Modern agriculture has become heavily dependent on fossil-fuel-based commercial nitrogen (N) for maintaining soil fertility and high crop yields. In the past, farmers have not had the means to monitor soils routinely for N concentrations, nor have they been able to match yield goals to those concentrations. In addition, the nutrient value of manure applied to fields often is not taken into account before commercial N fertilizer is applied. As a result, N has been over-applied on many midwestern farms, prompting concerns about the cost of commercial inputs into farming and the amount of nitrate in groundwater

Goedhart, Christian L.

Vander Zee, Delmar

Vos, Ronald J.

English

Modern agriculture has become heavily dependent on fossil-fuel-based commercial nitrogen (N) for maintaining soil fertility and high crop yields. In the past, farmers have not had the means to monitor soils routinely for N concentrations, nor have they been able to match yield goals to those concentrations. In addition, the nutrient value of manure applied to fields often is not taken into account before commercial N fertilizer is applied. As a result, N has been over-applied on many midwestern farms, prompting concerns about the cost of commercial inputs into farming and the amount of nitrate in groundwater.</p

Vos, Ronald

Goedhart, Christian

Digital Repository @ Iowa State University (ISU)

88-19 COMPETITIVE GRANTLeopold Center FOR SUSTAINABLE AGRICULTURE LEOPOLD CENTER Monitoring and modeling cropping system nitrogen for a sustainable agriculture Goals Modern agriculture has become heavily de­pendent on fossil-fuel-based commercial ni­trogen (N) for maintaining soil fertility and high crop yields. In the past, farmers have not had the means to monitor soils routinely for N concentrations, nor have they been able to match yield goals to those concentrations. In addition, the nutrient value of manure applied to fields often is not taken into account before commercial N fertilizer is applied. As a result, N has been over-applied on many midwestern farms, prompting concerns about the cost of commercial inputs into farming and the amount of nitrate in groundwater. This study assessed ways to minimize nitrate loss and optimize N uptake by plants, thus reducing adverse effects on groundwater and maximizing benefit to the crop. Nitrate is a mobile form of N that is easily taken up by crop plants but that can also leach (percolate) to groundwater. Specific goals included moni­toring N use and movement in soils, analyzing soils and vegetation, assessing alfalfa's role in a cropping rotation to regulate soil nitrate concentrations, and demonstrating that farm­ers can develop the expertise to make soil management decisions by testing nitrate lev­els in their own soils. Approach The study, located at the Dordt College Agri­culture Stewardship Center in northwestern Iowa, included two conventionally tilled fields in upland and lowland soils, one alfalfa field, and one ridge-tilled field. In addition to the sampling described below, researchers con­sidered factors such as rainfall, cropping, land use history, and topography. Other sites at local farms included newly planted alfalfa fields, established alfalfa fields, and alfalfa fields that had been rotated to corn. All were monitored for soil nitrate. Researchers collected soil samples at two-week intervals from June 1988 to June 1991. During the first year, ten soil samples taken at each of three depth ranges—0-0.3 meter (m), 0.3-0.6 m, and 0.6-1.0 m, or about 0-12 in., 12­24 in., and 24-39.4 in.—were combined for analysis. Samples were taken early in the season with a tractor-mounted soil sampler and later with a hand sampling probe. Sam­pling was suspended during portions of some summers because the extremely dry soil would not allow probe insertion. During years two and three, researchers also analyzed individual core samples from each collection site to esti­mate soil-nitrate concentration variability. In 1988, eight shallow wells placed in areas corresponding to four cropping histories (in­cluding corn, alfalfa, soybeans, and triticale/ alfalfa) were drilled 16.5-41 feet (ft) deep to sample the upper, shallow aquifer. Water levels were recorded prior to sampling. Groundwater was also sampled every two weeks throughout the project except for three winter months each year. A specially devel­oped vacuum kit drew water samples. In addition to analysis at Dordt College, dupli­cate samples from all facets of the study were analyzed by a commercial laboratory as an independent confirmation of results. Suction lysimeters buried at 10-in., 20-in., and 63-in. depths near the wells sampled soil water REPORT Principal Investigator Delmar Vander Zee Biology Dordt College, Sioux Center, Iowa Co-investigators Ronald J. Vos and Christian L. Goedhart, Agricul­ture, Dordt College and Dordt Agriculture Stewardship Center Budget $24,730 for year one $15,605 for year two $17,250 for year three Leopold Center Progress Reports 19 Volume 1 (1992) solution for nitrate in the root zone. Soil samples were dried and then refrigerated in preparation for one of two analysis methods. To determine a crop's N uptake, researchers sampled plant tissue from corn, alfalfa, and/or soybeans under various tillage practices dur­ing the 1988 and 1989 growing seasons. After determining the amount of dry matter for all samples, researchers determined the dry matter's crude protein content, which in turn helped them to calculate its N values. Because researchers observed low and rela­tively stable soil-nitrate concentrations in the single alfalfa field sampled in 1989, in 1990 they broadened their investigation of alfalfa to include its potential for reducing cropping-system soil nitrate. The study examined soil nitrate in three alfalfa management scenarios: (1) newly established alfalfa with tissue samples taken at each of three cuttings; (2) existing 2-4 year alfalfa stands; and (3) fields where corn had been planted following the previous year's crop of alfalfa (to help determine how much an alfalfa stand's decomposition might contribute to soil nitrate). Findings All samples of well water from the study had some nitrate. Two were continuously above 10 parts per million (ppm), which is consid­ered the limit for safe drinking water. Three additional wells had concentrations above 10 ppm at some point during the study. Samples from wells having water tables closer to the surface showed more nitrate soon after periods of intense rainfall. Wells located down-slope also showed higher nitrate readings than oth­ers, possibly because nitrate-loaded water pooled during surface runoff. Cropping and manure fertilization history strongly affected soil nitrate concentrations in the fields studied. For example, nitrate con­centrations were lowest in the alfalfa (less than 5 ppm) and up to eight times higher in the row-cropped fields, whether tilled conventionally or ridge-tilled. Fields with a history of heavy manure application showed nitrate concentra­tions equal to or higher than that required for corn production. Moreover, the concentration of soil solution nitrate in some fields indicated that significant amounts of leachable nitrate existed below the root zone. For example, in the final season of the project, one conventionally tilled corn field with a history of alfalfa cropping and heavy manuring showed a nitrate soil solution of 45­75 ppm at the 1.5 m depth. The ridge-tilled field had an average of 162 ppm nitrate— among the highest of any of the fields in this study. Table 1. Estimated savings to area farmers following reduced commercial N fertilizer application in spring 1989. Operator Acres involved Savings/acre ($) Total savings ($) 1. 6200 $13.70 $84,940 2. 1500 30.00 45,000 3. 700 10.00 7,000 4. 62 13.50 837 5. 260 15.00 3,900 TOTALS 8722 $141,677 Note: $ 16.24 average savings per acre; corn yields in fields with reduced N were equal to or greater than adjacent fields/plots with conventional N application rates. In addition to using the late-spring soil test (now sold as the N-Trak kit), these farmers are using the fall season stalk test to determine how well the corn plants were using available N. Even on fields where N rate was reduced, the stalk test showed high N levels. Vander Zee, D. 20 Volume 1 (1992) This project also clearly demonstrated that alfalfa will remove nitrate from the root zone (down to one meter, or about 39 in.), even during its first year, and that established stands can rapidly take up nitrate from manure appli­cations. Finally, local, on-farm soil testing for nitrate provided savings estimated at $16 to $20 per acre (see Table 1). Such information about potential savings effectively encourages farm­ers to recognize the economic value of soil testing. In addition, such testing promotes prudent accounting of the value of on-farm N sources such as manures. The information provided by this testing encourages greater stewardship in crop and soil management— especially given soluble nitrate's propensity to run off or leach. Implications The researchers conducting this project have concluded that past land use and manuring practices contribute significantly to soil N economy and that knowledgeable application of N can be accomplished with the late-spring soil nitrate test and the fall-season stalk test. Furthermore, alfalfa in rotation or strip crop­ping can not only contribute to soil N (via decomposition of biomass); it can also miti­gate deep soil N levels. Thus, including alfalfa in rotations and monitoring soil in late spring for nitrate has great potential for reducing groundwater pollution by nitrate. This project has raised northwestern Iowa farm­ers' awareness of the importance of proper N fertilizer use. Early in the project, researchers became distributors and trainers of the late-spring soil nitrate test and worked with selected farmers; when a test kit became avail­able commercially, other local farmers ac­cepted it more readily as a result. A survey of five farmers within a 25-mile radius of the project location who used the prototype kit suggests that as a group they saved almost $142,000 in fertilizer costs the first year; follow-up has revealed that these savings are ongoing. Farmers are relying on the test kit to help them assess their N needs and 'fine tune " their Napplications. This has reduced routine fertilizer application on pre­viously manured soils. As these farmers ex­pand the amount of acres on which they fertilize according to the kit recommendations, their savings increase; moreover, they work in har­mony with the environmental and economic goals of sustainable agriculture. For more information contact R. J. Vos, Agriculture Stewardship Center, Dordt College, Sioux Center, Iowa, 51250, (712) 722-6285. The nitrogen cycle. Adapted from L.G. Bundy, Soil and applied nitrogen, A2519, CES, UW-Extension, 1985 Leopold Center Progress Reports 21 Volume 1 (1992) 

Monitoring and modeling cropping system nitrogen for a sustainable agriculture

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Monitoring and modeling cropping system nitrogen for a sustainable agriculture

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