• Topics: Horticultural Sciences  |  Sweet Corn  |  Hochmuth, George J  | Cordasco, Kim M 

A Summary of N, P, and K Research with Sweet Corn in Florida¹

Sweet corn production in Florida represents 8% of the total vegetable production value in the state or $123,760,000 (Fla. Agri. Stat. Serv., 1998). Based on statistics from the 1996-1997 crop season, 53% of the sweet corn grown in the state was harvested from the Everglades Agricultural Area, 19% from central Florida, 18% from southeast and southwest Florida, and 10% from west and north Florida. The heaviest harvest period occurred between April and July when 80% of the sweet corn grown in the state was harvested. The bulk of this sweet corn (56%) was shipped out of the state during this harvest. Over the ten-year period from 1987 to 1997, harvested acreage decreased from 55,000 to 42,000 acres while the number of crates harvested/acre increased from 232 crates/acre in 1987 to 330 crates/acre in 1997 (42 lb/crate).

The purpose of this publication is to summarize sweet corn fertilization research on mineral soils leading to current University of Florida recommendations for sweet corn fertilization and to summarize needs for continued research. In 1995, maximum nutrient recommendations were for 150 lb/acre nitrogen (N) and 120 lb/acre P₂O₅ and K₂O for mineral soils with very low Mehlich-1 (M-1) soil-extracted P and K (Hochmuth and Hanlon, 1995). The N rate was revised upward from 120 lb/acre recommended in 1989 (Kidder et al., 1989). When residual soil concentrations of P and K are interpreted as low, medium, or high, the recommendations for P₂O₅ and K₂O are decreased to 100, 80, and 0 lb/acre, respectively. A 1994 USDA-administered survey of fertilizer use by commercial farms documented an average application of 95, 80, and 215 lb/acre of N, P₂O_5, and K₂O, respectively, in Florida (Fla. Agr. Stat. Serv., 1995). The average N application appears low because much of the corn produced in the state is grown on organic soils that require little N fertilizer.

Sweet corn fertilization research has been conducted in Florida for more than thirty years. During the fifteen-year period from 1981 to 1996 yields have increased. Sustained high yields can be expected with fertilization practices designed to supply crop nutrient requirements and protect the environment from excessive fertilization. Use of the Mehlich-1 (M-1) soil test, initiated in 1979, refined the practice of fertilizer rate recommendation and resulted in a crop- and soil-specific guide to fertilizer application. Recommendations for N, P₂O₅, and K₂O fertilization were based on in-field experiments where optimum fertilizer rates were determined from yield responses to a range of fertilizer rates, and with varying climatic conditions, soil types, and sweet corn varieties. In terms of cost, fertilizer represented from 6% to 13% of the total operating and fixed costs of sweet corn production as estimated in the 1995-1996 crop season (Smith and Taylor, 1996).

Data Summary Method

To evaluate sweet corn yield responses to varying rates of fertilizer, a method was needed to standardize the numerous methods used for quantifying statewide yield results such as tons, boxes, crates, ear weight/acre, or metric tons/hectare. Relative yield (RY), a calculated percentage, was chosen as the unit to express sweet corn yield responses to fertilization. The highest yield for each fertilizer experiment received a 100% value and other yields were expressed as a percentage of the highest yield. The actual yield expressed in 42-lb crates/acre units, is presented for the treatment corresponding to 100% RY. The RYs were plotted against rates of nutrient to determine how sweet corn yields responded to fertilizer in Florida. The RY presentation allowed data from a variety of experiments with different cultivars, production locations, and crop seasons to be included in the graphical summary of yield responses to fertilization. For most studies, RYs of 95 to 100% were not significantly different.

Nitrogen

Early research with N fertilization of sweet corn focused on gaseous loss of N from unincorporated, surface applied N fertilizer (Volk, 1962). Yield responses were evaluated from plants fertilized with covered or uncovered side-dressed fertilizer and from plants fertilized with liquid N fertilizer. Experiments were conducted near Gainesville on Leon fine sand soils in both seasons. In 1961, 18 lb/acre N was applied “in-the-drill” at planting with additional N applied in one or two side-dress N applications at 47 lb/acre each from urea, NH₄NO₃, or Uran 32 solution (16.5% urea - N, 15.5% NH₄NO₃). Experimental plots were established on beds treated with 0, 1900, or 4800 lb/acre lime in 1959.

Results with corn yield response to N fertilization are presented in Fig. 1. Yields averaged over the three N-source treatments, on previously limed and unlimed beds (pHs 4.5, 5.2, and 5.7), and covered or uncovered N fertilizer were optimized when fertilizer was side-dressed twice for a total of 112 lb/acre N (239 crates/acre, 100% RY). A lesser yield resulted from plants fertilized with the single side-dress N application of 65 lb/acre N, 77% RY. With the higher N rate, plants with covered fertilizer yielded 14% more than plants where fertilizer was uncovered, 227 and 200 crates/acre, respectively. Researchers observed that more late-developing ears were harvested when plants received the second fertilizer side-dressing than those side-dressed once and that lime treatments had similar effects on yield. Yields were not different between liquid and dry N sources, but, urea fertilized plants produced yields 6% to 8% lower than plants fertilized with other N sources.

Figure 1. 

Relative yield of sweet corn for experiments, years, and seasons as a function of added N.

A second experiment was conducted in 1962. The same N sources were used as in the previous experiment with two additional treatments, Feran 21 (NH₄NO₃ solution) and Pril-Cal (plastic coated Ca(NO₃)₂. Fertilizers were applied once at planting, 18 lb/acre N, followed by a single side-dress application of 75 lb/acre N. Dolomitic limestone was applied uniformly to all beds (fall 1961) for low, medium, and high pH plots (5.2, 5.9, and 6.2, respectively). Plants fertilized with non-urea fertilizer yielded 3% more when the fertilizer was covered than when it was uncovered. Plants fertilized with urea produced similar yields with either covered or uncovered fertilizer. Uran fertilized plants, however, produced more when the fertilizer was uncovered, 256 crates/acre, compared to covered, 242 crates/acre. Researchers concluded that covering the fertilizer improved N availability regardless of soluble or dry N source. As in the previous season, lime increased the yield of late developing ears.

Simultaneous experiments were conducted in 1961 and 1962, in Gainesville with N sources NaNO₃, NH₄NO₃, Uran, Pril Cal, and urea applied in single or split applications (1961) and NaNO₃, NH₄NO₃, and urea applied single or split totalling 50, 100, or 150 lb/acre N (1962) (Robertson, 1962). Dolomitic lime was applied and incorporated at 1 ton/acre. Iona sweet corn seed was planted both seasons on Ona-Kanapaha fine sand complex soils fertilized with 12 lb/acre N at planting. The corn was cultivated when plants were 10, 18, and 24 inches tall. Irrigation was not needed in 1961, but two irrigations of 1 inch/acre each were applied in 1962.

In 1961, plants received no N or 50 lb/acre N from each N source side-dressed and incorporated when plants were 10 inches tall. Total N application including the 12 lb/acre N applied at planting was 62 lb/acre. Marketable yields were similar with all applied N sources resulting in an average high yield of 165 crates/acre from fertilized plants compared to 55 crates/acre from unfertilized plants. Nitrogen sources also did not affect the number of ears/acre, the weight/ear, or tissue N concentrations sampled from whole leaves at tasseling. Tissue N concentration, averaged over all N source treatments, was 1.43%, which was below the adequate range of 1.5 to 2.5%.

In 1962, fertilizer was applied and incorporated when plants were 10 inches tall or applied half at this height and half when plants were 24 inches tall. Sweet corn yields responded linearly to N rate with NaNO₃ and urea N sources through 150 lb/acre (306 and 286 crates/acre, respectively, 100% RY) but, responded quadratically with NH₄NO₃ - N. Equal yields resulted from plants fertilized with 100 lb/acre NH₄NO₃ - N (305 crates/acre, 100% RY) as those fertilized with 150 lb/acre NaNO₃ or urea - N. Yield from NH₄NO₃ fertilized plants was reduced to 93% RY with 150 lb/acre N. Researchers noted that reduced stands resulted from the single application of 150 lb/acre NH₄NO₃ - N. Yield responses were generally higher with all N sources when N was split applied compared with the single N application. Yield responses to increasing N rates were graphed by N source in Fig. 2. As in 1961, plants fertilized with covered NH₄NO₃ or NaNO₃ yielded more than urea fertilized plants through 100 lb/acre N. Leaf- tissue N concentrations at tasseling averaged 2.4% and 2.5% with 100 and 150 lb/acre N, respectively, and were within the adequate range of 1.5% to 2.5% (Hochmuth et al., 1991).

Figure 2. 

Relative yield of sweet corn for experiments, years, and seasons as a function of N source and N rate.

Experimentation with N fertilization of sweet corn was continued the summer of 1976 and spring of 1977 when field experiments were conducted near Gainesville (Rudert and Locascio, 1979a). Nitrogen sources, rates, time of application, and the nitrification inhibitor nitrapyrin were evaluated for their effects on yield and tissue nutrient concentrations. Kanapaha fine sand soils with 1.0% organic matter and an after-lime pH of 6.5 received factorial applications each year of 50, 100, or 200 lb/acre N from (NH₄)₂SO₄, 0, 0.5, or 1 lb/acre nitrapyrin, and fertilizer application at preplant or 50% preplant, 50% side-dress (5 weeks after seeding). An adjacent experiment was conducted with the above N rates from Ca(NO₃)₂ - N and 0 or 1 lb/acre nitrapyrin in 1976 and no nitrapyrin in 1977. The fertilizer was banded 3 inches from the row and 3 inches below the bed surface and Silver Queen seeds were planted.

Quadratic yield responses occurred with (NH₄)₂SO₄ fertilized plants with yields leveling off above 100 lb/acre N in 1976 and 1977. Relative yields, respectively, corresponding to 50, 100, and 200 lb/acre N were 62%, 86%, and 100% (208 crates/acre, 1976) and 62%, 88%, and 100% RY (310 crates/acre, 1977). Nitrogen leaching, due to heavy weekly rainfall, was cited for the lower summer yields in 1976 compared to the dry, irrigation-supplemented spring 1977 season. During the wet season, plants fertilized with (NH₄)₂SO₄ yielded 65% more than Ca(NO₃)₂ fertilized plants. Plants fertilized with Ca(NO₃)₂ - N had similar yields with all N rate treatments. Soil analysis revealed that most of the soil N had leached below the top 12 inches 2 weeks after application of Ca(NO₃)_2.

Under non-leaching conditions in the spring of 1977, a linear yield response to N rate resulted with Ca(NO₃)₂ fertilizer (289 crates/acre with 200 lb/acre N, 100% RY). Leaf-tissue N concentrations from ear leaves sampled at tasseling (11 weeks) were below the adequate range of 1.5 to 2.5% with all N rates and both N sources. Lower tissue N concentrations occurred in (NH₄)₂SO₄ fertilized plants compared to Ca(NO₃)₂ fertilized plants. Yield responses to N rates and N sources for these experiments were presented graphically in Fig. 2.

Yields responded similarly in both seasons to single or split N applications regardless of fertilizer source or soil moisture conditions. Use of nitrapyrin, intended to inhibit the conversion of NH₄⁺ to NO₃^- and stabilize soil NH₄⁺ - N content, did not influence yields of sweet corn in either experimental season. Researchers suspected high soil temperatures acted to denature the nitrapyrin but, in later research, NH₄ - N was found to leach at a faster rate than nitrapyrin in sandy soils (Rudert and Locascio, 1979b). Maximum nitrapyrin effectiveness occurred within 2 to 4 weeks of application on Kanapaha fine sand soils and thereafter NH₄ - N had moved below nitrapyrin in the soil.

Researchers concentrated on finding the optimum N rate and N placement method for sweet corn production in an experiment in Quincy, North Florida Research and Education Center, in 1990 (Rhoads, 1990). As with previously summarized research, the objective was to minimize N movement in the soil and maximize yields. Nitrogen treatment rates of 125 and 200 lb/acre were achieved with application of 50 lb/acre at plant emergence and 75 lb/acre five weeks before harvest (for 125 lb/acre) and an additional 75 lb/acre N applied 7 weeks before harvest (for 200 lb/acre). Fertilizer was broadcast between the rows (row middles) or banded to one side or both sides of the row. The row middles were alternately compacted by wheel traffic or non-compacted.

Sweet corn yields, averaged over all placement methods, were optimized with 125 lb/acre N (283 crates/acre) 99% RY, a 50% increase in yield over plants with the zero N treatment. Leaf tissue sampled 5 weeks before harvest had N concentrations within the adequate range of 2.5 to 4.0% with N treatments of 125 and 200 lb/acre, but N concentrations were inadequate with the zero N treatment. Analysis of whole-plant N content revealed that from each N respective treatment, 125 and 200 lb/acre, plants absorbed 85 and 70 lb/acre N with 40 and 130 lb/acre N unrecovered. Nitrogen recovery increased when fertilizer was banded near the root zone. From a single band of fertilizer, with 125 lb/acre of applied N, 88 lb/acre was recovered and when fertilizer was banded on both sides of the plant row, 100 lb/acre N was recovered. Less N was recovered from 125 lb/acre of applied N where fertilizer was broadcast across compacted or non-compacted row middles, 74 to 79 lb/acre N, respectively. Researchers concluded that plant recovery of N decreased with N rates above 125 lb/acre, that N fertilizer applied in bands near the root zone increased N recovery, and that the 120 lb/acre recommended N rate (Kidder et al., 1989) resulted in optimum yields.

Researchers at Suwannee Valley Agricultural Research and Education Center near Live Oak applied N, at rates in 50 lb/acre increments, from 0 to 250 lb/acre to field test the N recommendation of 120 lb/acre (Hochmuth et al., 1992; Kidder et al., 1989). Preplant NH₄NO₃ - N was applied at 30 lb/acre, except with the zero N treatment, to Lakeland fine sand soil with 2.0% organic matter content. The remaining N was applied as a single side-dress application with the 50 lb/acre treatment, and in two equal side-dress applications with the other treatments. Side-dress applications were made when plants were 6 inches tall and when plants were 18 inches tall. Sprinkler irrigation was applied to maintain soil moisture at -12 centibars as measured by tensiometer.

Marketable corn yields increased quadratically resulting in optimum yields with 150 lb/acre N (330 crates/acre, 100% RY). As with yield, N concentrations of leaf tissue sampled at tassel emergence increased quadratically from 2.1% with 0 lb/acre N (deficient) to 3.3% with 100 and 150 lb/acre N (adequate) and leveled off thereafter to 3.5% with 200 lb/acre N (adequate). For this experiment, a leaf-tissue N concentration of 3.3% and total N rate at 150 lb/acre resulted in optimum yield. The quadratic equation and linear plateau model, predicted maximum yield with N rates of 187 and 83 lb/acre, respectively. The midpoint of these two rates corresponded to the experimental high-yield rate of 150 lb/acre N.

Ear quality factors including ear diameter, ear uniformity, and ear tip uniformity also increased quadratically with increased N rate, leveling off above 50 lb/acre N (ear diameter), above 100 lb/acre N (ear uniformity), and above 150 lb/acre (tip uniformity). With N treatments of 150 lb/acre and above, yield of cull corn was minimized and yield of U.S. No. 1 grade ears leveled off. Yield of No. 2 grade ears was unaffected by N rate while ear length increased only with the first increment of applied N.

Sweet corn yield responses to N fertilization presented thus far were conducted in northern areas of the state where high yield resulted with 100 to 150 lb/acre N. Research was conducted in southern Florida during the winter season to evaluate sweet corn response to N fertilization and fill in gaps in research on sweet corn fertilization for southern production areas (Hochmuth et al., 1995; Hochmuth et al., 1998) Hochmuth et al., 1998. Nitrogen treatments, in 50 lb/acre increments from 0 to 200 lb/acre from NH₄NO₃, were applied on rockland soils (1994) and on rockland and marl soils (identified as sites one and two, respectively) in 1995. All experiments were conducted on commercial farms near Homestead, Florida and included a grower fertilization program at each location of 170, 340, and 470 lb/acre N, respectively. Half of the applied N and K fertilizers was banded along the plant row at plant emergence and the remaining N and K were divided in equal band side-dressings applied at 4 and 8 inch plant height stages.

Applied N had no effect on yield response for sweet corn grown on rockland soil in 1994. The average yield with N treatments from zero to 200 lb/acre was 227 crates/acre. Yield with the grower N rate of 170 lb/acre was 232 crates/acre. Nitrogen concentrations measured in the whole plant at the 6-leaf stage increased linearly within the sufficiency range of 3.0 to 4.0%. Leaf-tissue N concentrations sampled at the 30-inch plant height and at full silk were above sufficiency ranges for treatments where no N was applied. At these sample dates, leaf-tissue N concentrations did not change with increased N. Sweet corn ear quality characteristics; uniformity, diameter, length, and the amount of blank ear tips were similarly unaffected by N rate treatments.

Sweet corn yields responded linearly to applied N in field tests on rockland soils at site 1 in 1995. Highest, 100% RY, occurred with 150 lb/acre N (205 crates/acre) while yields with the grower treatment of 340 lb/acre N were 180 crates/acre. Overall yields were lower this season due to wind damage compared to an average area yield of 288 crates/acre (Fla. Dept. of Agric. and Cons. Serv., 1997). Parallel linear increases occurred in leaf-tissue N concentrations at the 10-week sample date. Nitrogen concentrations from the leaf tissue of unfertilized plants taken 6 and 10 weeks from planting were 3.4% and 3.1%, respectively. These leaf- tissue N concentrations were well above the minimum concentrations of 2.5% and 1.5% N for each sample date and researchers questioned the published sufficiency ranges (Hochmuth et al., 1991). Increased N fertilization from 0 to 200 lb/acre resulted in a linear increase in corn ear diameter from 1.4 to 1.6 inches.

Sweet corn planted on marl soils at site 2, grower 2 (1995) resulted in a quadratic yield response to applied N. Yields leveled off above 150 lb/acre N (285 crates/acre, 97% RY) while yields with the grower fertilization treatment of 470 lb/acre N resulted in 290 crates/acre. Leaf-tissue N concentrations at the 8-week sampling were at the lower end of the adequate range and increased linearly with fertilization through 200 lb/acre N. At the 11-week sample date, leaf-tissue N concentrations leveled off above sufficiency concentrations with 150 lb/acre N and researchers suggested sufficiency leaf N concentrations of 3.0 to 3.2% were more accurate for this sample date than the published 2.5%. All ear quality factors were affected by N fertilization this season with quadratic increases in ear diameter to 1.8 inches and ear length to 7.2 inches with 150 lb/acre N. The amount of “blank ear tip” decreased linearly from 1.6 inches/ear with 0 lb/acre N to 0.8 inches/ear with 150 and 200 lb/acre N. The number of 3-inch flags/ear increased linearly from 2.5/ear to 6.2/ear with 0 to 200 lb/acre N, respectively.

Based on the above experiments, optimum yields for sweet corn grown on marl or rockland soils occurred with 150 lb/acre N with no yield advantage from grower N rates of 170, 340, or 470 lb/acre. Tissue N concentrations associated with high yields were 5.0% from 8-inch whole plants and 4.0% and 3.0% from whole leaf samples of 30-inch plants and plants at full silk, respectively. Nitrogen was a factor in yield increases in two of the three experiments and a factor in improved ear quality at site 2 in 1995.

Additional winter sweet corn experiments near Homestead were conducted with growers 1 and 2 in the 1995-1996 season. These yield results were presented in a subsequent unpublished paper (Hochmuth et al., 1998). Soils were rockland (grower 1) and marl (grower 2). Both experiments received overhead irrigation. Fertilizer practices were the same as in the 1993-1994 experimental season. Yield responses were quadratic in both seasons leveling off above 100 lb/acre N (309 cartons/acre, 100% RY, and 349 cartons/acre, 97% RY, respectively). Nitrogen treatments ranged from 0 to 200 lb/acre. Similar yields resulted with the grower fertilization programs of 334 lb/acre N, grower 1, and 470 lb/acre N, grower 2. Ear uniformity (size, shape, and tipfill) was rated on a scale of 1 (low) to 5 (high). An optimum ear uniformity rating of 4 occurred with 0 and 150 lb/acre N in the experiment with grower 1 while ear uniformity was unaffected by increased N with the grower 2 experiment (3.5 average ear uniformity). Ear length and diameter increased linearly with N (grower 1) and increased quadratically (grower 2) leveling off above 50 lb/acre N. The length of the unfilled ear tips and the cumulative (5 ears) length of ear flag leaves decreased linearly with increased N in both experiments.

Field experiments with N fertility were conducted in a spring 1996 experiment on Myakka fine sand in Sanford, Central Florida Research and Education Center (White et al., 1996). On March 25, seeds of supersweet sweet corn cultivar XP-7' were planted and fertilized with one-third of the 0, 75, 150, and 225 lb/acre N treatment rates. Due to 4.23 inches of rainfall in early April, the crop was replanted and fertilized with the second one-third fertilizer application on April 12. Researchers expected the initial fertilizer application was lost to leaching. The remaining one-third of the N fertilizer was applied when plants were between 4 and 8 inches tall. No marketable corn ears were harvested from plants that received 0 lb/acre N, but yield increased significantly through 225 lb/acre N (326 crates/acre, 100% RY). Leaf-tissue N concentrations also increased above 3.0% at sampling dates 4, 5, and 7 weeks after planting with 150 and 225 lb/acre N. These concentrations were above the adequate range of 1.5% to 2.5% N. Ear quality characteristics were optimized with N treatments of 150 and 225 lb/acre for an optimum ear weight of 0.7 lb, ear length of 7.1 inches, and ear width of 1.8 inches. With these N rates, ear kernel-fill was complete and ears had tight and complete husk coverings.

Growth enhancers were evaluated for their effect on sweet corn yield in two Gainesville experiments in the spring of 1993 (Hochmuth, 1994). Nitrogen rates were applied factorially with other treatments in experiment one, but rates of mixed N and K fertilizers prohibited evaluation of yield response to N in experiment two. Sweet corn was planted in double rows 15 inches apart in beds 24 inches wide with fertilizers and growth enhancers banded 3 inches deep in the bed center. Factorial applications of N at 75 or 150 lb/acre were banded 30% at planting and 70% at the five-leaf stage in experiment one. Growth enhancers, Agronomix (vitamin) in experiment one and GR-1 in experiment two, were applied 50% at planting and 50% side-dressed at 0 or 5 lb/acre Agronomix and 0, 1.5, and 4.5 lb/acre GR-1. Rates of fertilizer and growth enhancer were calculated on an average 30-inch row spacing (17,424 linear row feet/acre) and overhead irrigation was applied to maintain tensiometer readings of -10 centibars.

Sweet corn yield, ear quality criteria, and leaf N concentrations were unchanged by Agronomix or GR-1 in experiments one and two. No interactions occurred among factors with respect to yield or ear quality in either experiment one or two. Optimum yield in experiment one resulted with 150 lb/acre N (367 crates/acre, 100% RY), significantly different at 5% probability from yield with 75 lb/acre N, 65% RY. Yields in experiment two were optimized with the combined 150 lb/acre N, 100 lb/acre K₂O (342 crates/acre, 100% RY) compared to 73% RY with 75 lb/acre N and 50 lb/acre K₂O. In experiment one, more fancy, more No. 1 grade, and fewer cull ears were harvested with the 150 lb/acre N treatment compared to the lower N rate. Ear quality characteristics, size, tip fill, and uniformity were also significantly improved with higher fertilization rate, compared to ears harvested from plants with the 75 lb/acre N treatment. Leaf-tissue N concentrations measured at the early silk stage were adequate in both experiments with both N rates, but increased significantly (1% probability) with increased N in experiment 1.

Experiments with growth enhancers continued in Gainesville, spring 1997, where researchers tested the hypothesis of higher yield results with lower N rates following application of Grow-Plex SP humate (Earthgreen Products Inc., Dallas, TX) (Hochmuth, 1997a). Grow-Plex humate was suspended in water with Earthgreen Synfactant and sprayed in the seed furrow at 0, 1, and 2 lb/acre humate. Nitrogen rates were 0, 25, 50, 75, and 100% of the recommended 150 lb/acre N or 0, 40, 75, 115, and 150 lb/acre N. Fertilizers were incorporated preplant at 20 lb/acre N and the remainder was banded in a center furrow when plants were 2 and 6 inches tall. Early plant vigor ratings, 1 for yellow and 5 for dark green, were made when sweet corn plants were at the 2 and 4 leaf stages. Overhead irrigation was applied to maintain soil moisture at -10 centibars using tensiometers.

Nitrogen and humate did not interact in their effects on sweet corn yields. Plant vigor ratings were not affected by increased N or increased humate, but yield increased significantly (1% probability) with added N to 150 lb/acre (454 crates/acre, 100% RY) and with added humate to 1 lb/acre (5% probability).

Gainesville experiments with Growplex humate were repeated in the fall, 1997, with additional experiments testing foliar humate applications (Hochmuth, 1997b). Growplex humate was applied in the seed furrow at 0, 1, or 2 lb/acre as a solution of water and product: Earthgreen Synfactant was not used in the solution. Nitrogen treatments of 0, 40, 75, 115, and 150 lb/acre N were applied factorially with the three humate treatments for 15 total treatments. Preplant application of NH₄NO₃ - N and K₂SO₄ - K were incorporated at 20 lb/acre N and K₂O except in plots that received no N. Side-dress N and K applications were made between the two plant-rows/bed when plants were thinned and again when plants were 8 inches tall. Additional treatments consisted of foliar Grow-Plex SP sprayed on the plants at 4 oz/acre in 30 gallons of water to plants in separate plots fertilized with 115 lb/acre N and each of the three furrow-applied humate treatments. Foliar sprays were made when the sweet corn was 12 and 24 inches tall. Overhead irrigation was used to maintain soil moisture at -8 to -10 centibars.

Greatest average yields (1% probability) resulted where plants received 150 lb/acre N, 291 crates/acre (100% RY) as opposed to 87% RY where plants received 115 lb/acre N. Yields were not generally affected by Growplex humate except in one of the 15, N rate x humate, treatments where plants fertilized with 115 lb/acre N and 1 lb/acre Growplex humate resulted in yields near those of plants fertilized with 150 lb/acre N alone, 298 crates compared with 319 crates/acre, respectively. Based on these results, researchers indicated high yields may be possible at lower N rates with furrow-applied humate. Significant yield differences (1% probability) resulted with foliar-humate applications, though yields of 177 crates/acre with two foliar humate applications, 2 lb/acre humate applied in the furrow, and 115 lb/acre N, were lower than yields with 150 lb/acre N and no humate, 319 crates/acre. Yields were lower overall with the foliar applied humate compared to plants that received humate in the furrow with 115 lb/acre N. Plant vigor was unaffected by furrow-applied humate but, was significantly (1% probability) improved with increased N.

Nitrogen Summary

Nitrogen research summarized here applies to the state s sweet corn production that occurs on the mineral soils of the north, west, southwest, and central regions of Florida. Of the fifteen summarized experiments, 14 resulted in optimum yields with N rates at or below the recommended 150 lb/acre N, as indicated by the dashed line in Fig. 1. Five of these experiments, however, did not evaluate N rates above 150 lb/acre. Additional studies are needed to evaluate yield responses to N rates above 150 lb/acre. The remaining 50% of sweet corn production occurred on organic soils in the Everglades, and this research was summarized in a review of fertilization practices on organic soils (Hochmuth et al., 1996).

Plants fertilized with 170, 340, or 470 lb/acre N on marl and rockland soils resulted in yields equivalent to those fertilized with 150 lb/acre. Split N application increased yield 14% in a 1962 experiment compared to yields from plants fertilized in a single application. The remaining experiments were fertilized with the split method, recommended for unmulched crops where leaching and fertilizer burn might occur with the single application method. Nitrogen recovery was improved when fertilizer was banded in the root zone to one side or to both sides of the plant row. Yield responses from plants fertilized with NH₄NO₃ or (NH₄)₂SO₄ were generally quadratic and leveled off above 100 lb/acre N. Yield responses as a function of N source and rate are presented graphically in Fig. 2. Quadratic responses to increased N also were found in leaf-tissue N concentrations and ear quality characteristics, ear length and diameter, with peak responses between 100 and 150 lb/acre N. In some experiments, the length of ear blank-tip decreased with N rates from 0 to 150 lb/acre, yield of cull ears decreased, and yield of fancy and No. 1 grade ears increased with 150 lb/acre N compared to yields with lower N treatments. Treatments added to inhibit nitrification or enhance growth with vitamin application (Agronomix or GR-1) were generally not effective.

Phosphorus and Potassium Soil Testing

Knowledge of soil nutrient levels, particularly P and K, before planting is the starting point to predicting sweet corn response to varying rates of applied nutrient. Mehlich-1 (M-1) soil extractant is used on mineral soils to determine preplant soil nutrient concentrations and provide information so research results may be reviewed for degree of support of existing recommendations established by M-1.

Mehlich-1 extractant indices (expressed as ppm soil-extracted nutrient) are classified as very low, low, medium, high, and very high, and a crop specific fertilizer recommendation is made from that classification (Hochmuth et al., 1995). The M-1 solution became the accepted extractant standard in 1979 at the University of Florida. Prior to M-1, ammonium acetate and water extractants were used. Indices recorded from these methods cannot be directly equated with M-1 indices or fertilizer recommendation rates but the review of these studies presents a profile of sweet corn response to fertilizer under varying conditions. Water management practices, fertilizer sources, and application methods are also summarized.

Phosphorus

Experimentation with phosphorus (P) fertilization of sweet corn was limited to work in Homestead, Florida. Phosphorus was applied on rockland soils in 1994 and on rockland and marl soils in 1995 (Hochmuth et al., 1995; Hochmuth et al., 1998). No soil test has been calibrated for the calcareous soils in south Florida, soil P was extracted using the ammonium bicarbonate-diethylenetriamine pentacetic acid extractant (AB-DTPA), the Mehlich-3 extractant (M-3), and the water extractant procedures for the purpose of comparison. Results from the water extractant analysis proved the most variable and therefore least reliable of the extractants used. Based on data from AB-DTPA and M-3 extractants, no yield response was expected with P or K fertilization. Plants fertilized with 0, 50, 100, 150, or 200 lb/acre P₂O₅ (banded at planting) produced similar yields, ear quality, and leaf nutrient concentrations (above adequate) in all three experiments. Yields averaged over all P treatments resulted in 237, 190, and 274 cartons/acre each respective experiment. Both the AB-DTPA and M-3 extractants accurately predicted sufficient soil P concentrations to support high yield, but researchers questioned the use of M-3 due to potential reaction with high soil carbonate concentrations. The AB-DTPA soil-extracted P concentrations at these sites were 70, 77, and 75 ppm, respectively. Experimentation is needed on calcareous soils with varying P concentrations to calibrate the AB-DTPA extractant at lower concentrations of soil P.

Two additional Homestead experiments were conducted in 1995-1996 with banded P and

overhead irrigation (Hochmuth et al., 1998). Soil P concentrations extracted with AB-DTPA were 547 ppm (grower 1) and 317 ppm (grower 2. As with the above experiments, sweet corn yields did not respond to P application rates of 0, 50, 100, 150, and 200 lb/acre P₂O₅. Average yields for each season were 290 and 332 cartons/acre. Ear quality factors were also not affected by increased rates of P fertilizer. Results are graphed in Fig. 3.

Figure 3. 

Relative yield of sweet corn for experiments, years, and seasons as a function of added P₂O5.

Potassium

Experimentation with phosphorus (P) fertilization of sweet corn was limited to work in Homestead, Florida. Phosphorus was applied on rockland soils in 1994 and on rockland and marl soils in 1995 (Hochmuth et al., 1995; Hochmuth et al., 1998). No soil test has been calibrated for the calcareous soils in south Florida, soil P was extracted using the ammonium bicarbonate-diethylenetriamine pentacetic acid extractant (AB-DTPA), the Mehlich-3 extractant (M-3), and the water extractant procedures for the purpose of comparison. Results from the water extractant analysis proved the most variable and therefore least reliable of the extractants used. Based on data from AB-DTPA and M-3 extractants, no yield response was expected with P or K fertilization. Plants fertilized with 0, 50, 100, 150, or 200 lb/acre P₂O₅ (banded at planting) produced similar yields, ear quality, and leaf nutrient concentrations (above adequate) in all three experiments. Yields averaged over all P treatments resulted in 237, 190, and 274 cartons/acre each respective experiment. Both the AB-DTPA and M-3 extractants accurately predicted sufficient soil P concentrations to support high yield, but researchers questioned the use of M-3 due to potential reaction with high soil carbonate concentrations. The AB-DTPA soil-extracted P concentrations at these sites were 70, 77, and 75 ppm, respectively. Experimentation is needed on calcareous soils with varying P concentrations to calibrate the AB-DTPA extractant at lower concentrations of soil P.

Two additional Homestead experiments were conducted in 1995-1996 with banded P and

overhead irrigation (Hochmuth et al., 1998). Soil P concentrations extracted with AB-DTPA were 547 ppm (grower 1) and 317 ppm (grower 2. As with the above experiments, sweet corn yields did not respond to P application rates of 0, 50, 100, 150, and 200 lb/acre P₂O₅. Average yields for each season were 290 and 332 cartons/acre. Ear quality factors were also not affected by increased rates of P fertilizer.

Potassium Summary

Sweet corn yields responded to K rates below the M-1 soil test recommendation in Gainesville and Sanford experiments on soils low in M-1 soil-extracted K. Yields were optimized with half to nearly half, 50 and 60 lb/acre K₂O, of the recommended 100 lb/acre K₂O in each experiment. Certain ear quality characteristics were also optimized with the minimum K treatment, these were ear length and weight in the Gainesville experiment and husk cover, tip fill, length, and width in the Sanford experiment. No yield advantage resulted from increased rates of applied K. Yields leveled off in Gainesville and decreased in Sanford with K rates above 50 and 60 lb/acre, respectively. Yield responses are presented graphically in Fig. 4 where the dashed line indicates the maximum recommended K rate of 120 lb/acre for soils very low in M-1 soil-extracted K. Additional experiments are needed to evaluate the K needs and efficiency of this crop in extracting soil K. Further research is also needed on the calcareous soils of south Florida to calibrate AB-DTPA soil extractant with sweet corn yields.

Figure 4. 

Relative yield of sweet corn for experiments, years, and seasons as a function of added K₂0.

Overall Summary

Most fertilization research for sweet corn has been done for N. Most of this work supports an N recommendation of 150 to 200 lb/acre. Since sweet corn is grown without polyethylene mulch, careful management of N fertilization is critical to minimize leaching losses of N. More research is needed to study the relationship of N management to N leaching. Very little research with P and K fertilization of sweet corn has been conducted.

Literature Cited

Florida Agr. Statistics Serv. 1995. Vegetable Chemical Use. 8 pp. Fla. Agric. Stat. Serv., Orlando, FL.

Florida Dept. of Agriculture and Consumer Services. 1998. Florida Agricultural Statistics - Vegetable Summary 1996 - 1997. 70 pp. Fla. Agric. Stat. Serv., Orlando, FL.

Hochmuth, G., D. Maynard, C. Vavrina, and E. Hanlon. 1991. Plant tissue analysis and interpretation for vegetable crops in Florida. Fla. Coop. Ext. Serv. Spec. Ser. SS-VEC-42.

Hochmuth, G., B. Hochmuth, and M. Donley. 1992. Nitrogen fertilization of sweet corn on a sandy soil in northern Florida. Fla. Agr. Expt. Sta. Res. Rep. Suwannee Valley AREC 92-.

Hochmuth, G. 1994. Sweet corn response to N and K fertilization and to vitamins as growth enhancers. Fla. Agr. Expt. Sta. Res. Rep. Suwannee Valley AREC 94-01.

Hochmuth, G. J., and E. A. Hanlon. 1995. IFAS Standardized fertilization recommendations for vegetable crops. Fla. Coop. Ext. Serv. Circ. 1152.

Hochmuth, G. J., E. Hanlon, S. O Hair, J. Carranza, and M. Lamberts. 1995. On-farm evaluations of Univesity of Florida N, P, and K recommendations for sweet corn on rockdale and marl soils. Proc. Fla. State Hort. Soc. 108:184-192.

Hochmuth, G., E. Hanlon, G. Snyder, R. Nagata, and T. Schueneman. 1996. Fertilization of sweet corn, celery, romaine, escarole, endive, and radish on organic soils in Florida. Fla. Coop. Ext. Serv. Bull. 313.

Hochmuth, G. 1997a. Response of sweet corn and snapbean to Growplex Humate. Fla. Agr. Expt. Sta. Res. Rep. Suwannee Valley AREC 97-7.

Hochmuth, G. 1997b. Snapbean and sweet corn response to N rate and furrow-placed Growplex humate. Fla. Agr. Expt. Sta. Res. Rep. Suwannee Valley AREC 97-21.

Hochmuth, G. J., E. Hanlon, S. O Hair, J. Carranza, and M. Lamberts. 1998. Report to South Florida Water Management District. Unpublished.

Kidder, G., E. A. Hanlon, and G. J. Hochmuth. 1989. IFAS standardized fertilization recommendations for vegetable crops. Fla. Coop. Ext. Serv. Spec. Ser. SS-SOS-907.

Rhoads, F. M. 1990. Sweet corn research in north Florida. Fla. Agri. Expt. Sta. Research Report, Quincy. (Vegetable Field Day).

Robertson, W. K. 1962. A study of sources, rates and methods of application of nitrogen on sweet corn. Proc. Fla. State Hort. Soc. 75:249-253.

Rudert, B. D., and S. J. Locascio. 1979a. Growth and tissue composition of sweet corn as affected by N source, nitrapyrin, and season. J. Amer. Soc. Hort. Sci. 104:520-523.

Rudert, B. D. and S. J. Locascio. 1979b. Differential mobility of nitrapyrin and ammonium in a sandy soil and its effect on nitrapyrin efficiency. Agronomy Journal. 71:487-489.

Smith, S. A., and T. G. Taylor. 1996. 1995-96 Production cost for selected vegetables in Florida. Fla. Coop. Ext. Serv. Circ. 1176. 65 pp.

Volk, G. M. 1962. Comparison of covered to non-covered side dressing of urea, ammonium nitrate, uran, feran and calcium nitrate on sweet corn. Proc. Fla. State Hort. Soc. 75:170-175.

White, J. M., R.V. Tyson, E. A. Hanlon, G. J. Hochmuth, and C. A. Neal. 1996. Plant petiole sap testing for nitrogen and potassium in sweet corn grown on mineral soil. Proc. Fla. State Hort. Soc. 109:149-151.

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U.S. Department of Agriculture, Cooperative Extension Service, University of Florida, IFAS, Florida A. & M. University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Millie Ferrer-Chancy, Interim Dean.

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