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Thirteen states associated with the Southern Extension and Research Activities Information Exchange Group-6 (SERA-IEG-6) agreed to share their soil test based P and K rate recommendations for nine major crops. The objectives were to compare fertilizer P and K rate recommendations, to look for opportunities to rationalize similar recommendations across state lines, and to examine challenges to the development of a cooperative regional approach to P and K recommendations. Mehlich-3 (eight states), Mehlich-1 (five states), or Lancaster (one state) extractions were the basis of plant available soil P (STP) and K (STK) assessment. Fertilizer recommendation philosophies (sufficiency, build and maintain, and/or hybrid) variation among the states might be the main reason behind such discrepancies. Although a few similarities in P and K rate recommendations were found, the different philosophies, numerical presentations, and extraction procedures drove important recommendation differences. Widespread adoption of the Mehlich-3 extraction procedure has not reduced variation in fertilizer P and K rate recommendations among the states. Instead, for states using Mehlich 3, soil test critical concentrations ranged from 30 to 75 mg P kg(-1) and 60 to 175 mg K kg(-1) for corn (Zea mays L.) grain and warm-season grass hay production. The adoption of uniform soil testing terminology, sample collection guidelines, extraction methods, and interpretations across common physiographic regions, soils, and state lines remains a challenge. Differences arise because of the different soil orders and properties, climate conditions, and resulting crop responses to added P and K fertilizers. Such differences in soil-test-based fertilizer P and K recommendations are state specific and highlight needs to examine the soil testing and recommendation process, make soil test results end-user friendly, and, when appropriate, standardize fundamental information used in the soil testing guidelines. USDA-NRCSUnited States Department of Agriculture (USDA) [69-3A75-17-45]; USDA-ARS Award [58-8070-8-016] Published version The authors are thankful for the partial funding provided by USDA-NRCS Grants 69-3A75-17-45 (CIG), as well as USDA-ARS Award 58-8070-8-016.

Measuring biological net soil nitrogen mineralization (NSNM) can assist nitrogen (N) fertility management in agroecosystems, but the time and apparatus required for this assessment limits measurement frequency. Stable NSNM predictors that could be rapidly measured to compare systems would be extremely useful. We compared the effects of crop and soil management on NSNM from 2011 to 2013 and evaluated several carbon (C) and N indices including particulate organic carbon (POC), particulate organic nitrogen (PON), permanganate oxidizable carbon (POXC), soil organic carbon (SOC), and soil total nitrogen (STN) as NSNM indicators. Soil (0 to 10 and 10 to 20 cm depths) was collected from six long-term burley tobacco (Nicotiana tobacum L.) tillage and rotation systems. Net soil nitrogen mineralization was estimated by a 120-d laboratory incubation. The relative effects of management on NSNM among these systems were stable across 3 yr, which suggested NSNM measurement can be reduced to that frequency when NSNM is used to compare different crop and soil management. The soil C and N fractions contributed variably to predicting soil N mineralization in the different rotation systems, but SOC (which can be calculated from soil organic matter, a common index in the routine test package of many soil testing laboratories) was the best overall NSNM predictor in this study.

Burley tobacco (Nicotiana tobacum L.) production in Kentucky is typically tillage intensive and is often mono-cropped. Intensive tillage has been reported to degrade soil structure and deplete soil organic carbon (SOC) and total soil nitrogen (TSN) stocks. This study assessed the effect of tillage, crop rotation, and N fertilizer application on soil structure, water-stable aggregation, and aggregate-associated SOC and TSN concentrations and stocks for rotations including burley tobacco. Four burley tobacco production systems (main plots) were established in 2007 on a Bluegrass-Maury (fine, mixed, active, mesic Typic Paleudalf) silt loam soil including: (i) no-tillage continuous tobacco (NT-Tobacco); (ii) conventional tillage continuous tobacco (CT-Tobacco); (iii) 2-yr sod (Festuca arundinacea Schreb.) 1-yr no-tillage tobacco (NT-Rotation); (iv) 2-yr sod and 1-yr conventional tillage tobacco (CT-Rotation). In 2012, two N rates (0 and 280 kg N ha⁻¹) were applied to split plots. Soil samples were taken in Spring 2013 to determine water-stable aggregate-size distributions and associated SOC and TSN concentrations and stocks. Compared with conventional tillage and tobacco monoculture, no-tillage and rotation with sod significantly increased macroaggregate fractions (>250 μm), and associated SOC and TSN stocks at the expense of the silt-clay size class (

Tillage impacts many components and functions of the soil ecosystem. Thus, the prediction of soil structures produced by tillage may be regarded as a major objective in soil science. Brittle fracture is the desired mode of failure in most tillage operations. Mechanistic or phenomenological models based on the probabilistic Weibull "weakest link" theory are commonly applied to model brittle fracture of air-dry aggregates. The overall objective of this study was to develop a Weibull model to describe the strength of different-sized soil aggregates across a wide range of water contents. Rupture energy data were obtained for aggregates sampled in three field experiments. These included two soil compaction experiments (Bygholm I and II, sandy loam) and a long-term tillage and fertilization experiment (Maury, silt loam). Aggregates were subjected to a crushing test after having been adjusted to matric potentials ranging from -10 kPa to -163 MPa (air dry). Water content strongly affected the characteristic rupture energy (Weibull α parameter), and this relationship was successfully modeled with a power law function. In contrast, water content had little or no effect on the spread of aggregate strengths (Weibull β parameter). Based on these results, we proposed a three-parameter Weibull brittle fracture model for the tested sandy loam and silt loam soils that takes account of the effect of water content for a single aggregate size fraction. This model, in which only α depends on water content, explained on average 89% of the total variation in rupture energy. Further research is needed to fully investigate the influence of water content on the rupture energy of different-sized aggregates.

We combined water activity meters and fractal modeling to facilitate rapid, physically based characterization of the soil water retention curve. Desorption data (6 points per sample) were collected in the tension (h) range 2.0 x 10 2 to 1.5 × 10 5 kPa using the gravimetric method to measure water content (w) and water activity meters to measure h. Thirty-two disturbed samples from a long-term nitrogen fertilization and tillage comparison study on a silt loam soil were analyzed. A new form of an established fractal equation was derived: w = ah D-3 - ρ n /ρ s , where a is a compound parameter including the bulk density (ρ b ) and air entry tension (h a ), D is the mass fractal dimension, ρ w is the density of water, and ρ s is the particle density. This model was fitted to the measured water retention curves by nonlinear regression analysis. The a and D parameters were estimated, while ρ w and p, were fixed at 1.00 and 2.65 Mg m -3 , respectively. Convergence was always achieved and the equation fitted the data extremely well; residual sums of squares ranged from 1.2 × 10 -6 to 6.7 x 10 -5 , with a median value of 2.2 X 10 -5 . Estimates of a (0.62-0.74) and D (2.948-2.963) were physically reasonable, and sensitive to soil management practices. The a parameter increased, r = 0.80 (P < 0.01), (signifying decreasing ρ b and/or h a ) whereas D decreased, r = -0.75 (P < 0.01), (signifying more rapid capillary drainage) with increasing soil carbon content. Additional research is needed to test this approach on other soil types, and to assess the influence of soil disturbance and variations in p, on the model's performance.

The compactability of a soil can be determined from parameters derived from laboratory compaction curves generated using the Proctor test. However, this is a destructive, time-consuming, and labor-expensive procedure. Our objective was to evaluate a new more rapid test to determine the maximum dry bulk density (ρ Max b and soil water content at ρ Max b (SWC Max ) from laboratory compaction curves using a mold with smaller dimensions than established for the standard procedure. Laboratory compaction curves were developed for nine soils with clay contents ranging from 232 to 385 g kg -1 using the standard Proctor test procedure and the proposed procedure, which uses a 54-mm diam. mold and a 24.4 N rammer dropped from a height of 305 mm to produce a compactive effort (CE) of 109 kJ m -3 drop -1 . At a CE of 545 kJ m -3 , which is similar to the 540 kJ m -3 produced by the standard test, the ρ Max b and SWC Max parameters derived from the proposed procedure were positively and significantly correlated with those derived from the standard Proctor test. With both procedures, ρ Max b decreased and SWC Max increased as the soil clay content increased. The small mold method did not affect the linear relationship between soil clay content and ρ Max b , However, different relationships between SWC Max and clay content were observed, depending on the compaction procedure. Use of the small mold procedure requires less dry sieved soil, saves time, and labor in evaluating soil compactability. Based only on the reduction in rammer drops, use of the small mold procedure involves 15 times less labor requirements than the standard Proctor test.