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Fluid Journal : Fluid Journal 1993-1995
75 lbs/A. Plots were 6 by 30 feet. A plot combine was used for harvesting on June 24.After harvest, grain samples were weighed and moisture and test weight were determined. Samples were dried in a forced- air oven at 60 degrees C for 48 hours. Subsamples were ground to 0.1 mm, digested in sulfuric acid/peroxide and the digests analyzed for total N and P. Soils were sampled to 4- to 6-inch increments. Samples were air dried, extracted with 2M KCl and the extracts analyzed for inorganic N (NO3- and NH4+) on an autoanalyzer. Ecological considerations The experiments described were con- ducted to quantify the N and P rate and placement interaction effects on dryland winter wheat and grain sorghum yield, apparent nutrient recovery and soil profile NO3- content after harvest. The impact of nitrogen fertilizer use on groundwater and environmental quality has become a major concern for the public and the fertilizer industry. Adoption of fertilizer "best management practices" is essential for I) conserving limited resources used to produce agricultural inputs, 2) reducing the environmental impact of chemical inputs applied to agroecosystems, 3) increasing input efficiency and agricultural profitability and 4) ensuring long-term sustainability. The most important factor in reducing the quantity of N remaining in the soil after harvest is to apply the "correct" fertilizer N rate. Fertilizer N rates are determined by models generally represented by: N recommendation = abc In the model, a = yeild goal, b = soil test N and c = factors. The term "factors" includes adjustments or corrections for previous crop (i.e., legume), manure applications or other soil/crop management factors. "Soil test N" represents extractable inorganic N deter- mined prior to planting. Soil profile NO3- is highly correlated to yield response to fertilizer N and is routinely used in making N recommendations in the Great Plains. This test quantifies the NO3- content at sampling time and is subject to considerable error in fields where NO3- is lost by leaching plant uptake. In the above model, "yield goal" influences the quantity of fertilizer N recommended more than any other term. Thus, accurate determination of optimum fertilizer N rates requires realistic yield goals for each field. Yield goal estimates that are too low will underestimate N needs and reduce yield potential and profit. Yield goals that are too high will result in above- optimum N applications and will, in addition to reducing profitability, greatly reduce crop recovery of applied N and increase the probability of N leaching. Among other management factors, N placement can greatly influence crop recov- ery of applied N and reduce the quantity of fertilizer N in the soil profile after harvest. In Kansas studies on no-till dryland sorghum, ANR was 40 and 70 with broadcast and knife applications of 75 lbs/A of N, respec- tively. On P responsive soils, positive interaction of N and P fertilization on grain yield and fertilizer N recovery has been observed. In long-term N and P studies with irrigated sorghum, yields increased 30 to 40 percent with 40 lbs/A P2O5 compared to no P on a low-P soil fertilized with 160 lbs/A of N. In studies with winter wheat, ANR increased from 25 to 50 per-cent by increasing P rate from 20 to 40 lbs/A of P2O5, respectively. In other Kansas studies, ANR in winter wheat was 40 and 50 percent with 32 lbs/A of P2O5, applied broadcast and with the seed, respectively. These studies demonstrate that both P rate and placement greatly influence recovery of fertilizer N and reduce quantity of N remaining in the soil profile after harvest. Conclusions Adoption of N management practices that optimize yield and recovery of fertilizer N will reduce the quantity of potentially leachable fertilizer N in the soil after harvest. Our studies show that subsurface and surface band application of N and P increased grain yield and ANR, compared to broadcast N and P. At the no-till wheat site, grain yield with surface band N was greater than broadcast N, but was less than subsurface N. On P responsive soils, P fer- tilization increased grain yield and ANR. In general, increasing ANR decreased soil profile N concentration after harvest. Dr. Havlin is associate professor, Depart- ment of Agronomy; Dr. Schlegel is associate professor, Southwest Res. & Est. Center, Tribune, KS, and Dr. Pierzvnski is assistant professor, Department of Agronomy. All are associated with Kansas State University in Manhattan, KS. ! Table 3. Soil Characteristics and selected soil properties at the Ford County sites in 1992. Parameter* Grain Winter Sorghum Wheat Soil Type Ulysses sil Harney sil Soil Class Aridic Hapulstoll Typic Argiustol pH (1:1) 7.4 7.2 Bray 1-P (lbs/A) 14.0 11.0 NH4OAc-K (lbs/A) 1,152.0 865.0 OM% 2.2 2.1 DTPA-Fe (lbs/A) 12.4 8.4 DTPA-Zn (lbs/A) 2.8 2.1 Soil N** (lbs/A) 45.0 29.0 * Soil analyses: 0 to 6 inches **NO3- + NH4+, 0 to 4 feet Rate (lbs/A) Placement ANR* Soil N* ANR* Soil N* NP 2O5 Method % lbs/A % lbs/A 0 0 - 41 - 25 40 0 Broadcast 22 70 31 44 40 20 " 36 59 44 40 40 40 " 43 52 54 36 80 0 " 26 86 26 57 80 20 " 30 66 32 50 80 40 " 34 64 33 48 40 0 Knife 37 61 46 41 40 20 " 53 50 66 39 40 40 " 60 48 78 33 80 0 " 31 76 35 49 80 20 " 36 58 50 43 80 40 " 38 57 49 40 40 0 Dribble 35 64 43 45 40 20 " 51 48 55 41 40 40 " 61 50 60 35 80 0 " 29 79 42 54 80 20 " 34 55 51 41 80 40 " 37 51 50 40 *ANR = apparent N recovery; Soil N = inorganic N content, 0 to 4-foot depth Table 2. Fer tilizer management effect on ANR and soil N content after harvest. Grain Sorgum Winter Wheat Late Spring 1993
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