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Legume Green Manure Nitrogen Best Management Program for Potato Production in Environmentally Sensitive Florida Watersheds. Fernando Muñoz, and Chad M. Hutchinson Horticultural Sciences Department. Institute of Food & Agricultural Sciences, University of Florida Abstract Potato production (8-10,000 ha) in the Tri-county Agricultural Area (TCAA) around Hastings, Florida is valued at approximately $65,000,000 annually. The IFAS recommended fertilizer rate is 212 kg N ha-1 although some potato farmers apply more N to reduce nutrient stress during wet seasons. Excess soluble N in potato increases the risk of nitrate leaching and/or runoff in the St John River watershed. Legumes in crop rotation programs with potato could supply N to the potato crop and reduce the need for soluble fertilizers. The project was located at three grower fields in the TCAA over a two year period. The six treatments were planted following a rotation pattern of winter/spring, summer, and fall. The treatments were: Potato-Sorghum-Fallow, Potato-Cowpea-Greenbean, and Potato-Cowpea-Fallow. The potato crop was fertilized at either 168 or 212 kg N ha-1. Soil NH4-N and NO3-N were higher in potato plots following cowpea and green bean. The rotation exhibited higher NO3- N concentration in the soil solution and water table than rotation with sorghum and cowpea. Green bean yield averaged 6,316 kg ha-1. Marketable potato yield in the cowpea-green bean rotation averaged 41.8 MT ha-1 over the three sites and was significantly higher than the sorghum or cowpea treatments (average 31.6 MT ha-1). A low incidence of internal heat necrosis was observed with no significant differences between treatments. Preliminary results indicate that green bean increased soil N before the potato season. However, these plots also exhibited higher leaching potential compared to other plots. Introduction Potatoes grown in Florida for the fresh and chip markets dominate national markets at specific times of the year because of Florida’s unique production season (spring potatoes). Advances in breeding programs and storage techniques have enabled northern state potato production to encroach on Florida’s winter/spring production window. As a result, there has been a decrease in available production contracts. In addition, the St. Johns River Water Management District (SJRWMD) and Florida Department of Agriculture and Consumer Services (FDACS) are in the process of developing Best Management Practices (BMPs) for potato production in the TCAA and across the state. Although the BMPs in consideration do not restrict farming, growers are concerned that the nitrogen rate recommendations could reduce productivity further eroding their economic viability. To stay competitive, farmers need to diversify their crops and/or plant more than one cash crop per year. The concern is that increased crop production would negatively impact the water bodies close to production areas. Any further expansion of production should be done cautiously with the environmental impacts fully understood. One BMP currently under consideration by the SJRWMD is the use of a cropping sequence that would utilize legumes to supply nitrogen to the potato crop. Legumes can fix nitrogen from the atmosphere for use by subsequent crops, recycle un-used nutrients, reduce soil erosion, and add organic matter to the soil. It is proposed that legumes could be used both as a summer cover and a second cash crop during the year without increasing nutrient load in the surrounding water bodies. The use of legume cover crops and cash crops may reduce the need for inorganic fertilizer in the potato crop by providing a “slow-release” organic nitrogen source. Legumes have been estimated to contribute up to 75 kg/ha of nitrogen to a following potato crop (Porter and Sisson, 1991). In addition, an organic nitrogen source is less likely than an inorganic nitrogen source to move out of the rows during heavy rains thereby reducing the impact of farming practices on the area’s water resources. However, in order to be adopted as a cost shared BMP, the crop sequence BMP needs verification at field scale. A three-year research project was funded by the Nitrate Bill BMP Program and is currently underway to document the impact of legume rotation crops on potato plant growth and tuber production in the TCAA. Materials and Methods The project was located on three grower sites in Hastings, Florida. Lower St. Johns River watershed. Soil are Entisols (Hyperthermic, uncoated Typic Quartzipsamments) Total area was 3.5 ha distributed in two 0.9 ha and one 1.7 ha fields. The 6 treatments described in Table 1 were established in each site. The sites were named Bradbury, House and Kelly. The potato variety planted was Atlantic. Soil samplings at preplant and Full flowering were taken during potato season and at preplant for summer cover crops and green bean. Biweekly water samplings from wells and lysimeters were collected during the potato season. Plant tissue samples were collected when potato plants reached 25 cm and at full flowering. Cover crop samples were collected at time of soil incorporation. Green bean samples were collected at first flowers and at harvest. T6 T5 T4 T3 T2 T1 Treatment ID Fallow Cowpea Potato-168 kg ha -1 N (3/4-IFAS N recommendation) Fallow Cowpea Potato-224 kg ha -1 N (IFAS N recommendation) Green Bean Cowpea Potato-168 kg ha -1 N (3/4-IFAS N recommendation) Green Bean Cowpea Potato-224 kg ha -1 N(IFAS N recommendation) Fallow Sorghum Potato-168 kg ha -1 N (3/4-IFAS N recommendation) Fallow Sorghum Potato-224 kg ha -1 N (IFAS N recommendation) Fall Summer Spring Results and Discussion The project was initiated with cover crop planting in summer 2003 followed by green bean planting in fall (Table 1). Tissue analysis of sorghum and cowpea at soil incorporation time showed that average the average N by the summer cover crops was 130 and 80 kg ha -1 TKN for sorghum and cowpea respectively. This result shows sorghum as a better cover crop to recover residual N from the potato fertilization (Fig. 1). Average soil NO 3 -N values at potato planting were 10 and 15 mg kg -1 of soil for treatments including a fallow period and green bean in fall respectively. The increase in soil NO 3 -N in treatments including green bean can be due to the effect of the incorporation of the green bean residues in fall. After a leaching rain, a peak in NO 3 -N concentration in the soil solution was observed in all of the 3 fields (Fig. 2). In Bradbury and Kelly fields the effect was not as high as in the House field except for T3 and T4 in Bradbury field where higher NO 3 -N concentration was observed after a leaching rain. The House field was planted late and the N sidedressing was applied few days before the leaching rain, then high solubilization of fertilizer occurred. Low initial values of NO 3 -N concentration (20-40 mg L -1 ) in the water table (Fig. 3) were observed in Bradbury and House fields but in Kelly initial values were as high as 120 mg L -1 and T3 exhibited a fluctuating pattern with a high peak after the leaching rain (Fig. 3). This increase in NO 3 -N concentration is suggesting that the soil at Kelly field maybe present favorable conditions for nitrification due to fast drainage. The peak in NO 3 -N concentration after the leaching rain in the water table at Kelly field is evidencing a leaching event. The leaching rain had no effect on the NO 3 -N concentration in the water table at Bradbury and produced a small increase in the House field (Fig. 3). These 3 different situations are showing that the risk of NO 3 -N leaching depends on the specific soil conditions at each site. Marketable and total yield combined over site showed a positive effect of the inclusion of cowpea and green in rotation with potato but a negative effect on specific gravity (Fig. 4). This result suggests rotation with legumes as a good alternative for fresh market potatoes but not for chip potatoes. Table 1. Treatment identification and chronological order of the crop rotation. 0 50 100 150 200 250 300 350 4-Mar 13-Mar 17-Mar 30-Mar 13-Apr 27-Apr mg L-1 t1 t2 t3 t4 t5 t6 Bradbury 0 50 100 150 200 250 300 350 4-Mar 13-Mar 17-Mar 30-Mar 13-Apr 27-Apr mg L -1 t1 t2 t3 t4 t5 t6 Kelly 0 50 100 150 200 250 300 350 4-Mar 13-Mar 17-Mar 30-Mar 13-Apr 27-Apr mg L -1 t1 t2 t3 t4 t5 t6 House NO 3 -N NO 3 -N NO 3 -N Figure 2. Observed NO 3 -N concentration in the soil solution (lysimeters) by each site. 0 20 40 60 80 100 120 140 4-Mar 13-Mar 17-Mar 30-Mar 13-Apr 27-Apr mg L -1 t1 t2 t3 t4 t5 t6 0 20 40 60 80 100 120 140 4-Mar 13-Mar 17-Mar 30-Mar 13-Apr 27-Apr mg L -1 t1 t2 t3 t4 t5 t6 0 20 40 60 80 100 120 140 4-Mar 13-Mar 17-Mar 30-Mar 13-Apr 27-Apr mg L -1 t1 t2 t3 t4 t5 t6 NO 3 -N NO 3 -N NO 3 -N Bradbury House Kelly Figure 3. Observed NO 3 -N concentration in the water table (wells) by each site. N uptake by Cowpea 0 50 100 150 200 250 BRADBURY KELLY HOUSE kg ha -1 T5 T6 N uptake by Sorghum 0 50 100 150 200 250 BRADBURY KELLY HOUSE kg ha -1 T1 T2 Figure 1. Nitrogen recovery by the summer cover crops. Total Kjeldahl Nitrogen (TKN). 0 10 20 30 40 50 60 1 2 3 4 5 6 MT ha -1 Mark yield Total yield Figure 4. Yield and specific gravity of potatoes combined over sites. Spec ific Gravity 1.082 1.083 1.084 1.085 1.086 1.087 1.088 1.089 1.090 1.091 1 2 3 4 5 6 Contact information Dr. Fernando Muñoz [email protected] Dr. Chad M. Hutchinson [email protected] University of Florida Plant Science Research and Education Center Hastings, Florida
