Five long-term tillage studies in Kansas were evaluated for changes in soil properties including soil organic carbon (SOC), water holding capacity (WHC), bulk density, and aggregate stability. The average length of time these studies have been conducted was 23 yr. Soil properties were characterized in three depth increments to 30 cm, yet changes due to tillage, N fertility, or crop rotation were found primarily in the upper 0-to 5-cm depth. Decreased tillage intensity, increased N fertilization, and crop rotations that included cereal crops had greater SOC in the 0-to 5-cm soil depth. Only one of five sites had greater WHC, which occurred in the 0-to 5-cm depth. Aggregate stability was highly correlated with SOC at all sites. No-tillage (NT) had greater bulk density, but values remained below that considered root limiting. Soil organic C levels can be modified by management that can improve aggregate stability, but greater SOC did not result in greater WHC for the majority of soils evaluated in this study. MATERIALS AND METHODS Five long-term study sites were selected across the state of Kansas as described in Tables 1 and 2, and located in Fig. 1.

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Reduced-and no-tillage seedbed preparation methods coupled with broadcast P applications lead to an accumulation of available P in the surface 0-to 5-cm soil layer and a depletion of available P deeper in the profile. A 3-yr study determined the effects of tillage and fertilizer P management on P uptake and grain yield for P-stratified soils. Tillage practices were moldboard plow (once at the start of the study followed by reduced tillage), reduced tillage (disk followed by field cultivation), and no-tillage. Four P management methods were imposed: (i) no P; (ii) 20 kg P ha 21 applied as a surface broadcast; (iii) 20 kg ha 21 applied as a banded starter, 5 cm to the side and 5 cm below the seed; or (iv) 20 kg ha 21 applied in a deep placed band, 13 to 15 cm on 0.7-m centers. The one-time moldboard plowing produced higher early season dry matter yields for corn (Zea mays L.), wheat (Triticum aestivum L.), and soybean [Glycine max (L.) Merr.] compared with the no-tillage system, but tillage effects on final grain yield were inconsistent. Subsurface placement of P generally increased P uptake and grain yield of corn and sorghum [Sorghum bicolor (L.) Moench], but had little effect on grain yield of soybean. Results indicate that subsurface applications of P fertilizers should be considered if soil test P is highly stratified within the surface 0-to 15-cm layer and the 15-cm composite is medium or below for available P.

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Process-based computer models have been proposed as a tool to generate data for Phosphorus (P) Index assessment and development. Although models are commonly used to simulate P loss from agriculture using managements that are different from the calibration data, this use of models has not been fully tested. The objective of this study is to determine if the Agricultural Policy Environmental eXtender (APEX) model can accurately simulate runoff, sediment, total P, and dissolved P loss from 0.4 to 1.5 ha of agricultural fields with managements that are different from the calibration data. The APEX model was calibrated with field-scale data from eight different managements at two locations (management-specific models). The calibrated models were then validated, either with the same management used for calibration or with different managements. Location models were also developed by calibrating APEX with data from all managements. The management-specific models resulted in satisfactory performance when used to simulate runoff, total P, and dissolved P within their respective systems, with > 0.50, Nash-Sutcliffe efficiency > 0.30, and percent bias within ±35% for runoff and ±70% for total and dissolved P. When applied outside the calibration management, the management-specific models only met the minimum performance criteria in one-third of the tests. The location models had better model performance when applied across all managements compared with management-specific models. Our results suggest that models only be applied within the managements used for calibration and that data be included from multiple management systems for calibration when using models to assess management effects on P loss or evaluate P Indices.

Core Ideas Claypan soil properties were minimally affected by tillage and N fertilization after 20 years. Soil organic matter distribution was affected by tillage, but with no apparent net difference. Long‐term tillage and N fertilization did not affect soil bulk density or penetration resistance. Subtle long‐term soil changes may not influence producer choices for tillage and N fertilization. Tillage and nitrogen (N) fertilization systems may affect soil chemical and physical properties, especially long term. This study's objective was to examine the effect of three tillage systems (conventional tillage, reduced tillage, and no‐till) with four N fertilizer treatments on properties of a claypan soil in the eastern Great Plains after 20 yr of cropping. Soil chemical and physical properties were little affected by long‐term tillage and N fertilization. The lack of tillage to distribute soil phosphorus (P) and potassium (K) nutrients within no‐till treatments resulted in lower P and K values below the surface. In no‐till, the accumulation of organic matter in the surface soil (0 to 7.5 cm) was countered by lower organic matter in the 7.5‐ to 15 ‐cm zone, resulting in no net increase compared with the other two tillage systems. Soil bulk density and penetration resistance in the surface 30 cm were unaffected by tillage or N fertilization.

The Agricultural Policy Environmental eXtender (APEX) model is capable of estimating edge-of-field water, nutrient, and sediment transport and is used to assess the environmental impacts of management practices. The current practice is to fully calibrate the model for each site simulation, a task that requires resources and data not always available. The objective of this study was to compare model performance for flow, sediment, and phosphorus transport under two parameterization schemes: a best professional judgment (BPJ) parameterization based on readily available data and a fully calibrated parameterization based on site-specific soil, weather, event flow, and water quality data. The analysis was conducted using 12 datasets at four locations representing poorly drained soils and row-crop production under different tillage systems. Model performance was based on the Nash-Sutcliffe efficiency (NSE), the coefficient of determination (r 2 ) and the regression slope between simulated and measured annualized loads across all site years. Although the BPJ model performance for flow was acceptable (NSE = 0.7) at the annual time step, calibration improved it (NSE = 0.9). Acceptable simulation of sediment and total phosphorus transport (NSE = 0.5 and 0.9, respectively) was obtained only after full calibration at each site. Given the unacceptable performance of the BPJ approach, uncalibrated use of APEX for planning or management purposes may be misleading. Model calibration with water quality data prior to using APEX for simulating sediment and total phosphorus loss is essential. (P) and sediment loss from agricultural fields continues to degrade fresh water quality despite decades of efforts to understand loss processes and implement management practices to reduce nonpoint pollution (e.g., Sharpley et al., 1994Sharpley et al., , 2015Sims and Kleinman, 2005;Jarvie et al., 2013). Failure of conservation practices to produce expected improvements in water quality has renewed appreciation for the complexity of P movement within landscapes and to waterbodies Sharpley et al., 2013). Processbased watershed-and field-scale models offer the prospect of integrating knowledge to assess and quantify impacts of conservation and management practices.The P Index was developed in the mid-1990s to assess risk of P loss from agricultural land (Lemunyon and Gilbert, 1993) and is now an integral part of the NRCS 590 Nutrient Management Standard and other state and federal programs (Sharpley et al., 2003(Sharpley et al., , 2017. However, the diversity in P Index ratings and P management recommendations for similar conditions led and Osmond et al. (2006) to emphasize the need for science-based assessment and improvement of existing P Indices.Extensive testing of P Indices requires water quality data from field-scale watersheds from a broad range of soils and management scenarios with a sufficient number of monitoring years to estimate long-term average annual losses. Such extensive datasets are rare. Hence, computer models must ...

Annual row crop production on the naturally occurring claypan soils of the eastern Great Plains may require field operations during somewhat wet conditions and this potentially results in soil compaction by the commonly-used, heavy-weight tractors and equipment. The objectives of this experiment were (i) to determine if compaction reduced yield and growth of soybean [Glycine max (L.) Merr.] and grain sorghum [Sorghum bicolor (L.) Moench] grown on a claypan soil (fine, mixed, thermic Mollic Albaqualf) and (ii) to determine the effect of wheel tracks on selected soil properties and whether chisel plow tillage could reduce wheel-track compaction. Compaction treatments were (i) ALL-all of the plot compacted, (ii) WT-wheel-track compaction, (iii) WTC-wheel-track compaction followed by a chisel tillage operation, and (iv) NO-no intentional compaction. In general, it took until the third year of annually repeated compaction in the ALL treatment to reduce crop growth and yields compared with the NO compaction treatment. Even though nearly half of the area was compacted each year in the WT treatment, few measured crop parameters decreased. In wheel tracks, soil penetrometer resistance and bulk density increased and air permeability decreased compared with out of tracks. However, chisel tillage appeared to eliminate the compaction by reducing penetration resistance and bulk density and increasing air permeability to values similar to out of tracks. Thus, compaction of claypan soils may not often be a problem for producers in this area, especially if occasional chisel tillage is included to remove possible compacted zones.