Research report from OSU's North Willamette Research and Extension Center
Delbert Hemphill
OSU Dept of Horticulture, NWREC
John Selker
OSU Dept of Biological and Ecological Engineering
Richard Dick
OSU Dept of Crop and Soil Science
Introduction
Nitrate pollution of groundwater from the application of high rates of N fertilizers to vegetable crops is a concern in the Willamette Valley. Excess N not taken up by the crop remains in the soil and can be leached to groundwater during the wet winter months. Such concerns led us to initiate in 1990 a study of the cycling and availability of N in vegetable cropping systems. These are the eighth and ninth years of a study in which winter cover or "catch" crops have been seeded following vegetable crops and in which the N uptake of the cover crop and its contribution to a succeeding vegetable crop has been measured in comparison to a winter-fallow control. In 1994, sweet corn was grown on these long-term rotation plots at NWREC and fertilized at three rates of N. Following harvest the plots were seeded to cereal rye or a mixture of cereal rye and Austrian winter pea. In 1995, broccoli was grown on these plots at three rates of N to determine the cover-crop contribution to broccoli yield and N uptake. Following harvest, the plots were again disked, harrowed, and seeded (drilled) to triticale or a mixture of triticale and Austrian winter pea. In addition, other plots in both years were overseeded (relay intercropped) to cereal rye, triticale, or red clover about one month after sweet corn or broccoli emergence. These cover crops were permitted to grow through the winter. In 1996, sweet corn was again grown on the plots and fertilized with three rates of applied N.
In autumn 1993, passive capillary wick samplers were installed beneath the winter-fallowed plots and fall-planted cereal rye (later triticale) plots. All three N rates were also represented. The samplers have allowed us to collect leachate on a continuous basis and determine both the nitrate concentration of the leachate as well as the total nitrate loss on an area basis. Details of the installation and use of the capillary wick samplers can be found in Brandi-Dohrn et al., 1997.
Objectives in 1997 and 1998 were 1) to evaluate effects of several winter cover crops, including fall-seeded and overseeded triticale, fall-seeded triticale plus winter pea, and overseeded red clover on yield and quality of broccoli (1997) and sweet corn (1998) at three rates of N and 2) to evaluate the effect of these cover crops and the N applied to the vegetable crops in 1996 and 1997 on the amount of nitrate leached below the root zone.
Methods, Broccoli, 1997
During winter, the plots had been fallow or in the cover crops listed under Objectives. The cover crops had been overseeded into standing sweet corn in July 1996 or were broadcast-seeded and scratched into the soil in early October, 1996. While we did not consider this an outstanding cover crop in terms of biomass accumulation, the plots seeded to cover crops in October had significantly (P=0.01) greater ground cover than did fallow plots, when measured in January (Table 1).
'Packman' broccoli was seeded on 3 July in rows 30 inches apart. Two previous seedings of 'Pirate' failed because of inadequate stands. Plot size was 600 square feet. Nitrogen rates were 0, 125, and 250 pounds/acre, with half the N applied just after seeding, and the remainder applied 5 weeks after seeding. At this time, the appropriate plots were overseeded to 'Celia' triticale or 'Kenland' red clover in preparation for the 1998 experiments. Harvest was on 16 September.
Methods, Sweet Corn, 1998
During winter the plots had been fallow or in the cover crops listed under Objectives. The cover crops had been overseeded into the standing broccoli crop in July 1997 or were broadcast-seeded and harrowed into the soil in early October, 1997. 'Jubilee' sweet corn was seeded on 1 June in rows 30 inches apart. The stand was thinned to 6-7 inches between plants in the row. Plot size was 600 square feet. Nitrogen rates were 0, 50, and 200 lb/acre, with half the N applied just after seeding and the remainder applied 5 weeks after seeding. At this time the appropriate plots were overseeded to common cereal rye or 'Kenland' red clover in preparation for the 1999 experiments. Harvest was on 9 September.
Results, N Rate and Cover Crop on Broccoli Yield, 1997
Both yield and mean head weight tended to decline following an overseeded triticale cover crop, regardless of N rate (Table 2). This result is consistent with those dating back to 1990 for both triticale and cereal rye cover crops (Hemphill, 1993, 1995, 1997). The overseeded clover cover crop tended to increase yield but the effect was not statistically significant. However, mean head weight was significantly increased with the overseeded clover. Yield with fall-seeded triticale tended to be higher than following fallow, and mean head weight was significantly improved by the fall-seeded triticale. In past years, fall-seeded cereal cover crops have tended to depress yield. Results from this year may indicate that soil available N content has increased sufficiently with continuous cover- cropping to provide some yield boost to the following vegetable crop. Initial pea stand in the triticale/pea cover crop was adequate, but the stand was lost during the late winter. Consequently, yield did not increase with this cover crop over that obtained with triticale alone. There was no response to the rate of N applied when averaged over cover crop treatments. The highest-yielding treatment combination was the intermediate rate of applied N following the fall-seeded triticale/pea cover, with 5.2 tons/acre (Figure 1). Largest mean head size was obtained with the combination of fall-seeded triticale and no applied N (Figure 2). Lowest yield and head weight were with overseeded triticale and no applied N.
Winter Cover Crop on Nitrate Leaching, 1996-1997
A fall-seeded triticale cover crop significantly reduced the nitrate concentration of leachate reaching the 4- foot depth in the soil profile following the 1996 corn crop (Figure 3). The difference in nitrate-N concentration in the leachate was small during the high-rainfall months of December and January, but the amount of water percolating past the root zone was also reduced by the cover crop, resulting in a dramatic effect on the amount of nitrate being leached (Figure 4). By April, the amount of nitrate lost from the root zone was 33 pounds N/acre for the fallow plots versus 18 pounds/acre for the cover crop plots at the high rate of N applied to the previous vegetable crop. This is very consistent with results obtained in the winters of 1992-93 through 1995-96 (Hemphill, 1995, and 1997). Over the five winters that we have obtained data from the passive capillary lysimeters, the reduction in mass of nitrate leached has averaged 45 percent. This demonstrates the ability of even a relatively sparse cover crop to significantly improve ground and surface water quality.
Cover Crop Yield and N Uptake, 1997-1998
While the cover crops, again, were rather sparse and slow-growing, all plots seeded to cover crops had significantly (P=0.01) greater ground cover than did fallow plots when measured in February (Table 3). In addition, most of the biomass on fallowed plots was attributable to annual bluegrass, which is shallow-rooted and does not recover much N. Cover-crop biomass accumulation and nitrogen uptake were below average for these plots, but some interesting trends emerged. The yield of overseeded crops was generally larger than for the fall-seeded crops, a reversal of the previous trend (Table 4). This may have been due to the late planting date of the fall-seeded crops, saturated soil during much of the winter, and a soil pH which was getting lower than desired. Except for the overseeded red clover, cover-crop biomass and N accumulation did tend to increase with increasing rate of N applied to the preceding broccoli crop.
N Rate and Cover Crop on Sweet Corn Yield, 1998
Although not statistically significant, yield, mean ear weight, ear length, and tipfill tended to decline following an overseeded triticale cover crop, regardless of N rate (Table 5). This result was consistent with those dating back to 1990 for both triticale and cereal rye cover crops. In past years, fall-seeded cereal cover crops have also tended to depress yield, but this was not the case in 1998, perhaps because of the poor stand and growth of the fall-seeded triticale. Initial pea stand in the triticale/pea cover was adequate, but the stand was lost during the late winter because of a root disease. Therefore, yield did not increase with this cover crop over that obtained with triticale alone. The response to rate of N applied was normal when averaged over cover crop treatments. The highest-yielding treatment combination (10.0 tons/acre) was the high rate of applied N following the overseeded clover cover, indicating some N contribution from the clover, despite the relatively low N uptake (Table 3) of this cover. This treatment combination also produced the largest mean ear size. Lowest yield and mean ear weight was with overseeded triticale and no applied N.
Winter Cover Crop on Nitrate Leaching, 1997-1998
Averaged over rates of N applied to the preceding broccoli crop, a fall-seeded triticale cover crop significantly (P=0.05) reduced the nitrate concentration of leachate reaching the 4-foot depth in the soil profile for two sampling dates and tended to do so for all but the last sampling date (Table 6). The difference in nitrate-N concentration in the leachate between fallow and covered plots was larger during the fall and early winter months than in late winter. In fact, at the high rate of applied N, but not at the zero and intermediate rates, nitrate concentrations were higher in leachate collected beneath the triticale cover crop than below fallow plots for the last four sampling dates (Figures. 5-7). This is a reversal of the trends seen in previous years (Hemphill, 1997). In past years, nitrate concentrations of collected leachate have varied up and down during the rainy season but in 1997-1998, the concentrations tended to increase steadily during the winter. The reason for this is not known, but perhaps significant mineralization of organic N was occurring during the mild winter.
Summary
Consistent with past results, winter cover crops reduced leaching of nitrate from the root zone in the winter of 1996-1997. Leguminous cover crops made N available to the following vegetable crop, but a cover crop consisting only of an overseeded winter grain tended to depress yield of the following broccoli crop. In contrast to previous years, a fall-seeded triticale cover crop tended to increase broccoli yield.
In the winter of 1997-1998, a triticale cover crop reduced nitrate concentrations in leachate consistently at the zero and intermediate rates of applied N, but not at the high rate of N. This is in contrast to results for the previous five winters. A clover cover crop made N available to the succeeding sweet corn crop in 1998, but a cover crop consisting only of an overseeded winter grain tended to depress yield of the following sweet corn crop.
Literature Cited
Brandi-Dohrn, F.M., R.P. Dick, M. Hess, S.M. Kauffman, D.D. Hemphill, Jr., and J.S. Selker. 1997. Nitrate leaching under a cereal rye cover crop. J. Environ. Qual. 26:181-188.
Hemphill, D.D., Jr. 1993. Vegetable research at the North Willamette Research and Extension Center, 1991-1992. Oregon Agric. Expt. Sta. Spec. Rep. No. 908.
Hemphill, D.D., Jr. 1995. Vegetable research at the North Willamette Research and Extension Center, 1993-1994. Oregon Agric. Expt. Sta. Spec. Rep. No. 944.
Hemphill, D.D., Jr. 1997. Vegetable research at the North Willamette Research and Extension Center, 1995-1996. Oregon Agric. Expt. Sta. Spec. Rep. No. 975.
Table 1. Effect of cover crop on percentage of ground covered, as determined by the string method, 6 January, 1997. Cover crop % Triticale % Legume % Weeds % Total Fallow 0 1 34 35 Overseeded triticale 12 0 30 42 Overseeded clover 0 21 25 46 Fall-seeded triticale 36 1 30 67 Fall-seeded triticale/pea 20 29 18 66 LSD (0.05) 14 7 15 17 Table 2. Main effects of preceding cover crop and rate of applied N on yield of broccoli, NWREC, 1997. Treatment Yield Mean head wt. tons/acre (g) Cover crop (avg. over N rates) Fallow 3.5 349 Overseeded triticale 2.6 236 Overseeded clover 4.3 407 Fall-seeded triticale 4.1 466 Fall-seeded triticale/pea 4.4 408 LSD (0.05) NS 22 N rate, lb/acre (avg. over covers) 0 3.8 383 125 4.0 375 250 3.9 361 LSD (0.05) NS NS Table 3. Effect of cover crop on percentage of ground covered, as determined by the string method, 4 February, 1998. Cover crop % Triticale % Legume % Weeds % Total Fallow 0 0 34 34 Overseeded triticale 28 9 51 88 Overseeded clover 0 28 68 96 Fall-seeded triticale 26 2 25 53 Fall-seeded triticale/pea 34 19 21 74 LSD (0.05) 12 9 12 10 Table 4. Interaction of cover crop and rate of N applied to preceding broccoli crop on cover crop biomass and N uptake, NWREC, 1998. Cover crop N rate Cover dry biomass N uptake ----------------lb/acre---------------- Overseeded triticale 0 1712 26 125 3090 46 250 3759 58 Overseeded clover 0 1893 46 125 2141 37 250 2249 38 Fall-seeded triticale 0 796 17 125 743 16 250 1128 28 Fall-seeded triticale/pea 0 716 13 125 607 10 250 1054 20 LSD (0.05) 700 14 Table 5. Main effects of preceding cover crop and rate of applied N on yield of sweet corn, NWREC, 1998. Treatment Yield Mean ear wt. Ear length Tipfill tons/acre (g) (inches) Cover crop (avg. over N rates) Fallow 7.5 203 8.2 2.1 Overseeded triticale 5.6 164 8.0 1.6 Overseeded clover 7.3 210 8.1 1.9 Fall-seeded triticale 7.5 210 8.1 2.3 Fall-seeded triticale/pea 7.0 198 8.1 2.2 LSD (0.05) NS NS NS NS N rate, lb/acre (avg. over covers) 0 4.5 140 7.3 1.2 125 7.6 206 8.2 2.0 250 8.9 245 8.8 2.9 LSD (0.05) 0.8 20 0.2 0.3 Table 6. Main effects of rate of N applied to 1997 broccoli crop and a triticale cover crop on nitrate concentration of leachate collected during the 1997-1998 rainy season. Collection date Nov. 5 Dec. 3 Dec. 19 Jan. 9 Jan. 21 Feb. 13 Mar. 24 -----------------------ppm Nitrate-N------------------------- Cover crop Triticale 3 4 6 7 10 15 20 None 6 7 9 9 12 16 18 Significance NS ** * NS NS NS NS N rate, lb/acre 0 3 3 3 4 4 6 7 125 4 4 6 6 9 13 16 250 8 10 13 15 19 28 34 Significance * ** ** ** ** ** ** **,*,NSSignificant differences at 1% and 5% probability levels, and no significant differences, respectively.