Nitrogen fertilizer is a fundamental input for production of corn (Zea mays L.) that can move to ground and surface waters when overapplied. Previous research has shown that chlorophyll meter (CM) readings can indicate N stress in corn, but has not addressed whether the amount of N needed can be predicted by CM readings. Our objective was to evaluate whether CM readings can predict corn N need and yield response to N. Sixty-six N rate experiments were conducted in seven north-central states over a 4-yr period. Linear regression was used to relate absolute and relative CM readings over a range of growth stages to economically optimal N rate (EONR) and yield response to N applied at growth stage V7 or earlier. Chlorophyll meter readings at all growth stages from V5 to R5 were significantly related (P , 0.0001 in 22 of 24 models, P , 0.01 in 2 models) to EONR and yield response to N. Relationships were stronger for relative than for than absolute CM readings, and also were stronger when the corn had received no N fertilizer at planting. Coefficients of determination ranged from 0.53 to 0.76 for relative CM reading as a predictor of EONR or yield response to N, and were lower for the V5 to V9 stage than for later stages. Earlier research has indicated that measurements with this level of predictive accuracy can produce N rate recommendations that are more profitable than current N management practices. Our findings suggest that CM readings (and potentially other measures of corn color) are quantitatively related to early-season EONR and yield response to N over a wide range of environments with enough accuracy to be helpful in making management decisions.

Soybean isoflavone concentrations vary widely, but the contribution of soil fertility and nutrient management to this variability is unknown. Field experiments from 1998 to 2000 on soils with low to high exchangeable potassium (K) concentrations evaluated K application and placement effects on isoflavone concentrations and composition of soybean in various tillage and row-width systems. Soybean seed yield and concentrations of daidzein, genistein, glycitein, leaf K, and seed K were measured. Significant increases in daidzein, genistein, and total isoflavone were observed with direct deep-banded K or residual surface-applied K on low-K soils. Positive effects of K fertilization on isoflavones were less frequent on medium- to high-testing K soils. Both individual and total isoflavones were often positively correlated with seed yield, leaf K, and seed K on low-K soils. Appropriate K management could be an effective approach to increase isoflavone concentrations for soybeans produced on low- to medium-K soils.

Phosphorus and K fertilization increases alfalfa (Medicago sativa L.) yield and stand persistence, but the changes in yield components as affected by P and K fertility level are not known. Our hypothesis is that P and (or) K fertilization will increase one or more alfalfa yield components, and those component responses may change with stand age. The objectives of this field study were to determine the impact of P and K fertilization on alfalfa forage yield and yield components during the initial 3 yr after establishment. Treatments were the factorial combinations of four P rates (0, 25, 50, and 75 kg P ha−1) and five K rates (0, 100, 200, 300, and 400 kg K ha−1) arranged in a randomized complete block design with four replications. Forage harvests occurred four times annually, and yield, mass shoot−1, and shoots area−1 were determined. Plant populations were determined in early December and late May each year. Incremental additions of P and K increased alfalfa yield in each year. Potassium fertilization did not influence plant population, while robust P‐responsive alfalfa plants apparently crowded out smaller, less vigorous plants thus decreasing plants m−2 Stand assessments based on shoot counts, or aboveground plant counts may not accurately indicate alfalfa yield potential. Shoots plant−1 was not affected by application of either nutrient, while shoots m−2 generally declined with increased P and K fertilization. Improved forage yield of P‐ and K‐fertilized plots was consistently associated with greater mass shoot−1 Because fertilizer‐responsiveness is closely associated with greater mass shoot−1, cultivars possessing this trait may be relatively more productive under well‐fertilized conditions.

The conventional 1 M NH 4 OAc-exchangeable potassium (K ؉) soil The neutral 1 M ammonium acetate (NH 4 OAc) test is inadequate in soils where nonexchangeable K ؉ contributes method, which extracts both solution and exchangeable significantly to crop nutrition. Studies were conducted (i) to compare K ϩ , is the most common soil test method used to develop the abilities of the 1 M NH 4 OAc method with a modified NaBPh 4 method to estimate critical soil K ؉ levels, (ii) to estimate the contribu-K ϩ fertilizer recommendations. This method does not tion of nonexchangeable K ؉ to plant-available K ؉ , (iii) to compare measure plant-available nonexchangeable K ϩ nor the the abilities of the 1 M NH 4 OAc method and the modified NaBPh 4 relationships among different pools of soil K ϩ. When method to estimate plant dry matter yield and plant-available K ؉ , exchangeable K ϩ is depleted to its critical level, further and (iv) to compare the abilities of both methods to measure soil K ؉ plant K ϩ uptake is regulated by the rate of K ϩ release balance. Winter wheat (Triticum aestivum L. 'Abe') was grown in from the nonexchangeable pool (McLean and Watson, eleven Midwestern soils in a greenhouse using consecutive 28-d defoli-1985). The inadequacy of the NH 4 OAc soil test has been ation and regrowth cycles. Soils also were incubated for 6 mo with clearly demonstrated in illitic (McLean, 1976; Portela, five K ؉ rates (0-809 mg K ؉ kg Ϫ1). Ammonium acetate-and NaBPh 4-1993; Eckert and Watson, 1996) and vermiculitic (Cassextractable K ؉ (5-min extraction period) were determined in soil man et al., 1990) soils. Nair et al. (1997) showed that samples taken after every three defoliation cycles and after incubation. NH 4 OAc-extractable K ϩ alone was a poor indicator of Critical soil K ؉ levels could not be determined by either method alone but could be predicted by including cation-exchange capacity (CEC) K ϩ availability to cardamom [Elettaria cardamomum and illitic K ؉ content in regression models. Nonexchangeable K ؉ rep-(L.) Maton] in kaolinitic soils, unless it is integrated resented a significant portion of plant-available K ؉. Plant-available with soil K ϩ buffer power. Obviously, NH 4 OAc-extract-K ؉ and dry matter (DM) yield were well related to NH 4 OAc-extractable K ϩ alone is not always a reliable estimate of plantable K ؉ only in soils with low nonexchangeable K ؉ contribution (r 2 ϭ available K ϩ. 0.889 and 0.915, respectively), but they were well related to NaBPh 4-Richards and Bates (1988) found that StepK (loosely extractable K ؉ in all soils (r 2 ϭ 0.984 and 0.874, respectively). Slopes bound nonexchangeable K ϩ released by repeated exfor NH 4 OAc-extractable K ؉ vs. soil K ؉ balance varied widely among traction with 1 M HNO 3) was a more reliable index soils (0.16-0.68) depending on NH 4 OAc-extractable K ؉ , illitic K ؉ , and of the K ϩ-supplying power of Canadian soils than is clay content, but for NaBPh 4-extractable K ؉ slopes were near unity. NH 4 OAc-extractable K ϩ. They concluded that a simple These studie...

The Soil and Water Assessment Tool (SWAT) is increasingly used to quantify hydrologic and water quality impacts of bioenergy production, but crop-growth parameters for candidate perennial rhizomatous grasses (PRG) Miscanthus 9 giganteus and upland ecotypes of Panicum virgatum (switchgrass) are limited by the availability of field data. Crop-growth parameter ranges and suggested values were developed in this study using agronomic and weather data collected at the Purdue University Water Quality Field Station in northwestern Indiana. During the process of parameterization, the comparison of measured data with conceptual representation of PRG growth in the model led to three changes in the SWAT 2009 code: the harvest algorithm was modified to maintain belowground biomass over winter, plant respiration was extended via modified-DLAI to better reflect maturity and leaf senescence, and nutrient uptake algorithms were revised to respond to temperature, water, and nutrient stress. Parameter values and changes to the model resulted in simulated biomass yield and leaf area index consistent with reported values for the region. Code changes in the SWAT model improved nutrient storage during dormancy period and nitrogen and phosphorus uptake by both switchgrass and Miscanthus.Abbreviations ACRE = agronomy center for research and education BIO_E = radiation use efficiency 9 10 BLAI = maximum leaf area index CMN = rate of humus mineralization CYLD = nutrient fraction at harvest DLAI = point of the growing season when senescence begins HEFF = harvest efficiency HI = harvest index HU = heat unit LAI = leaf area index OAT = one-at-a-time method PAR = photosynthetically active radiation PLTFR = plant nutrient fraction PLTNFR = plant nitrogen fraction PLTPFR = plant phosphorus fraction PRG = perennial rhizomatous grasses RUE = radiation use efficiency SWAT = soil and water assessment tool T_BASE = base temperature WQFS = water quality field station.