Influence of Straw Application Rates, Plowing Dates, and Nitrogen Applications on Yield and Chemical Composition of Sugarbeets

Fertilizer N applied at ever-increasing rates sometimes accumulates in the soil. The practice of fertilizing grain straw with N to stimulate decomposition is questionable, but decomposition of straw immobilizes N that must be compensated for in fertilizing the succeeding crop. Too much N decreases the sucrose content of sugarbeets and decreases sucrose recovery. Experiments were conducted to determine the relative value of early and late straw applications, plowing with N applied in the fall or spring, and the amount of N needed to compensate for straw applications in obtaining optimum beet and sucrose yields with maximum quality. Sugarbeets (Beta vulgaris L.) were grown following winter wheat (Tritwum aestivum L., var. `Nugaines9 ) in 1970 and 1971 on a Portneuf silt loam soil near Kimberly, Idaho. Straw was applied to the beet plots at rates of 6.7, and 13.4 metric tons/ha, and the plots were plowed either in early September or mid-November. Nitrogen was applied at 67 kg N/ha in the fall and at 67 and 134 kg N/ha in the spring. The treatments were arranged in a split-split lot design with 4 replications. Control plots were used with all experiments. N fertilization increased beet, top, and sucrose yields, as well as amino N, Na, K concentrations, and impurity index. It decreased the t sucrose percentages of the beets. Straw applications decreased beet, top, and sucrose yields, Na and amino N concentrations, and impurity index, but they did not influence K content of the beets. Early plowing increased sucrose percentage and yield and decreased Na, K, and impurity index. Interactions between straw applications and plowing dates were significant for sugarbeet and beet top yields. Approximately 7.5 kg N fertilizer per metric ton of straw were required to compensate for the deleterious effects of the straw. Additional index words: Sucrose percentage, K, Na, Amino N, Impurity index. T HE role of nitrogen in straw decomposition has been investigated extensively, and many papers have been published on the subject. Reviews by Allison (1), Bartholomew (2), and Harmsen and VanSchreven (7) present the classic concepts relating C:N ratios to plant material decomposition and the immobilization and release of nitrogen during decomposition. Allison (1) cited references indicating that plant residues with C:N ratios greater than 25 to 30 require additional N for most rapid decomposition. Conversely, N may be available as fertilizer from plant residue with an initially wide C:N ratio after the residue has decomposed and lost enough carbon to narrow the ratio. Because of these immobilization theories, N is frequently applied to compensate for the addition of straw. Experimental and practical experience indicates that the concepts are basically correct. Increased N fertilization on many fields has increased soil fertility and residual nitrogen levels. 'Contribution from the Agricultural Research Service, USDA, and the Idaho Agricultural Experiment Station. Presented at Agronomy Meeting, Miami Beach, Florida, Nov. 1, 1972. Received Feb. 3, 1973. 'Soil Scientists, Snake River Conservation Research Center, and Superintendent, Idaho Branch Agricultural Experiment Station, Kimberly, Idaho 83341. James et al. (9) reported residual N fertilizer with nitrate concentrations up to 725 kg/ha in farmer's fields used for sugarbeet experiments. Many fields that they sampled had high residual nitrate. Crops grown under conditions of such fertility buildup will utilize residual fertilizer and may also have available considerable nitrogen from mineralization during decomposition of soil organic matter and crop residues. Field studies by Smith and Douglas (15) showed that with normal straw applications in fertile soils, the addition of N does not hasten straw decomposition. Residual nitrogen fertilizer and mineralization will supply enough N under many field conditions to meet decomposition requirements. However, N will be immobilized during straw decomposition and will influence the availability of N to succeeding crops. Sugarbeets (Beta vulgaris L.) grown following plowdown of grain straw may develop N deficiency. Several researchers have reported excellent responses to N fertilizer on sugarbeets treated with crop residues or manure (3, 5, 8, 16). Decreases in sucrose percentage in the sugarbeets with increasing N fertilization (12, 13) and increasing sucrose percentages resulting from straw applications have been reported (3, 4, 5). The amino N content of the beets is affected by N fertilizer (10) and 'influences sucrose recovery. Because of increasing fertility levels in soil, it is necessary to evaluate established practices of fertilizing to enhance decomposition of crop residues. N immobilization that accompanies decomposition of low N crop residues must be compensated for adequately but without adding excess N that decreases sucrose content of sugarbeets. The experiments reported here were conducted to determine 1) the fertilizer value of N applied to `Nugaines' wheat (Triticum aestivum L.) straw preceding early and late fall plowing, compared with spring N application, and 2) the N requirements of sugarbeets following incorporation of wheat straw. MATERIALS AND METHODS Nugaines wheat was grown in 1969 and 1970 on Portneuf silt loam soil near Kimberly, Idaho. Each crop was fertilized with 112 kg N/ha as ammonium sulfate. All of the straw was removed and plots 4.86 by 12.15 meters were established for growing sugarbeets. Straw from the area was applied to the plots by hand at rates of 6.7 and 13.4 metric tons/ha (3 and 6 tons/ acre), and treated with 67 kg N/ha sprayed on the straw as ureaammonium nitrate solution immediately before plowing. The straw contained an average of 0.47 and 0.36% total N in 1969 and 1970, respectively. The rates cover the range of expected straw yields for Nugaines wheat in this area. The plots were plowed either in early September or mid-November. Nitrogen was applied by hand to specified plots in the spring at rates of 67 and 134 kg N/ha as NFI.NC4. The statistical design was a split-split block arrangement with four replications. Straw treatments comprised the major plots, plowing dates the split plots, and nitrogen treatments the split-split plots. The sugarbeets were planted in rows 61 cm apart and were thinned by hand to an average spacing of 25 cm. The beets and previous wheat crop were irrigated in furrows according to a computer-scheduled irrigation program (11). Beet petiole sam