Malheur Experiment Station Information for Sustainable Agriculture

IMPROVED IRRIGATION EFFICIENCY AND EROSION PROTECTION BY
MECHANICAL FURROW MULCHING SUGAR BEETS

C.C. Shock, J. Hobson, J. Banner, L.D. Saunders and B. Townley

Malheur Experiment Station
Oregon State University,
Hobson Manufacturing Inc.
Ontario, Oregon, 1992
 

Summary

    Sugar beet yield, quality, and recoverable sugar were measured for the HM-PM9 sugar beets grown on a Nyssa silt loam soil at 3 percent slope, with and without mechanically applied wheat straw mulch in irrigation furrows. Water inflow, outflow, and sediment loss were measured over time on each of 24 plots for all thirteen irrigations. Infiltration was calculated, and irrigation durations were managed so that total water infiltration would be the same in strawed and non-strawed sugar beets. Runoff water and sediment from each plot were independently analyzed for nitrate, ammonium, total N, phosphate, and total P during one irrigation. Irrigation efficiency increased with furrow mulching. Mechanically applied straw mulch increased the beet yield by 2.5 t/ac and recoverable sugar by 866 lb/ac. Furrow mulching decreased the loss of sediment from 78.8 to 6.7 tons per acre, decreased estimated total P loss from 133 to 12 lb/ac, and decreased total estimated N loss from 334 to 75 lb/ac. Most N losses were in the form of organic N, and most P losses were in the form of insoluble P in the sediment.
 
 

    This study sought to measure the potential of mechanically applied wheat straw mulch to reduce nutrient, pesticide, and sediment losses from furrow irrigated sugar beets, and increase water use efficiency. Water shortages over the last six years have increased awareness of the importance of conserving water through increased irrigation efficiency. Nitrogen and phosphorus losses were of particular interest, both because of their economic importance as farm inputs, and their roles as environmental contaminants.

    Measured losses of phosphorus included phosphate-P dissolved in runoff water, phosphate-P present in the sediment, and total P in the sediment. Measured nitrogen losses included ammonium and nitrate, both in runoff and in the sediment, and total reduced N in the sediment.
 
 

    Sugar beet (Beta vulgaris variety HM-PM9) seed was planted in rows 22 inches apart, with one seed every 1.9 inches in the row, on April 1, 1992. The Nyssa silt loam with 3 percent slope had received 0, 100, and 400 lb P₂O₅/ac in the form of triple superphosphate before planting. Treatments consisted of two wheat straw mulch rates, 0 and 800 lb/ac, each with three rates of applied phosphate in a complete factorial design with four replicates. Straw mulch was applied to the full length of irrigation furrows (furrow-mulching) using a Hobson Mechanical Straw Mulch Applicator. The site was located at the Malheur Experiment Station, OSU, Ontario, Oregon, and followed exactly on top of identical phosphate and mechanical straw mulch treatments for potatoes in 1990, and onions in 1991. Soil was sampled to 4 feet in 1-foot increments in the fall of 1991 before planting and in the fall of 1992 after harvest, and analyzed for nitrate-N, ammonium-N, and phosphate -P. Sugar beets were sidedressed with 100 lb N/ac as urea on May 19.

    Half of the straw was applied to the furrow bottoms prior to the first irrigation on May 4, and half of the straw was applied after cultivations and lay bye prior to the third irrigation on May 23. Sugar beets were thinned to one plant for every 6 to 7 inches of row. Irrigation furrows were 235 feet long and all furrows of all plots received 13 irrigations during the season, with water inflow in each furrow set at 120 gal/h. Crop evapotranspiration (ET_c) was calculated by an Agrimet weather station at the experimental site. All irrigation furrows were front wheel tracks of a John Deere 2040 tractor. Irrigation duration in furrows with and without mulch depended on the anticipated time necessary to meet ET_c. Estimated irrigation duration was based on the accumulated ET_c in the crop to date (less infiltration and rainfall), and the rate of water infiltration of the previous irrigation for each treatment.

    Weeds were controlled using 1.875 lb ai/ac Nortron preplant on March 30, a post-emergence mixture of 25 oz/ac Poast and 2.5 oz/ac Stinger on April 20, and a lay bye treatment of one pint Treflan per acre on May 21. Mildew was controlled by aerial spraying of 8 oz/ac Bayleton July 24, and sulfur on July 27.
 
 

    Onset of inflow and outflow, and hourly measurements of inflow, outflow, and sediment yield were recorded for every irrigation. For each water outflow rate reading, a one liter sample of the runoff was placed in an Imhoff cone and allowed to settle for 15 minutes. Sediment content in the water, (y in g per liter) was found to be related to the Imhoff cone reading after 15 minutes (x) by the empirical equation:

y = 1.015x
r² = 0.98
p < 0.0001.

    Composite water samples were collected in 20-liter buckets to obtain sediment samples for nutrient analysis during one irrigation. Sediment was analyzed for nitrate-N, ammonium-N, total N, phosphate-P, and total P. Total inflow, outflow, infiltration, and sediment loss were integrated from hourly measurements using the LOTUS 1-2-3 software program, InfilCal 4.0 (Shock and Shock).

    During one irrigation, hourly inflow water samples and hourly outflow water samples were collected from every plot. The collection time of the water was recorded, and composite water samples were made in proportion to the sample represented in the water inflow or outflow calculated by using InfilCal 4.0. Composite water samples were analyzed for nitrate-N, ammonium-N, and phosphate-P. Net nutrient losses of N and P were calculated by comparing the nutrient content in inflow water with the nutrient content in the outflow, plus sediment. Average nutrient concentrations from the single sampled irrigation were used to estimate the nutrient concentrations in the other twelve irrigations where no water or sediment samples were collected for analysis.

    Ten beets were sampled October 21 from each plot, and they were evaluated for leaf, crown, and beet fresh weight, dry weights, N content, and P content. Beets were topped October 22 and dug October 23. For the middle 50 feet of each of the two middle rows, beet stand was counted, beets were dug and weighed, and a subsample of seven beets was analyzed for sugar content, conductivity, and nitrate-N. The percent extraction and total recoverable sugar was calculated based on the industry's empirical formulas.
 
 

    During the first irrigation, greater lateral movement of water was evident in mulched furrows. Splitting the application of straw mulch allowed cultivation for weed control. Pronounced differences in sediment yield continued throughout the season. Mechanical furrow mulching decreased runoff, increased infiltration, increased irrigation efficiency, and decreased sediment yield (Table 1). Water infiltration in plots with and without furrow mulch was managed to match the crop's evapotranspiration water requirement.

    Furrow mulching increased sugar beet yield and recoverable sugar (Table 2). Lower economic responses would be expected on a less erodible site, but greater economic responses would occur if the producer had limited irrigation water supplies.

    Phosphate-P additions in the irrigation water amounted to 1.94 lb P/ac in the check plots compared with 1.25 lb P/ac in the furrow mulched plots because 36 percent less water was applied. Net phosphate-P losses were 3.6 lb P/ac from the non-strawed plots and 0.03 lb P/ac from the strawed plots. Net phosphorus losses in the runoff were estimated to be 131.5 lb P/ac from non-mulched plots, and 10.6 lb P/ac from the furrow mulched plots averaged over all levels of applied phosphate (Table 3). Straw mulch reduced the losses of dissolved and soluble phosphate-P and insoluble-P lost in the sediment. The large difference in P losses was composed mostly of insoluble P in the sediment.

    Because nitrate and ammonium content of the irrigation water, 4.6 and 2.0 ppm N respectively, the irrigation water was estimated to add 142.3 lb N/ac in the check plots and 91.2 lb N/ac in the furrow mulched plots, (Table 4). Less N was contributed by irrigation in the furrow mulched plots because less water was used. Furrow mulching reduced organic nitrogen losses in the sediment by 89.8 percent. Losses of nitrate-N and ammonium-N, were reduced by furrow mulching. Total nitrogen loss was reduced from 333.5 lb N/ac to 74.6 lb N/ac.

    Sediment, phosphorus, and nitrogen losses were particularly large for sugar beets grown on this site. Sediment and nutrient losses may be very high without furrow-mulching because the sugar beet plant presents little in the way of roots or leaves to interfere with erosive processes, especially early in the season.

    Phosphate applications increased beet yield (93 percent confidence level) and recoverable sugar (81 percent confidence level) only at the 100 lb rate. Phosphate applications increased phosphate-P losses dissolved in the water and phosphate-P present on the sediment, particularly at the high rate of applied phosphate without furrow mulching (Table 5).
 
 

    Irrigation efficiency was greatly increased by furrow mulching. Beet tonnage and recoverable sugar increased with straw mulch. Mechanically applied straw mulch at 800 lb/ac decreased sediment yield by 91.5 percent. Large losses of sediment, nitrogen, and phosphorus occurred with the production of sugar beets under furrow irrigation without mulch on a three percent slope. Most of the phosphorus lost was in the form of insoluble phosphorus in the sediment. Most of the nitrogen lost was in the form of soil organic material in the sediment. High levels of applied phosphate aggravated losses of phosphate in the water and on the sediment.
 
 

Table 1. Furrow irrigations, water infiltration, potential consumptive water use, and soil loss with and without wheat straw furrow mulching. Malheur Experiment Station, Oregon State University, Ontario, Oregon, 1992.

¹ Taking into account 2.06 inches of rainfall.
 
 

Table 2. Performance of sugar beets on bench ground with and without furrow mulching. Beets were irrigated so that mulched and non-mulched beets had similar and adequate water for growth. Malheur Experiment Station, Oregon State University, Ontario, Oregon, 1992.

Table 3. Average phosphorus accumulations and losses in furrow irrigated sugar beets with and without furrow mulching. Malheur Experiment Station, Oregon State University, Ontario, Oregon, 1992.

Table 4. Surface nitrate accumulations and losses in furrow irrigated sugar beets with and without furrow mulching. Malheur Experiment Station, Oregon State University, Ontario, Oregon 1992.

† 16.6 lb N/ac net accumulation
 
 
 

Table 5. Influence of applied phosphate and furrow mulching on sugar beet productivity and phosphate losses in runoff water and in sediment lost. Malheur Experiment Station, Oregon State University, Ontario, Oregon 1992.

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