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Research Report  2.5

Variable Rate Technology For N Fertilizer Application

Dr. Gary Kachanoski, Dept. of Land Resource Science,
U. of Guelph, Guelph, ONT N1G 2W1
COESA Report No.:  RES/FARM-002/97

Objectives & Expected Outputs
Executive Summary

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Objectives and Expected Outputs
Objectives: To develop variable rate technology for N fertilizer application by trying different methods of obtaining a field map for variable application of N; to determine the economic benefits of variable rate technology for N; and to determine the change in potential nitrate loading to the groundwater from variable field application of N fertilizer compared to constant-rate application.
Expected Outputs: Maps of field variations of crop response to applied fertilizer N, estimates of the spatial distribution of the entire N response curve, the acquisition and testing of a variable rate fertilizer applicator linked to a Global Positioning System, and mapping of the N soil test, soil texture, soil organic matter, soil water regime and landform shape are expected to meet the objectives outlined above.
Type: Contribution Agreement, University
Spending Profile: 93-94: $70 K; 94-95: $70 K; 95-96: $37.4 K; Total: $177 K
Status: Available Fall 1997


Executive Summary

The goal of the study is to determine the feasibility of variable rate technology for N fertilizer application, to maximize economic crop response while minimizing environmental impacts on water quality. Specific objectives include:

  1. To assess different methods of obtaining the field map for variable application N of fertilizer,

  2. Determine the economic benefits of variable application of N fertilizers, and

  3. Determine the change in potential nitrate loading to the groundwater from variably applying N fertilizer. Two sites were established in the spring of 1993 in Huron Co. near Londesboro, Ontario on the farm of Bruce Shillinglaw.

Each site consisted of 4 adjacent blocks of no-till planted corn. Each block consisted of 2 treatments; (1) Fertilizer added (F) at 160 kg N ha-1, and (2) No fertilizer N added (NF). Each treatment was 8 rows of corn with 75 cm row spacing and a length of approximately 325 m. Spatial patterns of yield with fertilizer added and yield with no fertilizer were obtained from detailed hand harvesting (approx. 250 hand yield samples per field). Yield patterns were also obtained using a commercial on-the-go yield sensor attached to a combine. Soil cores were taken in a dense grid from each field to obtain the spatial pattern of the soil N test. Extensive soil sampling to a 90 cm depth was also carried out in the fall period to obtain the spatial patterns of residual mineral soil N, and the subsequent loss of N by leaching. All of the instrumentation and sampling was referenced to a detailed elevation map of the site obtained from a detailed survey of each site.

The data from the year of results was used in the second year to construct two variable rate maps for fertilizer N application for each of the two field sites (S1, S2). The 2 variable rate maps were based on 1) the N soil test and 2) a differential yield map (fertilizer yield - check yield). Thus, in 1994 each of the two field sites had N fertilizer rate treatments consisting of: check (0 Kg N ha-1), constant rate at 150 Kg N ha-1, variable rate from soil test prediction, and variable rate from the differential yield map. In 1995 the site was planted to soybeans and hand sampled yields were taken in representative areas to examine any carry-forward influences from the variable N treatments. In 1996 the site was seeded to barley and fertilizer applied (67 kg N ha-1) in the original blocks that had fertilizer in 1993. Hand sampled yields were taken in the same locations as in 1993. Throughout the study soil samples were taken to examine the influence of the fertilizer treatments on soil N storage and losses. Soil solution samplers were installed to measure the concentration of nitrate in drainage water.

Major findings of the study are:

  • A yield map based on one fertilizer N application rate is not enough information to determine the spatial pattern of N application for site specific management.

  • A new yield index called the Delta yield dYF was developed to estimate the spatial pattern of N fertilizer response. It is based on the difference between yield with and without fertilizer N.

  • Yield measurements with an on-the-go combine monitor were significantly correlated to hand yield measurements, but the monitors may not be accurate enough to estimate dYF . Robust spatial interpolation methods are needed for yield monitor data.

  • Site specific application based on dYF used 40 $ ha-1 less N fertilizer, but resulted in a decrease in crop yield valued at 36 $ ha-1. Application based on soil test used 63 $ ha-1 less N fertilizer, but lost 92 $ ha-1 from crop yield decline.

  • The soil N test map did not stay constant with time. The dYF pattern was quite constant and 1996 YF values for barley were similar to 1994 dYF values for corn.

  • Variable N application significantly decreased subsequent soybean yields in areas with low or no fertilizer N. This cost must be incorporated into economic models of site specific management.

  • Constant fertilizer application resulted in nitrate in drainage water that exceeded the Ontario drinking water objectives. However, at one site the water standards were exceeded where no fertilizer N was applied. Drainage losses increased in sites with high spatial variability.

  • Fertilizer N applied using site specific methods was used more efficiently by the crop compared to constant fertilizer N application. Drainage N losses were reduced with site specific N application proportional to the decrease in average fertilizer N applied.

  • The dYF relationship with recommended N and the spatial patterns of YF measured at the two study sites indicate it may be better to only vary N application for major changes in N requirements. A simple 3-rate (0 %, 50 %, 100 % of recommended) fertilizer N applicator was built.


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