Controlled-release fertilizers (CRFs) are often called slow-release fertilizers (SRFs) or timed-release fertilizers. However, the terms CRF and SRF should not be used interchangeably. The Association of American Plant Food Control Officials defines CRFs as fertilizers that contain a plant nutrient in a form in which the plant uptake is delayed after application, or that provide a longer duration of nutrient availability compared to other quick-release fertilizers, such as urea. The main difference between CRF and SRF is that in CRF (usually coated fertilizer), the factors affecting the rate, pattern, and duration of release are well known and controllable, whereas in SRF, they are not well controlled. At soil temperatures of 25°C, a CRF must meet three criteria: (1) less than 15% of the CRF nutrients should be released in 24 hours, (2) less than 75% should be released in 28 days, and (3) at least 75% should be released by the stated release time (40–360 days) (Trenkel 1997). Widely used CRFs include Nutricote®, Osmocote®, Polyon®, and Agrocote®. This publication discusses the use of CRFs—namely, coated fertilizers for commercial potato production in Florida. Energy costs, and thus fertilizer costs, escalated dramatically since 2005 and, therefore, cost for either traditional or CRF fertilization for potato production needs to be updated accordingly.
The recent emphasis on the development of vegetable production best management practices (BMPs) has prompted a re-examination of fertilization practices for potato production in Florida. Considering the goals of the BMP programs, the use of CRFs has the potential to meet the production and environmental goals of both growers and regulatory agencies. Past UF/IFAS research has demonstrated that a CRF program can reduce nitrogen (N) rates by 25–50 lbs/acre without sacrificing crop yield or quality when compared to a traditional soluble N fertilizer program (uncoated urea, CO(NH2)2). Although CRF technology can improve N use efficiency, the high cost of the material has limited the adoption of CRF technology for potato production. However, the development of BMPs coupled with the cost-share potential of CRFs at the national, state, and/or local level has improved the chances for CRF use for potato production.
This publication compares the costs and benefits of a nitrogen CRF program with those of a traditional soluble N program in potato production. The cost of a soluble N fertilizer program can vary from grower to grower and from year to year based on manufacturing costs, N sources, and application rates. A range of possible costs and rates are detailed in Table 1. The BMP N rate for a soluble N fertilizer program is 200 lbs/acre N and is included for comparison. Table 2 lists several cost and rate combinations for a CRF fertilizer program. There are several unknowns with the CRF fertilizer program, the most important of which is material cost per ton. Other critical unknowns include the unpredictable climatic influences that may affect release timing of CRFs and their resulting effects on yield, as well as the number of leaching events, which can affect the efficacy of soluble source comparisons.
Soluble Nitrogen Source
Table 1 lists estimated costs for fertilizer application rates of traditional soluble N fertilizers in northeast Florida over the past years. A survey conducted in Flagler County in early 2011 indicated that the total production cost has sharply increased because of price increases in fuel and fertilizers. For example, the price of UAN-32 (urea ammonium nitrate solution 32-0-0) has increased from $152/ton in 2005 to $410/ton in 2008, to $445/ton in 2014, and to $492/ton in 2021 (https://www.dtnpf.com/agriculture/web/ag/crops/article/2021/10/27/nitrogen-fertilizer-prices-lead-hits). In March 2011, the same fertilizer formula was sold for $440/ton in Putman County. The N costs range from $158 to $192/acre, which is equivalent to 6.2%–7.5% of the total cost for potato production ($3,057 for chipping potato to $4,148 for russet potato). To find the N cost for a specific farm or program, locate the cost of N per acre for the previous season at the top of the chart and move down the column to the appropriate N fertilizer rate.
CRF Nitrogen Source
UF/IFAS research indicates that potato tuber quality and yield with a CRF program of 150–175 lbs/acre N are comparable to those with a standard soluble fertilizer program at the BMP rate (200 lbs/acre N). CRF prices used in this article are based on the fertilizer market in Florida. According to the listed prices from suppliers in Putman County, in March 2011 the prices of CRFs ranged from $1,200 to $1,300 per ton. Accordingly, the per-acre cost would be $209–$265 (Table 2). Published data from the UF/IFAS Food and Resource Economics Department's website indicates that the total cost of potato production is $3,057 and $4,148, respectively, for chipping and russet potatoes. Therefore, the projected cost of N from a CRF program is in the range of 6.9%–8.0% of the total cost of potato production (Table 2).
Benefits of CRF use
Tables 1 and 2 show the potential costs for both the soluble and CRF fertilizer programs. The most expensive highlighted CRF program ($285/acre) is 1.4 and 0.6 times more expensive than the least and most expensive highlighted traditional fertilizer programs ($120 and $180/acre), respectively. These have changed much since 2003 (Hutchinson and Simonne 2003). Although the costs are not yet comparable, the benefits of a CRF program compared to a traditional fertilizer program are as follows:
- A CRF fertilizer program requires only one pre-plant fertilizer application. However, a traditional fertilizer program needs multiple applications (application number dependent on season and/or farm). The Florida BMP program recommends at least a single split application (two trips) when traditional N sources are used. Each trip for fertilization across the field costs between $5 and $7/acre for broadcasting.
- A polymer-coated CRF releases nutrients at a rate that is dependent upon soil temperature rather than soil moisture. Therefore, during potato growing seasons with substantial rain, N in the CRF prill remains in the field and does not leach into the watershed. The current BMPs for the traditional fertilizer program allow up to 30 lbs/acre N to be added during the season after each leaching rain event to replace leached fertilizer. In the 2003 season, some growers applied an extra 90 lbs/acre N as part of the BMP program because of the substantial rainfall (total 290 lbs/acre N for BMP program in 2003). No additional N was necessary with the CRF program (150–175 lbs/acre N in 2003). In the Hastings area, CRFs can be used for "insurance: by using CRFs, potato can get sufficient nutrients regardless, there are heavy rains."
- The CRF program improves N use efficiency and potentially reduces N leaching risks. A greater percentage of applied N makes it into the crop when fertilized with a CRF fertilizer as compared to a traditional N fertilizer source. This improvement is because CRFs release N slowly over the season as the crop needs it. Accordingly, there is less opportunity for N to leach into the groundwater with a CRF program. CRF N rates of 175 and 150 lbs/acre translate into a yearly N savings of 450,000–900,000 lbs in the St. Johns River watershed production area compared to the BMP N rate of 200 lbs/acre. By reducing the CRF N rate below the BMP rate, potato producers and manufacturers can develop goodwill with the public while reducing the potential for nitrate to enter the watershed, thus improving water quality in the Lower St. Johns River Basin.
- The CRF program reduces the potential for ammonia (NH3) volatilization and nitrous oxide (N2O) emissions and improves air quality and operational efficiencies of N fertilization. With fewer trips across the field to apply fertilizer and less worry during rainy seasons, the producers can spend more time marketing potatoes and doing other things to improve the profitability of potato production. Appropriate soil placement of CRF is critical to ensuring the crop's effective use of it and to preventing CRF prills from being washed away during heavy rain events.
- Under the same amount of nitrogen, compared with soluble N, CRFs increased marketable tuber yield by 69%–80% (Table 3). Tuber yield produced by using equal amounts of the three CRFs at total N of 168 kg/ha (150 lbs/acre) was as much as 26% more than that obtained with conventional N fertilizer, ammonium nitrate, at total N of 224 kg/ha (200 lbs/acre). At 168 kg/ha N rate, the margin marketable tuber yield was only 89 kg/kg N for the conventional soluble fertilizer. The corresponding margin yield was as much as 201 kg/kg N for CRFs. The margin marketable yield means that marketable tuber yield increment is produced by an additional unit of N after a particular N rate. These data suggest that CRFs may increase potato yield potential under the same growth conditions.
In Florida, soil temperature is low at the beginning of the growing season. This low temperature can slow nutrient release from CRFs, but traditional soluble N fertilizers work well. Recent research results show that using half traditional fertilizer and half CRFs can reduce leaching potential, enhance potato tuber yield, and may be more affordable than the full CRF application rate.
A CRF fertilizer is more expensive than a traditional soluble fertilizer on a per unit basis, which limits adoption of the new technology. The use of CRF would likely increase if CRF costs were shared by all parties that have a stake in improving water quality in the St. Johns River watershed. In this simple model, the cost of a traditional N fertilizer program in most years falls between $120/acre and $180/acre based on the current N fertilizer price (Table 1). Estimated CRF program costs (Table 2) would be approximately $29–$105/acre more than the most expensive soluble N cost ($180/acre, Table 1). If this cost difference were supported 100% by local, state, or national regulatory agency funds, the cost-share program would require between $522,000 and $1,890,000 annually to be fully funded. The northeast Florida potato crop is valued at approximately $163 million. The cost-share program costs would be a relatively small cost to keep a northeast Florida business with a potential $163 million annual return solvent.
These numbers serve as a starting point for discussion regarding the value of using CRFs for commercial potato production in the St. Johns River watershed. There are approximately 18,000 acres of potatoes in the St. Johns River watershed that can benefit from a CRF program. There are more than 100,000 acres of other vegetable crops on seepage irrigation across Florida that could benefit from a CRF program as well. This acreage increases greatly if one considers all the production areas in the United States where N may be negatively impacting surrounding watersheds. A CRF program can be a win-win-win opportunity for producers, manufacturers, and regulatory agencies by helping all meet their production, business, and environmental goals.
UF/IFAS Food and Resource Economics Department. "Cost of Production for Florida Vegetables 2008–2009." University of Florida. Accessed November 11, 2021. https://fred.ifas.ufl.edu/pdf/iatpc/files/HastingsTablePotato09.pdf.
Hutchinson, C. M. 2005. "Influence of a Controlled Release Nitrogen Fertilizer Program on Potato (Solanum Tuberosum L.) Tuber Yield and Quality." Acta Hort. (ISHS) 684: 99–102. Accessed November 11, 2021. https://www.actahort.org/books/684/684_13.htm.
Hutchinson, C. M., and E. H. Simonne. 2002. "Development of Controlled Release Fertilizer Program for Potato Production." Vegetarian Newsletter, March. UF/IFAS Horticultural Sciences Department.
Hutchinson, C. M., E. Simonne, P. Solano, J. Meldrum, and P. Livingston-Way. 2003. "Development of a Controlled-Release Fertilizer Program for North Florida Irish Potato (Solanum tuberosum) Production." J. Plant Nutr. 26 (9): 1709–1723.
Hutchinson, C. M., W. A. Tilton, and E. H. Simonne. 2002. "On-Farm Demonstration of a Controlled Release Fertilizer Program for Potato Production." Vegetarian Newsletter, June. University of Florida Horticultural Sciences Department.
Hyatt, C. R., R. T. Venterea, C. J. Rosen, M. McNearney, M. L. Wilson, and M. S. Dolan. 2010. "Polymer-Coated Urea Maintains Potato Yields and Reduces Nitrous Oxide Emissions in a Minnesota Loamy Sand." SSSAJ 74: 419–428.
Mayer, H. 2010. "Nutrient Release Patterns of Controlled Release Fertilizers Used in the Ornamental Horticulture Industry of South Florida." Master's thesis. University of Florida.
Pack, J. E. 2004. "Controlled-Release Nitrogen Fertilizer Release Characterization and Its Effects on Potato (Solanum Tuberosum) Production and Soil Nitrogen Movement in Northeast Florida." Master's thesis. University of Florida.
Shaviv, A., and R. L. Mikkelsen. 1993. "Controlled-Release Fertilizers to Increase Efficiency of Nutrient Use and Minimize Environmental Degradation: A Review." Fertilizer Research 35: 1–12.
Trenkel, M. E.1997. Controlled-Release and Stabilized Fertilizers in Agriculture. Paris: The International Fertilizer Industry Association.
Worthington, C. M., K. M. Portier, J. M. White, R. S. Mylavarapu, T. A. Obreza, W. M. Stall, and C. M. Hutchinson. 2007. "Potato (Solanum tuberosum L.) Yield and Internal Heat Necrosis Incidence Under Controlled-Release and Soluble Nitrogen Sources and Leaching Irrigation Events." American Journal of Potato Research 84: 403–413.
Traditional soluble fertilizer program costs per acre using a blend of urea and ammonium nitrate with a final grade of 32-0-0.
Alternative CRF program costs per acre using a polymer-coated urea with a final grade of 43-0-0.
Differences in yield and tuber quality of 'Atlantic' potato grown with either conventional or CRF fertilizers at different N rates (Hutchinson 2005).