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, ridge-tillage and nutrients supplied by a mixture of organic amendments. Our objectives were to characterize yearly changes in surface soil-test parameters, including the stratification of P and K; compare soil-test methods (Mehlich III versus Bray P1 and 1 M NH4OAc exchangeable K); and evaluate soil-test interpretation strategies [sufficiency level versus basic cation saturation ratio (BCSR)] within a fanner-managed field. The study was conducted on the Clarion–Nicollet–Webster Soil Association near Boone, 1A, in a field that had not been moldboard plowed for 29 years. Soil samples were collected from the 0–5, 5–10, 10–15, and 15–20 cm depth increments before plowing and every autumn thereafter throughout the 5-year rotation. Samples were 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 was evident within 2 years after moldboard plowing. This was not considered a production problem, because, based on sufficiency-level soil-test interpretations, the animal manure/municipal sludge applications had increased soil P and K concentrations to levels where no crop response to additional fertilizer would be expected. Using Mehlich III extradant with simultaneous inductively coupled plasma–atomic emission spectrometer (ICP–AES) analysis resulted in soil-test interpretations that were the same as with Bray P1 and 1 M ammonium acetate (NH4OAc) extracting solutions. The Mehlich III solution did extract more Ca than 1 M NH4OAc, causing K saturation percentages to be very low when the data were interpreted using a BCSR approach. 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 warranted.
