A Cost Share Program to encourage Best Management Practices (BMP) has been implemented for growers in the Tri-County Agricultural Area (TCAA, St. Johns, Putnam, and Flagler counties) in Northeast Florida.  The program is managed by the St. Johns River Water Management District (SJRWMD).  The goal of the BMP program is to reduce non-point source nitrate pollution from the 38,000 acres in agricultural production in the lower St. Johns River watershed. 

The BMP nitrogen rate for potato production in the TCAA is 200 lb N/acre.  Growers are concerned, however, that this nitrogen rate may not be sufficient to produce historical yields in all years.  In years with heavy rainfall, nitrogen can be leached from the beds making it both unavailable to the potato plant and a potential pollutant.  Provisions to allow growers to apply more nitrogen have been made in the BMP program for such occurrences.  However, depending on when rainfall events occur, growers may not be able to side-dress the crop before the critical tuber bulking stage. 

Controlled release fertilizers (CRF) could overcome the concerns of growers and regulatory agency personnel by supplying nutrients to the crop over the entire season while reducing the chance of off-site movement of nitrogen.  This is possible because CRFs meter out nitrogen to the plant gradually.  The timing of nutrient release from the CRF prill is dependent on soil temperature and polymer coating thickness on the prill and not soil moisture.  CRFs blends can be developed that release nutrients based on the timing of crop demand.  Therefore, CRFs can be a more efficient method of fertilizing a crop i.e. more fertilizer ends up in the crop and less is available to move off-site.

A CRF program was developed for an on-farm trial in spring 2002 based on two years of results from small plot trials at the University of Florida’s Yelvington Farm. The goal of the demonstration was to compare potato production after application of a grower applied BMP fertilizer program (200 lb N/acre) to a CRF program (150 lb N/acre). 

The experiment was conducted on a 235 acre chip potato farm in the Hastings area.  The soil type was Ellzey fine sand and the crop was irrigated with sub-surface (seepage) irrigation.  The experiment was arranged in a randomized complete block design with three blocks.  Blocks were spaced at three varied locations on the farm.  The two treatments were CRF and grower applied BMP fertilizer program.  CRF plot sizes in each block were 0.87, 0.70, and 0.97 acres.  Grower plots were adjacent to each CRF plot and were of similar size.

Potatoes (var. ‘Atlantic’) were planted on February 6 and 11, 2002.  All production practices in both plots were the grower standard practices except for the application of CRF. 

The grower applied 225 lb P/A and 330 lb K/A in both the CRF and grower BMP beds prior to planting.  The nitrogen program for the grower BMP beds was 124 and 81 lb N/acre applied on February 26, 2002 and March 22, 2002, respectively.  The nitrogen source was liquid 32-0-0.

The CRF nitrogen program was a 50:50 (N rate) mix of 37-0-0 and 43-0-0 products (The Scotts Company, Marysville, OH) broadcasted and incorporated at layby, 21 days after planting.  A total of 150 lb N/acre was applied.

Eight sub-plots, each 20 feet long, were harvested from each plot on May 22, 2002.  Potatoes were transported to the University of Florida’s Yelvington Farm where they were washed and graded with commercial equipment into five standard size classes.  Specific gravity was measured by the weight-in-air/weight-in-water method.

This on-farm trial demonstrated the potential of a CRF program in potato production in the TCAA.  Potato plants fertilized with the CRF program (150 lb N/acre) produced 21 cwt/acre more tubers in total yield than plants fertilized with the grower BMP nitrogen program (200 lb N/acre) without negatively impacting quality or grade (Table 1).

Additional trials are planned for next season with “second-generation” CRFs.  These materials will be blended to more closely match the uptake requirement of the potato plant.  If production results remain constant after future testing, a savings of 50 lb N/acre without a yield loss would be a positive step forward for growers and regulating agencies in the TCAA.  This would result in one million pounds less nitrogen applied in the lower St. Johns River basin annually.

Material cost is the last challenge to be overcome before wide-scale CRF use becomes a reality in row crop production. Cost of CRFs to the growers in the TCAA will ultimately be determined by the rate of material used, pricing by the manufacturer based on large scale production (economies of scale), and whether CRFs are adopted as a reimbursable BMP in the SJRWMD Cost Share Program.