achieving and maintaining an optimum supply of available plant nutrients, but there is little information on the multi-year soil-test changes for farms using long-term crop rotations and nutrients supplied by a mixture of organic sources. Our objectives were to characterize yearly changes in surface soil-test parameters, compare soil-test methods (Mehlich III versus Bray P1 and 1 M NH4OAc Exchangeable K), and examine soil-test interpretation strategies [basic cation saturation ratio(BCSR) versus sufficiency level] within a farmer-managed field near Boone, IA. Soil samples were collected from the 0 to 20-cm depth in 5-cm increments every October, and analyzed for pH, Bray extractable P, exchangeable K, total C and N, NH4- and NO3-N concentrations, and Mehlich III extractable P, K, Ca, Mg, Na, Mn, and Zn. Stratification of P and K reappeared within two years after moldboard plowing. This was not considered a production problem, because based on sufficiency-level soil-test interpretations, the farm manure/municipal sludge applications had increased soil P and K concentrations to levels where no crop response to additional fertilizer would be expected. The Mehlich III solution extracted more Ca that NH4OAc causing K saturation percentages to be very low when the data were interpreted using a BCSR approach. However, this study was not designed to determine if the low K saturation was economically or nutritionally important. We conclude that stratification is not a production issue at this site, that either soil test methodology would be acceptable for P and K extraction, and that additional studies evaluating the BCSR soil-test interpretation philosophy are needed.