Sweet corn (vegetable crop management)
Main Genetic Types
Normal sugary (su) corn is the standard corn grown for processing and the fresh market. The seed germinates well at 55°-60°F. Varieties in this category are characterized as having great corn flavor and a creamy texture. Because sugar can rapidly convert to starch, postharvest storage is not recommended.
Sugary enhanced (se) corn results in slightly increased sugar levels and slower conversion of sugars to starch after harvest. Kernels are very tender with good "corn" flavor and often the consumer pick at the farmers' market and roadside stands. Seed germinates well at temperatures of 55°-60°F.
Supersweet or shrunken-2 (sh2) corn produces kernels with a very high sugar content compared to the normal (su) varieties. Texture is crispy rather than creamy as with the normal and sugary enhanced varieties. Fresh market shelf life is extended due to the ability of the kernels to retain moisture and sweetness for long periods of time. This trait makes these varieties great candidates for long distance shipping for the fresh market industry. Seeds are smaller, lighter in weight, and shrunken in appearance (giving the genetic type the name "shrunken"). The seed's reduced food reserves and delicate seed coat require that they not be planted until soil temperatures reach at least 60°F at the 2" soil depth.
Newer Genetic Types
Synergistic (su) corn varieties actually combine the desirable attributes of both the normal sugary and supersweet varieties. Each ear contains kernels that are 75% su and 25% sh2 giving the corn a "sugar spike" while maintaining overall tenderness and creamy texture. These types of varieties are intended for local distribution rather than long distance shipping.
Augmented (sh2) corn truly represents the next generation of combining desirable traits to achieve a new kind of sweet corn. While being a true supersweet type, kernels are also very tender. This has been a criticism of some of the earlier sh2 varieties because of their relatively thick seed coat. Although these might be considered the new and improved supersweet varieties by many, planting should only occur when soil temperatures reach at least 60°F at the 2" soil depth.
As new varieties continue to be introduced into the market, the best recommendation for variety selection is to consult your seed company representatives and variety trial results conducted at universities for the variety that will best fit your need.
Each ounce contains approximately 120-180 sweet corn seeds. About 10-15 pounds are used per acre. Seed sizing has been widely used in the seed industry to improve uniformity of emergence and stand establishment. In general, flat seed performs better than round seed and heavier seed performs better than lighter seed.
Large differences in seed vigor may occur between varieties, particularly under cool, wet, or compacted soil conditions. The difference in performance is not apparent from germination information on the identification tags because germination tests are frequently conducted under ideal conditions. Personal experience with a particular variety is often the only way to know its relative potential and limitations.
Use only seed that has been treated with appropriate fungicides and insecticides for maximum germination and stand establishment. Seed companies are now offering supersweet varieties that are specially coated with a polymer after being treated with pesticides. The polymer coating reduces user exposure and reduces the amount of pesticide used.
Seeding and Spacing
For early fresh market sugary varieties (su), seeding may start as soon as soil temperatures reach 50°F and after the danger of spring frost has passed. Use 10-15 pounds/acre of seed, depending on the variety and seed size. Seeding at a depth of 1 to 2 inches is generally satisfactory. A more shallow planting (½ inch) and maintenance of high soil moisture are recommended where head smut may be a problem and for supersweet (sh2) types. For processing, recommended stands are 26,000 to 27,000/A if irrigated or 20,000 to 24,000/A if not irrigated while for fresh market, where large ear size and good husk color are important, stands should be between 20,000 and 25,000 per acre.
For a rough planting schedule that will provide about 10-14 days between mid-season peak harvests between plantings, wait until most of the plants in the previous planting have three leaves before planting again.
Plant fast-growing, small-stature varieties in rows approximately 30 inches apart, with 6-8 inches between plants. Grow vigorous, tall-growing varieties in rows 30-36 inches apart, with 9-12 inches between plants. Plant processing varieties according to the row spacing and rate recommended by the processor.
Isolation is necessary for two reasons: color and kernel quality (sugars and texture). Two ways to achieve isolation are distance and time. If effective isolation is not achieved, significant outcrossing can occur, which can render a significant amount of product unsalable (up to 10 adjacent rows of each variety!). Color contamination when growing white varieties is a special concern as pollen from yellow or bi-color varieties will cause yellow kernels to form in white varieties.By Distance
- Locate the variety most affected by outcrossing upwind from other varieties.
- Plant varieties with the same genetic type together in a block -- at least 250 feet away from other genetic types.
- Since colored kernels in white varieties are very obvious, use an isolation distance of 500 feet or more between white and colored varieties.
If the two- to three-week pollination time difference is to be used as a means of isolation between genetic types, and plantings of different genetic types are adjacent, several things need to be considered:
- The later planting must not be planted based on calendar day difference but rather on growth stage or heat units (using a 50°F base). Obtain specifics regarding the variety from the individual seed company. The maturity difference between the two types of corn also needs to be figured into the planting date difference. To obtain an effective two- to three-week spread at pollination, the early planting must germinate uniformly. If germination is not uniform, late-germinating plants may cause problems.
A wide variety of soils is suitable. It is important that the soil be well drained and well supplied with organic matter. The optimum pH range is 5.8 to 7.0.
The minimum soil temperature for germination is 50°F. Temperatures should reach 60°F for the supersweet and improved supersweet varieties because germination is drastically reduced under cooler soil conditions. Generally, sweet corn takes about 14 days to emerge from 50°F soils, but only about 5 days to emerge at 70°F. Soil temperature is a key factor to consider when scheduling plantings.
A soil test is the most accurate guide in determining fertilizer requirements. Follow recommended soil sampling to estimate fertilizer needs. Your Univeristy of Minnesota Extension educator can provide you with soil sampling instructions, soil sample bags, and information sheets.
Good management practices are essential if optimum fertilizer responses are to be realized. These practices include the use of recommended varieties, selection of adapted soils, weed control, disease and insect control, good seed bed preparation, proper seeding methods, and timely harvest.
Primary NutrientsNitrogen (N)
Eastern Minnesota and All Irrigated Sweet Corn
Because of the mobility of nitrate in soils and the complex transformations from organic matter, soil tests for N are not reliable for predicting N fertilizer needs in the eastern half of Minnesota, particularly on sandy irrigated soils. Therefore, N recommendations are based on yield goal, previous crop, and soil organic matter content. Refer to Nutrient Management for Commercial Fruit & Vegetable Crops in Minnesota (BU-5886), pg. 9, for more information and reference map. The soil nitrate test has not been calibrated for sweet corn in eastern Minnesota and is therefore not used for making N recommendations.
|Table 1. Nitrogen Recommendations|
|Previous Crop and Organic Mater (O.M.) Level|
in Group 22
|Yield Goal||low 4,5||medium
|tons/A||N to apply (lb/A)6|
|1Crops in Group 1||2Crops in group 2|
|alfalfa -- poor stand (less than 4 crowns
per square foot) -- or new seedling
alsike clover or red clover
- 3 Organic soil = more than 19%
- 4 Low O.M. = less than 3.1% medium to high = 3.1-19%
- 5 The well-drained silt loam soils in southeastern Minnesota with low O.M receive the same N recommendation for soil with a medium to high O.M. content. All irrigated soils are included in the low O.M. category.
- 6 For irrigated sandy soils, split N applications are recommended: 10-20 pounds N per acre in the starter and the remainder in one or two more applications at the 4-6 leaf stage and the 10-12 leaf stage.
Because of less rainfall, residual soil nitrate from previous crops can be measured and used to adjust the N recommendation in western Minnesota. The soil nitrate test is based on a 0-2 foot sample collected in the spring before planting. Adjusted N recommendations for sweet corn based on the soil nitrate test are provided in table 2.
|Table 2. Nitrogen Recommendations When Nitrate-N (0-2 ft.)|
|Soil Test Is Used (Western Minnesota)|
|Yield Goal (ton/A)||Soil Nitrate-N (0-2 ft.)
Plus Fertilizer N to Apply (lb/A)
For coarse-textured and irrigated soils, apply N in two to three applications. At planting, apply 10-20 pounds N per acre with P and K fertilizers as a band (refer to section on starter fertilizers). Apply additional N in two equal applications at the 4-6 leaf stage and again at the 10-12 leaf stage. For nonirrigated, fine-textured soils, timing of N application during the season is not as important. In wet years, broadcast application prior to planting is sufficient to meet crop needs, particularly if anhydrous ammonia or urea is used. Splitting N applications after planting works well provided that timely rainfall moves the applied N to the root zone.Nitrogen Source
From a practical standpoint, the source of N to use for sweet corn production should be based on cost. At equal N rates, N source has little impact on sweet corn provided the N fertilizer is applied properly and that differential nitrate leaching or ammonia volatilization losses do not occur. To prevent excessive nitrate leaching, avoid early application of nitrate-based fertilizer such as ammonium nitrate. If urea is used, be sure that the fertilizer is incorporated or irrigated in within 24 hours after application. Ammonia volatilization can be substantial from surface-applied urea. If anhydrous ammonia is used, do not sidedress too close to young seedlings as injury can result. The use of a nitrification inhibitor such as N-Serve (with early-applied anhydrous ammonia) is a best management practice to reduce nitrate leaching.Phosphorus (P)
Phosphorus requirements for sweet corn are based on soil test P level and yield goal. Banding of P at planning increases P use efficiency compared to broadcast application and substantially reduces the recommended P application rates.
|Table 3. Phosphate Recommendations|
|Soil Test P (ppm)|
|tons/A||P2O5 to Apply (lb/A)|
|less than 6||70/40||40/25||30/20||10/10-15||0/10-15|
|10 or more||100/40||70/40||40/25||20/10-15||0/10-15|
If soil pH is 7.4 or greater, use the Olsen P test to determine P needs. If soil pH is less than 7.4, use the Bray P-1 test. For low P testing soils, a combination of broadcast and band applications are suggested. For high P testing soils, only banded applications are needed. Testing P levels is particularly useful for early-planted sweet corn where soils may be cool and wet.Potassium (K)
Potassium requirements for sweet corn are based on soil test K level and yield goal. On low K testing soils, a combination of broadcast and banded applications of K is suggested. On high K testing soils, all K fertilizer can be banded.
|Table 4. Potassium Recommendations|
|Soil Test K (ppm)|
|tons/A||- K20 to Apply (lb/A) -|
|less than 6||120/40||60/30||40/10-15||40/10-15||0/10-15|
|10 or more||180/40||120/40||80/30||60/25||0/10-15|
Secondary NutrientsCalcium (Ca)
Most soils in Minnesota are well supplied with Ca. In acid soils where Ca levels may be low, Ca requirements are met through a proper liming program. On acid soils where sweet corn is in rotation with potatoes and liming is not practiced, some supplemental Ca may be needed. Soil test Ca levels less than 300 ppm are considered low. In this situation, low rates (1,000 pounds per acre) of a fine lime should be applied during the year that potatoes are not grown.Magnesium (Mg)
As with Ca, most soils in Minnesota are well supplied with Mg. In acid soils, where Mg levels may be low, Mg requirements are met through a liming program using dolomitic limestone. If liming is not used in the rotation, for example with potato production, some supplemental Mg may be needed. Mg needs based on a soil test are provided in table 5.
|Table 5. Magnesium Recommendations|
|Magnesium||Relative||Magnesium to Apply|
Common sources of Mg fertilizer include potassium-magnesium sulfate (10 percent Mg) and Epsom salts (10 percent Mg). Finely ground dolomitic limestone can also be used to supply Mg.Sulfur (S)
Sulfur may be needed for sweet corn production on sandy soils. Soil tests for S are only reliable for low organic matter soils. Needs for S based on a soil test are provided in table 6.
|Table 6. Sulfur Recommendations|
|Sulfur||Relative||Sulfur to Apply|
For sweet corn, the need for S occurs early in the season. Therefore, a banded application of S in the sulfate form is suggested.
Of all the micronutrients, Zn is the one most likely to be needed in a fertilizer program for sweet corn. The need for Zn fertilizer should be based on a soil test.
|Table 7. Zinc Recommendations|
|Zn Soil Test(ppm)||Relative Level||Zn to Apply(lb/A)|
If Zn is required, application of Zn in a starter fertilizer is suggested to meet crop needs. However, carryover to succeeding years will be better with broadcast applications. Several fertilizer products can be used to supply Zn. Except for large particles of Zn oxides, all are equally effective. Base product selection on cost.Boron (B)
No consistent responses of sweet corn to B application have been observed in Minnesota. Where the soil test value for B is below 0.25 ppm, a trial broadcast application of two pounds B per acre is suggested.
Too much B fertilizer can be highly toxic; do not exceed suggested application rates. Fertilizer containing B should not be in contact with the seed.
Low rates of B (1/8 lb per acre) can be used in a starter fertilizer if the fertilizer is placed two to three inches below and two to three inches to the side of the seed.Copper, Manganese, and Iron
Sweet corn responses to copper, manganese, and iron have not been observed in Minnesota and are therefore not recommended in a sweet corn fertilizer program.Lime
The optimum pH for sweet corn production is 5.8-7. Lime is generally recommended if soil pH is less than 5.8. The need for lime can be determined from a routine soil test. Lime should be applied before planting and incorporated to a depth of six inches.
For further information on lime application rates, lime sources, and reference map, refer to Minnesota Extension bulletin, Nutrient Management for Commercial Fruit & Vegetable Crops in Minnesota (BU-5886), pgs. 2-4.
The amount of plant nutrients in a fertilizer program can be reduced if manure is used. The nutrient content of manure varies with type of livestock and methods used in storage, handling, and application. Many laboratories will measure the nutrient concentrations in manure. Manure analysis is strongly recommended if routine applications are made for crop production. The results of such an analysis will give a more precise measurement of the nutrient value of manure.
Using Starter Fertilizer
The use of a starter fertilizer at planting is an excellent management tool for corn production in Minnesota especially when soil conditions are cold and wet at planting. Yield increases are not always achieved with starter fertilizer, but this management practice is good insurance. For sweet corn, fertilizer should be banded two to three inches to the side and two to three inches below the seed. Banding fertilizer too close to the seed or at too high a rate can cause seedling injury due to high salt concentrations. In general, the N plus K2O application in a band should not exceed 90 pounds of nutrients per acre. Urea or diammonium phosphate forms of N may cause seedling injury if banded too close to the seed at planting, especially where the soil pH exceeds 7. In addition, urea, ammonium thiosulfate or fertilizer containing boron can cause significant injury if placed in contact with the seed.
Tissue analysis can be used during the growing season to monitor the nutrient status of the plant and help diagnose nutritional problems. Table 8 shows sufficiency levels in whole plant samples taken when plants are 12 inches high and in ear leaf samples taken during silking.
|Table 8. Sufficiency Levels|
|Stage of Growth||Plant Part Sampled||%||ppm|
|12" height||Whole plant||3.5||0.6||3.0||2.5||0.3||0.2||60||50||7||20||50||0.3|
Water management of irrigated sweet corn is an essential production practice to produce optimum yields and quality ears. Yield and quality of ears are significantly reduced if moisture stress occurs during tasseling, silking, and kernel fill. Short periods of moisture stress earlier in crop development usually will not affect yield unless poor germination occurs. However, it may delay harvest date. To manage the soil's moisture effectively, a regular in-field soil/water monitoring program should be established to assist in irrigation scheduling.
Establishment of an effective irrigation scheduling program involves being knowledgeable of several factors and then putting them into practice. The following discussion briefly outlines those factors.
Crop Water Use
Water use by the crop is referred to as evapotranspiration (ET). Sweet corn will use four to six inches less total water than field corn in a season, but generally will use similar daily amounts for the same conditions. Daily ET is dependent on several factors such as stage of growth, air temperature, and solar radiation. Table 9, taken from the University of Minnesota Extension bulletin Irrigation Scheduling (FO-1322), gives estimated daily crop water use in inches for different corn growth stages and several maximum daily air temperature ranges.
|Table 9. Estimated Daily Crop Water Use|
|Week After Emergence (growth stage)|
|(4 Leaf)||(12 Leaf)||(Tassel)||(Pollination)||(Milk)|
|Air Temperature (°F)||--inches of ET per day--|
The greatest daily water use will usually occur from tassel to harvest. It is not uncommon for sweet corn to use .25 inch per day or more for several days. To keep up with this use, your irrigation system must have a similar capability. For sandy soils, a system capacity of .25 inch or greater per day is generally recommended. The following table gives the system gallon per minute (GPM) capacity needed for each acre, irrigated for various pumping periods per day and crop water use rates. To determine the total system capacity needed to adequately meet the crop water needs for a given system, simply find the appropriate capacity and multiply it by your total corn acres (table 10).
|Table 10. System Gallon Per Minute (GPM) Capacity|
|GPM/ACRE at 75% Application Efficiency|
|Crop Water Use (inches/day)||12 hr/day||18 hr/day||24 hr/day|
Sweet corn has a relatively shallow rooting depth compared to field corn. Most literature suggests that under irrigation, only soil moisture in the top 18 to 24 inches of the rooting zone should be managed. If the soil is shallower or a restrictive layer prevents root growth, the root zone to be managed must be reduced.
Available Soil Water
Available soil water is the water stored in the soil that roots can extract. The amount of soil water available to a plant is dependent on both the rooting depth potential and the soil's available water-holding capacity (AWHC). The AWHC for a given soil is expressed in inches of water per inch of soil and is dependent on the soil's texture and organic matter. The AWHC for your fields can be estimated by SCS personnel or others having access to area soil maps. To determine the total available water capacity in a field, simply take the soil's AWHC times its rooting depth potential. For example, a sandy loam textured having an AWHC of .12 inch per inch and a 24-inch rooting depth will store a total of 2.88 inches for the sweet corn plants.
Knowing the amount of available water the soil is storing at any time is very important. It governs the amount of water that can effectively be applied at each irrigation without over-irrigating. Several tools are available to assist in monitoring the available water. A discussion of the typical monitoring methods can be found in the University of Minnesota Extension bulletins Irrigation Water Management Considerations for Sandy Soil in Minnesota (FO-3875) and Irrigation Scheduling--Checkbook Method (FO-1322).
Generally, optimum sweet corn growth will occur if the soil moisture level is maintained at about 85 percent of the available water capacity, although symptoms of moisture stress usually do not appear until the soil moisture has been reduced to 40-50 percent of capacity. Cycles of extreme wet versus dry conditions should be avoided as it may reduce ear quality. Sweet corn is most sensitive to moisture stress during pollination and ear formation.
Irrigation start-up should generally occur when the soil moisture level is between a ¾-inch deficit and 60 percent of field capacity. During the critical growth stages, keep the soil moisture as high as possible at all times. At germination time it is also important to start out with a full profile, but then allow the soil to dry down to possibly 50-60 percent of field capacity up to the 10-11 leaf stage to ensure a rapid and complete root development. Timing of irrigation may also be influenced by other factors, such as nitrogen application, insecticide spraying, disease suppression, forecasted rain, and harvesting.
Harvesting and Handling
Optimum processing quality and returns for normal sweet and sugary-enhanced (su and se) varieties occur when kernel moisture is about 73 percent and 76 percent for supersweet (sh2) varieties. Kernel moisture drops approximately 0.5 percent per day in normal sweet and sugary-enhanced corn varieties with considerable variation depending on season and variety. Kernel moisture of supersweet (sh2) varieties changes at a considerably slower rate. Fresh market corn usually reaches peak maturity from 21 to 25 days after silking.
Self-propelled and tractor-pulled harvesters are available from several manufacturers. These come in single-row or multiple-row units of up to eight rows. For fresh market corn harvest, some of the harvesters have to be slightly modified so that they do not damage the butt portion of the ear. These modifications are generally made easily and are usually offered as options from the manufacturer.
Corn for processing is generally machine harvested in Minnesota between July 25 and October 10. Harvest begins when kernels of standard varieties reach 70-74 percent moisture. Corn should be protected from overheating (kept in shade) and delivered to the processor as soon as possible. Supersweet and sugary enhanced varieties (grown for fresh market) maintain their sugar content longer after harvest than standard varieties but still require immediate cooling and refrigerated transport and handling. They are usually harvested at 76-78 percent moisture.
Fresh Market Postharvest Handling and Storage
Trim shanks and flag leaves at harvest. Remove field heat by spraying or immersing in cold, potable water or ice. Precooling to 50° F or lower will slow deterioration. Sweet corn has a high respiration rate, which results in a high rate of heat evolution. Keeping sweet corn in bulk piles will result in heating of cobs as the corn respires, and should be avoided.
Sweet corn is highly perishable, and should not be stored for more than about 3-5 days. Optimal storage conditions for sweet corn are 32°F and 95 to 98 percent relative humidity. Ice can be used to keep sweet corn cool. Loss of sugar is about four times faster at 50°F as at 32°F. At 85°F, 60 percent of the sugars may be converted to starch in a single day as compared with only 6 percent at 32°F. Deteriorating sweet corn may show denting, which is due to moisture loss. A loss of 2 percent moisture from sweet corn may result in objectionable kernel denting.
Some of the new, high sugar sweet corn cultivars should lose sweetness less rapidly than older varieties. Even if sugar loss occurs at the same rate as with older varieties, because they have more sugar to start with, loss over time will lead to more sugar left after a few days of storage compared with standard cultivars containing only 2-3 percent sugar.
Corn is packed in waxed cardboard boxes for wholesale. For farmers markets, it is often transported to market in bulk piles. These piles should be kept iced and as cold as possible.