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Comparison of commercial and reduced N fertilizer programs for impacts on vegetable crop yield, partial N balance, and soil and groundwater nitrate-N concentrations Authors: George Hochmuth, Don Graetz, and Wendy Graham Summary: Water quality has been the subject of concern and attention in the Suwannee River Basin for a number of years. Recent data have indicated increasing trends in concentrations of nutrients in ground water, spring water, and private drinking water wells in this region. This has brought a needed focus of agency efforts to find nutrient management solutions to the problem. In addition, the state’s participation in the Total Maximum Daily Load (TMDL) Program will require new initiatives for managing nonpoint sources (e.g., for nutrients) for agriculture, and for measuring the use and effectiveness of nonpoint source controls. An on-farm study was conducted to compare the effects of the commercial-farm nitrogen (N) management practice with a reduced-N program for effects on crop yield, and groundwater nitrate-N and soil pore water N concentration. A seven-year cropping cycle was monitored in one large field under center-pivot irrigation. Potato was grown in three of the seven years and yield was monitored. There was no effect of N management program on potato yield; a reduced-N program resulted in the same yield as with the commercial program. Nitrate-N concentration in the soil pore water was not affected by fertilizer treatment, but groundwater nitrate-N concentration was reduced slightly under the field with the reduced-N fertilizer program. Results measured over several years showed that adoption of best management practices can result in a reduction in groundwater nitrate-N concentrations. ____________________________________________ The Suwannee River, designated as an ”Outstanding Water Body” is the second largest river in Florida with a mean annual flow of 6.7 billion gallons per day. The Suwannee River basin comprises an area of 25,770 km2 with 43% of the total area located in north central Florida. The Suwannee River is used for recreation, fishing, potable water, and provides habitat for a diverse ecosystem. The primary land use in the Florida portion of the basin is agriculture, including forestry, pasture, row crops, and livestock such as dairy and beef cattle (Hornsby, 2000). The basin has karst characteristics that result in a direct linkage between the surface and ground waters (Katz et al., 1999). Over the past 40 years, the nitrate-N concentrations in many of the springs in the Suwannee River basin have increased from less than 0.1 mg/L to more than 5 mg/L (Hornsby and Ceryak, 1999; Katz, 1992; 2000; Maddox et al., 1992), leading to high nitrogen loading rates in the Suwannee and Santa Fe Rivers (Katz, 2000). Row crops, including vegetables are important agricultural enterprises in the Suwannee River basin. Potato, watermelon, and sweet corn are among the more important vegetable crops in the region. Potatoes are commonly planted in January to February in north central Florida and average potato yield in Florida is currently around 24.7 Mg/ha (Hochmuth et al, 2000). Fertilization rates and irrigation can vary significantly by cultivar and geographic location. Currently the Institute of Food and Agricultural Services (IFAS) recommends a target fertilization rate of 224 kg/ha (200 lbs/acre) (Hochmuth and 1 Cordasco, 2000; (Hochmuth et al, 2000). The N is split in several application in the growing season with some N applied at planting and the remainder between 20 and 50 days after planting. A survey of fertilization practices by Florida potato growers performed in the 1980s showed average N fertilization rates of about 300 kg/ha (Hochmuth and Cordasco, 2000). A more recent review of the potato fertilization research literature was made by Hochmuth and Hanlon. This review of the research supported the current recommendation of 200 lb/acre N as a target N rate for the season. The N amount can be adjusted up or down based on growing conditions and leaching rainfall. Recent research on new fertilizer materials was also reviewed, documenting the possibility of reducing the total amount of N applied with controlled-release fertilizers. There is a need to document the performance of new and revised N fertilizer best management practices in a commercial farm setting. The research summarized herein was conducted to evaluate the effects of two fertilizer management practices on crop production and soil and groundwater nitrate-N concentrations on a commercial farm in northern Florida. This farm rotates several crops during a annual growing period, for example a vegetable such as potato or sweet corn with peanut, silage corn and a winter cover crop of oats or cereal rye. The goal of this project was to implement BMPs at the farm level to reduce nutrient loadings to ground water from agricultural activities and to evaluate their water quality improvement effectiveness. The objectives to meet this goal were to: 1. Evaluate water quality impacts (primarily ground water) from a row cropping system on a commercial farm. 2. Evaluate the impacts Best Management Practices (BMPs) on potato crop yield for a row cropping farm, Materials and methods BMPs were evaluated on a 140-acre center pivot irrigated field where the landowner applied normal practices on half of the field and project-selected BMPs were used on the other half of the field. The project BMPs consisted of reductions in fertilizer N amounts and improved timing of fertilizer applications to anticipate crop needs. Wells were constructed in the field to monitor groundwater nitrate-N concentrations beneath the field. The wells are described in Table 1. Well positions in the field are shown in Figure 1. 2 Table 1. Well characteristics at the row crop farm. driller measured depth to depth of top of stand- bottom of open top of open hole Open reported well depth rock (ft) conductor pipe hole (ft above msl) interval well # well depth (ft) casing (ft) (ft above MSL) (ft above msl) (ft) 1 53 41 36 36 53 0 12 12 2 38 35 28 28 45 7 17 10 3 30 36.6 20 20 47 15 25 10 4 30 32.8 20.5 20 48 16 26 10 5 45 44.6 34.5 35 52 6 16 10 6 40 38.7 19 20 47 5 25 20 7 50 37.8 33.5 35 49 -3 12 15 8 40.5 40.3 19 20.5 46 5 25 20 9 40.5 39.8 25 20.5 44 4 24 20 10 40.5 41.7 20 20.5 50 8 28 20 11 40.5 41.4 27 23 49 7 24 17 12 38 38.8 22 20.5 53 13 31 18 13 38 38.6 20 20.5 48 8 26 18 Suction lysimeters were installed at selected points around the field for collecting soil pore water samples for nitrate-N analyses (Figure 1). Suction lysimeters consisted of a porous ceramic cup on the end of a PVC pipe with plastic sampling tubing allowing the sampling of soil pore water drawn into the cup by applying a vacuum to the apparatus. Suction lysimeters were installed at two depts., 1 and 2 meters. Soil samples were collected at over a 2 meter depth for an average nitrate-N concentration. Samples were collected with a bucket auger. Soil samples were extracted with 2M KCl and analyzed for nitrate-N. Potato crops were monitored for yield at the end of the growing season. Yields for each half of the field were obtained from the farm records. The row crop farm produced primarily vegetable crops such as potatoes and sweet corn plus cotton and peanuts (Table 2). The BMPs addressed N fertilization management practices. The landowner was encouraged to follow the fertilization and irrigation management practices recommended by the Institute of Food and Agricultural Sciences at the University of Florida. BMPs involving nitrogen and irrigation application rate and timing were evaluated with initial emphasis on potatoes (a heavily fertilized and irrigated crop) over three growing seasons. These activities resulted in the development of crop management guides for these two crops. These guides are farmer friendly and were developed to allow the farmer and his field supervisors to implement the plans with minimum input from external aid. BMPs generally consisted of reduction in fertilizer amounts and improved timing of fertilizer applications to anticipate crop needs (Table 2). These BMPs were demonstrated on a 140 acre 3 center pivot irrigated field where the landowner applied is normal practices on half of the field and the BMPs were used on the other half of the field. As part of the irrigation management program, new nozzles were installed on the center pivot irrigation systems. These new nozzles were designed to cause less bed erosion and yet have high efficiency and uniformity. These nozzles have produced the desired results as evaluated by the Suwannee River Mobil Irrigation Laboratory and are now being recommended by many of the irrigation equipment dealers in the watershed. Production practices at the row crop farm used a crop sequence that utilized a variety of crops annually. The general sequence was a high value vegetable crop (potatoes, sweet corn) in the Spring (February – June), followed by a crop such as cotton or peanuts in the Summer (July – September), and a cover crop for the winter. However, this sequence varied depending on market conditions for various crops. The actual sequence used on the demonstration field is shown in Table 2. For the project, more emphasis was given to the crops that received the most N, i.e., potatoes and sweet corn.

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