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FERTILIZATION
Gerald F. Peedin
Crop Science Extension Specialist - Tobacco
 
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The efficient use of nutrients, including those in the soil naturally or as a result of previous fertilization and management practices, can help reduce fertilizer costs and environmental concerns without reducing yield or quality. This requires a well-planned fertilization program based on soil testing, wise selection of nutrient sources based on needs and costs, and proper application. Overfertilization is expensive, wastes natural resources, and increases the potential for contamination of water resources.

Soil Testing

Soil testing is the first step in planning an economical and environmentally sound fertilization program and is provided as a free service by the Agronomic Division of the North Carolina Department of Agriculture and Consumer Services. It analyzes each soil sample to determine pH and the available levels of most major nutrients, such as phosphorus (P205), potassium (K2O), calcium (Ca), magnesium (Mg), and sulfur (S). It also determines soil levels of several micronutrients, such as manganese (Mn), copper (Cu), and zinc (Zn). The soil test report suggests application rates for lime and for each nutrient that should meet crop needs under good growing conditions.

The nutrient rates suggested on the soil test report reflect only what is found in the sample. Therefore, each sample should be taken properly so it adequately represents the field where the crop is to be grown. Take samples every three years (coastal plain) or four years (piedmont) from fields tended regularly by the same grower. Sample unfamiliar fields or those out of tobacco production for several years several months before the first tobacco crop. Submitting samples in the fall rather than winter or spring will enable you to receive soil test reports quickly and allow more time for planning fertilization programs. Soil boxes and instructions for taking samples can be obtained at your county Cooperative Extension center.

The information below, along with your experience and a soil test, will help determine reasonable application rates for most nutrients.

Primary Nutrients

Nitrogen (N)

Nitrogen has a greater effect on tobacco yield and quality than any other nutrient. Too little nitrogen reduces yield and results in pale, slick cured leaf. Too much nitrogen may increase yield slightly but may also make mechanical harvesting and curing more difficult, delay maturity, extend curing time, and result in more unripe cured leaf. Excessive nitrogen also stimulates sucker growth, which can lead to excessive use of maleic hydrazide (MH) and increase problems with hornworms and aphids. Nitrogen is also very leachable, and overapplication may contribute to groundwater contamination in deep, sandy soils.

Soil analysis is not used to estimate the nitrogen rate needed for a specific tobacco field in North Carolina. Rather, the 50- to 80-pound-per-acre range shown on the soil test report is based on information from numerous field tests conducted across the state. In these tests, a base nitrogen rate of 50 to 80 pounds per acre has given consistently good results on most soils in most seasons. This is the total amount of nitrogen supplied by normal applications of the N-P-K fertilizer and the sidedresser but does not include additional nitrogen sometimes needed for leaching adjustments. The lower portion of the range is suggested for fine-textured, fertile soils, especially where legumes such as soybeans or peanuts were grown the previous year. The higher portion of the range is suggested for coarse-textured soils with topsoils deeper than 15 inches to clay.

Suggested nitrogen rates for several average topsoil depths are shown in Table 5-1. Determine your portion of the nitrogen rate range primarily by topsoil depth, or depth to clay. Fields with deeper, sandier topsoils usually are more leachable and contain less nitrogen as humic matter than those with shallower, more heavily textured topsoils. Generally, you should reduce the nitrogen rates shown by about 5 to 10 pounds per acre if the previous crop was a legume or the variety to be planted is known to mature late or cure poorly when overfertilized with nitrogen. Even greater nitrogen rate reductions may be needed on dark soils with 1 percent or more humic matter. Also, when tobacco follows a heavily fertilized but poor corn crop (less than 75 bushels per acre), the residual nitrogen available for the tobacco may be as high as that left by soybeans or peanuts.

Only 15 pounds of extra nitrogen may reduce leaf quality, particularly in dry seasons. Both drought and excess nitrogen delay maturity and increase the amount of unripe tobacco. The first step to increasing the amount of ripe tobacco is to use a reasonable base nitrogen rate (particularly if irrigation is not available and mechanical harvesting is used), depending on topsoil depth, previous crop, variety to be grown, and experience. Also, be cautious and conservative with leaching adjustments for nitrogen. (See next section.) The second step is to delay harvest, if necessary, and make three or more primings so that each priming will have a high percentage of ripe leaves. The rate of ripening primarily depends on the amount and distribution of water, nitrogen rate, soil type, and variety, so base your harvest rate on these factors, not on the calendar date or how fast your neighbor's tobacco is being harvested.


Table 5-1. Base Nitrogen Rates for Tobacco in Relation to Topsoil Depth

Topsoil Depth
(inches)
Nitrogen Ratea
(lb/a)

5
10
15
20+
50
60
70
80

aDoes not include leaching adjustments.



The normal ripening process is caused by partial nitrogen starvation, which should begin about topping time. This means that nitrogen in the soil should be near depletion by that time. Overapplication of nitrogen and/or prolonged drought extends nitrogen uptake beyond topping time and therefore delays ripening because the crop is still absorbing nitrogen. Leaves harvested when they are high in nitrogen are more difficult to cure and often turn dark at the end of yellowing and into the leaf-drying stage. This problem is increased by dry, hot conditions, which cause the leaves to appear riper than they really are.

Leaching. Leaching is the movement of certain nutrients below normal rooting depth due to excessive water moving (percolating) through the root zone of deep, sandy soils. Leaching of nitrogen is more likely to reduce yield and quality than leaching of other nutrients. Although leaching losses of sulfur, magnesium, and potassium sometimes occur, their effects on yield and quality are relatively small.

More than 50 to 80 pounds of nitrogen per acre may be needed if leaching occurs, but determining the correct amount to replace is one of the most difficult and risky tasks in tobacco production. A general guide for making leaching adjustments for nitrogen is shown in Table 5-2. The amount of nitrogen to replace is expressed as a percentage of the suggested base rate that was applied before leaching occurred. If you used excess nitrogen before leaching occurred, subtract the number of excess pounds from the number of replacement pounds calculated. This guide is based on three major factors that influence the amount of leaching:

  1. topsoil depth to clay,
  2. the age of the crop when leaching occurs, and
  3. the estimated inches of water that move through the fertilized root zone.

Topsoil depth is used in the guide because water usually moves more freely and in larger quantities through deeper topsoils. Since the mass of tobacco roots normally occurs in the upper 12 to 14 inches of soil, the deeper the clay below rooting depth, the more likely nitrogen is to leach below the root mass.

Crop age is included in the guide because plants absorb more of the needed nutrients as they get older, and the amounts left in the soil and subject to leaching decrease as the crop grows. Too, as the plants get larger, their leaves form a canopy that sheds some of the water to the row middles, reducing the amount of water passing through the fertilized zone.

A reasonable estimate of the amount of water that enters the soil and ultimately percolates through the root zone is necessary for the adjustment procedure to be reliable. Amount of rainfall alone usually is not a good indication of how much leaching has occurred. Factors such as soil texture and slope, crust formation, duration of rainfall, and the amount of moisture already in the soil also are important.

Unfortunately, a practical method that includes these many percolation factors has not been developed, but growers who have experienced similar rainfall on their land in past years can make reasonable estimates. An invaluable tool in making leaching adjustments is an up-to-date record of daily rains and estimates of how much of each rain soaked into the soil.

Because phosphorus leaches very little in our soils, it is expensive to use phosphorus-containing fertilizers, such as 3-9-9 or 6-6-18, to make leaching adjustments. Some growers do this, however, to supply additional sulfur (S), magnesium (Mg), or both along with nitrogen for adjustments on deep, sandy soils. These nutrients can be supplied at lower cost by using 13-0-14 or an 8-0-24 that guarantees sulfur and magnesium, which is less expensive and just as effective as using any phosphorus-containing fertilizer for leaching adjustments. Another alternative is to mix equal amounts of Sul-Po-Mag (K-Mag) and one of the 1:0:0-ratio sidedressers. For example, an equal mixture of 16-0-0 fertilizer and Sul-Po-Mag gives an 8-0-11 N-P-K analysis, which also provides 5 percent magnesium and 11 percent sulfur. (If additional nitrogen is not needed, about 100 to 150 pounds of Sul-Po-Mag per acre usually will supply adequate sulfur and magnesium.)

Drowning. Distinguishing between drowning and leaching is often confusing because excess water causes both problems. Leaching is usually not a serious problem on soils that have clay within 10 to 12 inches of the surface because percolation through the root zone is restricted. If the soil becomes saturated, oxygen starvation and then root decay will begin unless the saturated condition is alleviated within about 24 hours. Usually, the plants yellow and partially or completely wilt. Wilting is a symptom of drowning and indicates that leaching losses are minimal because water remains in the root zone rather than moving through it. Although some nitrogen may be moved down to the clay, causing a temporary deficiency, it will be absorbed later as root growth resumes.


Table 5-2. Nitrogen Adjustments for Leaching

Topsoil Depth
(inches to clay)
Estimated Water
Percolated through Soil
(inches)
Weeks after Transplanting
1-3 4-5 6-7

(% of applied N to replace)a

Less than 10 1
2
3 or more
0
20
30
0
10
20
0
0
0

10 to 16 1
2
3 or more
30
45
60
20
30
40
0
10
15

17 or more 1
2
3 or more
50
75
100
25
35
45
15
20
25

aApply about 1 pound of potassium (K20) for each pound of nitrogen used as a leaching adjustment if the topsoil is deeper than 10 inches.



Adding 10 to 15 pounds of extra nitrogen in most drowning situations usually benefits the crop if it was not overfertilized with nitrogen before drowning. However, using the leaching adjustment procedure on a drowned crop often overestimates the amount of nitrogen to replace and results in overapplication.

Fertilizer Additions on Partially Drowned Tobacco. Heavy and frequent rains may cause drowning (i.e., root injury). Deep rooting is limited as long as the soil remains saturated, confining root development to the upper 6 to 10 inches. Many growers make at least one application of dry or liquid fertilizer following drowning in an attempt to reduce losses in yield and quality. Experiments were conducted on research stations near Kinston and Clayton in 1995 to evaluate the effects of soil-applied fertilizer materials on yield and quality of partially drowned tobacco; the term "partially drowned" is used because the tobacco remained wilted for only several days and then recovered. The fertilizers used are shown in Table 5-3; the results are averages of two nitrogen rates at Kinston (15 and 30 lb/a) and one nitrogen rate at Clayton (20 lb/a). All fertilizer treatments, made in one application on June 20, improved yield and value per acre compared to the nonfertilized control; the 16-0-0 and 30 percent liquid nitrogen fertilizers increased yield and value about 10 percent, while the 15-0-14 and 8-0-11 fertilizers increased yield and value about 15 percent. This indicates that the potassium supplied by the 15-0-14 and 8-0-11 fertilizers may have improved yield more than the 16-0-0 and 30 percent liquid nitrogen fertilizers that supplied only nitrogen. Compared to the control, none of the fertilizers improved grade index or average market price.

These same fertilizers were applied at two nitrogen rates at the Kinston station. The results in Table 5-4 indicate that using them at rates to provide 30 pounds of nitrogen per acre was no more effective than using them at rates to provide 15 pounds of nitrogen per acre. In addition, the nitrogen rate did not affect grade index or average market price. The plant roots in these tests never regained normal development and/or function following the extended drowned period. Therefore, the crops did not respond fully to the applied nutrients. Unfortunately, the results of these tests indicate that much of the extra fertilizers applied to drowned crops does not benefit them. Observations on farms in 1995 indicated that the more severe the drowning (i.e., root injury), the less likely the crops were to recover, regardless of the kinds or rates of fertilizers used.

Foliar spray treatments with 30 percent liquid nitrogen were also evaluated in these and other tests in 1995, but the results probably are not reliable due to frequent showers within several hours after application. In addition, several close observations later in the season did not reveal any greater visual response to the foliar spray treatments than to the fertilizers shown in Table 5-3.

Foliar Sprays to Supply Extra Nitrogen. When excessive rain falls between lay-by and topping, the crop may turn yellow when the plants are too tall to band-apply dry fertilizers with tractor-mounted applicators. Consequently, some growers consider applying foliar sprays or dry materials overtop to supply extra nitrogen. Research has shown foliar sprays on tobacco to be of questionable value because most contain so little nitrogen that several applications are needed to be beneficial (10 to 15 pounds per acre). Also, substantial leaf injury may occur on tender crops if high rates are used to reduce the number of applications. Other possible disadvantages for some foliar products include (1) the cost is high per unit of nutrient and (2) a major portion of the nitrogen may be in the ammonium form. Results from recent tests indicate that 16-0-0 fertilizer applied overtop or 30 percent liquid nitrogen applied as a wide band in the row middles is as or more effective than foliar sprays, and all of the needed nitrogen can be applied in one application without injuring leaves.


Table 5-3. Effects of Fertilizer Additions on Yield and Value of Partially Drowned Tobacco, 1995a

Fertilizer
Treatmenta
Applic.
Method
Yield
lb/a
Grade
Index
Price
$/Cwt
Value
$/a

None
---
1,714
77
173.5
2,974
16-0-0
BC-OT
1,887
77
174.6
3,294
30% N
WB-RM
1,873
79
175.5
3,288
15-0-14
BC-OT
1,961
76
173.8
3,408
8-0-11
BC-OT
1,996
77
174.5
3,483

aAverage results of tests conducted at research stations near Clayton and Kinston; N rates for each fertilizer were 15 and 30 lb/a at Kinston and 20 lb/a at Clayton. Adjustments were applied on 6/20/95; BC-OT = broadcast overtop of plants and WB-RM = wide band sprayed in row middle.



Table 5-4. Effects of Nitrogen Rate Adjustments on Yield and Value of Partially Drowned Tobacco, 1995

N Adj.
lb/a
Yield
lb/a
Grade
Index
Price
$/Cwt
Value
$/a

0
1,748
74
180.0
3,146
15a
1,946
74
179.3
3,489
30a
1,903
76
179.3
3,412

aResults averaged over 16-0-0, 30 percent liquid N, 15-0-14, and 8-0-11 fertilizers for each N rate. Test conducted at Lower Coastal Plain Research Station near Kinston.



Phosphorus (P205) and Potassium (K20)

Phosphorus is not very leachable, even in sandy soils, and a good tobacco crop only removes about 15 pounds per acre (as P205). However, many times this amount has been applied annually or biennially to most tobacco fields over the years, resulting in at least "high" levels of available phosphorus in about 85 percent of the fields used for tobacco.

Potassium is leachable, especially in deep, sandy soils, and a good crop removes about 90 pounds per acre (as K20). However, about 60 percent of our tobacco soils contain at least "high" levels of available potassium because of more abundant soil sources and excessive application. Also, subsoils in tobacco fields often contain substantial amounts of potassium and other leachable nutrients that are seldom measured by soil tests because only topsoils are usually sampled (Table 5-5).


Table 5-5. Average Soil Test Levels of Several Nutrients in Topsoils and Subsoils of 13 Flue-Cured Tobacco Fields, 1999-2000

Soil Horizon
Soil Nutrients

P K S Ca Mg


(Availability Index)a (% of CEC)
Topsoil
123 56 41 45 12.9
Subsoil
35 63 122 48 17.3

a 0-10 = very low; 11-25 = low; 26-50 = medium; 51-100 = high; 100+ = very high



These results represent primarily coastal plain soils and should be considered as preliminary at this point, but do provide additional evidence that application of several leachable nutrients above soil test recommendations usually does not improve tobacco yield and quality, but does increase production costs. In addition, overapplication increases the potential for these nutrients to reach our ponds and streams by soil and water movement.

Selecting the N-P-K Fertilizer

Growers with access to fertilizer blend plants can save money by having their N-P-K fertilizers blended according to specific soil test suggestions. However, standard-grade tobacco fertilizers are still predominantly used in North Carolina. Most contain three times more potassium than nitrogen but varying amounts of phosphorus relative to nitrogen and potassium. This is called the N:P:K ratio (Table 5-6). For example, analyses such as 6-6-18 and 8-8-24 are 1:1:3 ratios because they contain 1 pound of phosphorus and 3 pounds of potassium for each pound of nitrogen. An 8-0-24 analysis has a 1:0:3 ratio because it does not contain phosphorus. So the primary difference among ratios is that they supply different amounts of phosphorus for each pound of nitrogen applied. Therefore, if you select the proper ratio and application rate, you can obtain phosphorus rates very close to the soil test suggestion without altering the rates of nitrogen and potassium.

Generally, the N-P-K fertilizer should supply all of the suggested phosphorus, and possibly potassium, but no more than 40 pounds of nitrogen per acre; this is true for most soil types and rainfall conditions. Therefore, the amount of phosphorus suggested on the soil test report should determine which fertilizer ratio is selected from Table 5-6. For example, if a phosphorus rate between 0 and 40 pounds per acre is suggested, use 667 pounds of 6-6-18 or 500 pounds of 8-8-24 fertilizer per acre.


Table 5-6. Selection of Standard-Grade N-P-K Fertilizer Ratio, Analysis, and Rate Based on Soil Test Suggestions for Phosphorus

P205 Suggested on
Soil Test (lb/a)
N-P-K Fert. Selection
Fert. Costsa



Ratio Analysisb (lb/a)c
($/ton) ($/a)

Between 80 & 120 1:3:3 3-9-9
6-18-18
1,333
667

203
290
135
97
Between 40 & 80 1:2:3 6-12-18
8-16-24
667
500

240
276
80
69
Between 0 & 40 1:1:3 6-6-18
8-8-24
667
500

236
256
79
64
None 1:0:3 8-0-24 500
245 61

a Based on 2000 average costs of N-P-K fertilizers from surveys conducted by 21 county Extension service agents across North Carolina; prices are primarily for manufactured, bagged products.
b Analyses most commonly available; other suitable analyses of the appropriate ratio may be available in some areas.
c Each rate will supply 40 pounds of nitrogen and 120 pounds of potassium per acre and the highest rate of phosphorus listed to the left of each ratio.



The highest analysis available in a ratio will supply the nutrients at the lowest cost when used at the rates suggested in Table 5-6.

For some soils, particularly in the coastal plain, available phosphorus is so high that the soil test report does not suggest applying it. In these instances, applying 500 pounds of 8-0-24 fertilizer per acre will provide 40 pounds of nitrogen, no phosphorus, and 120 pounds of potassium per acre, plus some secondary nutrients. The zero phosphorus suggestion has caused some concern among growers. However, the results from many field tests conducted on soils with high to very high levels of soil test phosphorus do not reveal any reduction in yield and quality when 1:0:3-ratio fertilizers are used (i.e., when no phosphorus is applied). Remember, however, that phosphorus suggestions are made on the assumption that soil pH is maintained in the desirable 5.8 to 6.0 range. Therefore, failure to apply phosphorus on acid soils may reduce early growth and delay maturity, particularly when early root growth is reduced by unusually cool, wet conditions. These unfavorable soil and weather conditions are more likely to occur in the piedmont than the coastal plain.

Some growers use N-P-K fertilizers at rates higher than those shown in Table 5-6. This is not recommended because it increases costs and supplies excessive rates of phosphorus and potassium. Also, numerous field tests have shown that using the N-P-K fertilizer to supply more than 40 pounds of nitrogen per acre does not improve yield and quality (see Table 8 on page 41 of 1991 Tobacco Information). Additional nitrogen (and potassium if needed) can be obtained more economically from sidedress materials than from N-P-K fertilizers. For example, a pound of nitrogen from 6-6-18 fertilizer costs about $1.96, while a pound of nitrogen from 16-0-0 fertilizer costs only about 77 cents (based on average costs per ton in Tables 5-6 and 5-8).

Use of 3-9-9 fertilizer declined from 36 percent of the acreage in 1979 to 5 percent in 2000, while use of 6-6-18 fertilizer increased from 3 percent to 50 percent during the same period (Table 5-7). The combined use of 1:1:3 and 1:0:3 ratios increased from 5 percent of the acreage in 1979 to 69 percent in 2000. These changes occurred because many growers with high-phosphorus soils recognized the money they could save by using low-phosphorus, high-analysis fertilizers to meet soil test suggestions for phosphorus and other nutrients. The reduction in phosphorus application of approximately 45 percent since 1981 has also lowered the potential for phosphorus movement into ponds and streams without reducing tobacco yield and quality.

After choosing the fertilizer ratio based on the rate of phosphorus needed, the next step is to select a high-analysis grade of that ratio. Although high-analysis grades cost more per ton, they are less expensive per acre because much lower rates are needed to provide the same amounts of nitrogen and potassium (Table 5-6). Also, less fertilizer has to be transported, stored, and applied. These advantages may explain the trend in recent years toward greater use of the 8-8-24 analysis, with declining use of 4-8-12 and 6-12-18 analyses (Table 5-7). Initially, some growers experienced curing problems with high-analysis fertilizers because they used them at higher than suggested rates without reducing the rage of sidedress nitrogen accordingly. Fortunately, this problem has been largely corrected by most growers. But fertilizer calibration information, if needed, is available at your county Cooperative Extension center.








Table 5-7. N-P-K Fertilizer Use Estimated by County Agents, 1979-2000

Fertilizer
Percentage of Acres



Ratio Analysis
1979 1985 1990 1995 2000

1:3:3 3-9-9
36 17 13 7 5
1:2:3 4-8-12
6-12-18
8-16-24

20
28
5
8
14
5
5
13
8
2
13
6
<1
6
8
1:1:3 6-6-18
8-8-24

3
2
32
16
41
8
45
9
50
16
1:0:3 8-0-24
0 3 1 3 3
Various Liquids
- - 7 11 6
Various Drys
6 5 4 4 5




Dry Versus Liquid N-P-K Fertilizers. Tests have been conducted to compare the same analysis (6-4-10) of dry-blended and liquid fertilizers on tobacco yield and quality. Each fertilizer was applied broadcast, in two deep bands (about 5 inches deep and 4 to 5 inches to each side of the row), or in two shallow bands (about 0.5 to 1 inch deep and 4 to 5 inches to each side of the row). The broadcast applications were made about one week before planting and the band applications about one week after planting. The application rate (670 pounds per acre) was the same for both fertilizers and for each method of application, and all plots received the same rate of sidedresser about three weeks after planting.

The type of fertilizer (dry or liquid) did not affect yield, market price, grade index, or value per acre. However, deep fertilizer placement produced higher yields than shallow placement of both fertilizers when little rainfall occurred during the first six weeks after planting. When the early season was unusually wet, however, broadcast application reduced market price, grade index, and value per acre because of a higher proportion of pale, immature tobacco. These results indicate the following:

  • Fertilizer placement and time of application are more important than whether the fertilizer is dry or liquid.

  • Dry and liquid fertilizers should perform equally well if they have similar nutrient contents and are applied at the same time and by the same method.

  • Deep band application (4 to 5 inches) should produce good yields and quality more consistently than broadcast or shallow band application (such as that obtained with rolling cultivators), regardless of the fertilizer type.

The specific yield and quality data for these tests are on page 45 of 1994 Flue-cured Tobacco Information. Contact your county Cooperative Extension Service agent for more information.

Selecting the Sidedress Fertilizer

The soil test suggestion for potassium is useful in selecting the proper ratio of the sidedress material. When the soil test suggests applying less than 120 pounds of potassium per acre, a 1:0:0 ratio sidedresser such as 16-0-0 (soda), 15.5-0-0 (calcium nitrate), 34-0-0 (ammonium nitrate), or one of several liquid nitrogen sources can be used to supply the nitrogen needed in addition to the 40 pounds obtained from the N-P-K fertilizer. If the soil test suggestion is greater than 120 pounds of potassium per acre or if severe leaching occurs after the N-P-K fertilizer application, some potassium, in addition to nitrogen, may be needed in the sidedresser. In these instances, 1:0:1 ratio sidedressers such as 15-0-14 or 13-0-14 will usually supply all the extra potassium needed in addition to the 120 pounds obtained from the N-P-K fertilizer. In rare instances on deep, sandy soils, available potassium may be low enough to require use of a 1:0:3-ratio sidedresser such as 8-0-24 or 13-0-44. However, topsoil analyses and field tests indicate that less than 10 percent of the tobacco soils in North Carolina are low enough in potassium to require the l:0:3 ratio as a sidedresser if 100 to 120 pounds of potassium per acre are supplied by the N-P-K fertilizer. In addition, the accumulation of available potassium in the subsoils of many tobacco fields (Table 5-5) indicates that only 1:0:0 ratio sidedress fertilizers are needed in many instances, even when topsoil analysis suggests adding more than 120 pounds per acre of potassium.

The per-acre costs of several sidedressers and the amounts required to supply 30 pounds of nitrogen per acre are shown in Table 5-8. At present, the least expensive dry source of sidedress nitrogen is 34-0-0 fertilizer. However, take care to apply no more than half as much 34-0-0 fertilizer per acre as 16-0-0 or 15.5-0-0. Applying excessive rates of 34-0-0 fertilizer will drastically increase nitrogen application and reduce curability and quality of cured leaves. The most expensive sidedresser is 8-0-24 and should be used only when the soil test suggestion for potassium cannot be met with the 1:0:1-ratio sidedressers. An alternative and less expensive 1:0:3-ratio sidedresser is 13-0-44 fertilizer, which should be available at most blend plants.


Table 5-8. Average Costs of Sidedress Fertilizers in 2000 Based on Surveys by 22 County Extension Agents

Fertilizer 2000
Prices
Rates and Costs
for 30 lb N
K2O
Applied

Ratio Analysis ($/ton) (lb/a) ($/a) (lb/a)

1:0:0




1:0:1


1:0:3
16-0-0
15.5-0-0
34-0-0
30% N Liq.

15-0-14
13-0-14

8-0-24
247
242
210
113

293
250

246
188
194
90
9.2 gal.

200
230

375
23
23
9
6

29
29

46
0
0
0
0

28
32

90



The average performance of the most commonly used dry sidedressers in 11 tests conducted in 1991 through 1994 is shown in Table 5-9. The surface soils contained medium levels of available potassium in five tests, low levels in three tests, and high levels in three tests. All plots received 670 pounds per acre of 6-6-18 or 6-12-18 fertilizer (120 pounds of potassium per acre) banded five to seven days after transplanting. The sidedressers were banded about two weeks later at equal rates of nitrogen. The results indicate that:

  1. Yield and quality were similar for all sidedressers even though rainfall was relatively high at most sites in 1992 and 1994. Tobacco topsoils often test low to moderate in available potassium, but sidedressers containing potassium seldom increase yield or quality because adequate potassium is usually contained in subsoils of many tobacco fields that have clay within 10 to 12 inches of the soil surface.

  2. Using a phosphorus-containing fertilizer, such as 6-6-18 or 6-12-18, as the sidedress fertilizer did not improve yield and quality compared to using one of the less-expensive sidedresser materials.



Table 5-9. Agronomic Performance of Dry Sidedress Materials in 11 Tests, 1991 - 94

Sidedress Fert.a Yield
(lb/a)
Price
($/cwt)
Grade
Index

Ratio Analysis

1:1:3 6-6-18 or
6-12-18
2,722 173 62
1:0:0 16-0-0 2,702 172 61
15.5-0-0 2,785 172 62
34-0-0 2,701 172 62
46-0-0 2,711 173 64
1:0:1 13-0-14 2,669 172 62
15-0-14 2,728 173 61
1:0:3 13-0-44 2,698 173 63

a Initial fertilizer was 670 lb/a of 6-6-18 or 6-12-18 banded five to seven days after transplanting; all sidedressers were banded about two weeks later at equivalent rates of N.



Based on estimates by 26 county Extension agents, use of sidedress materials (expressed as a percentage of acreage) in 2000 was 16-0-0, 44 percent; 15-0-14, 21 percent; 15.5-0-0, 8 percent; 30 percent liquid nitrogen and 24 or 25 percent liquid nitrogen (+ sulfur), 7 percent; 13-0-14, 6 percent; 8-0-24, 4 percent; 34-0-0, 3 percent; and various other local analyses, 3 percent. Unfortunately, about 4 percent of the acreage was sidedressed with N-P-K fertilizers, the most expensive choice, which do not improve yield or quality (Table 5-9). This costly practice occurred in both the piedmont and coastal plain counties.

Liquid Nitrogen Sidedress Fertilizers. Some growers who usually need only nitrogen in their sidedress fertilizer want less expensive nitrogen sources that can be applied faster than dry materials. In 13 tests conducted in the 1996 through 1999 seasons, 30 percent liquid nitrogen and 24 or 25 percent liquid nitrogen (+ sulfur) were evaluated as sidedress fertilizers against more traditional dry sidedressers such as 16-0-0. All plots were fertilized with an N-P-K fertilizer such as 6-6-18 several days after transplanting, and the nitrogen sidedressers shown in Table 5-10 were applied two to three weeks later. Each sidedresser was applied at the same nitrogen rate in each test, ranging from 20 to 40 pounds of nitrogen per acre, depending on local soil and early rainfall conditions. The liquids were not diluted with water, and rates ranged from about 7 to 14 gallons per acre over the sites. All sidedressers were banded in furrows 5 to 6 inches from each side of the plants and 4 to 5 inches deep; they were covered with soil immediately after application to guard against possible volatilization of the urea nitrogen contained in both liquid fertilizers.

No noticeable growth and maturity differences were observed among the sidedressers in any test. Further, yield and grade index values were similar for the sidedressers in all tests (Table 5-10). While the yield for the 24 or 25 percent nitrogen sidedresser appears to be slightly higher than that for the 30 percent liquid nitrogen or 16-0-0, the differences were not statistically significant in any of the 13 tests, indicating that factors other than the sidedress fertilizer were most likely responsible for the yield differences shown. Since these tests were conducted over a wide range of soil and climatic conditions, growers who use these sources of sidedress nitrogen should not experience adverse effects on maturity, yield, and quality if the nitrogen rate is in the normal range for a given field and if the liquid fertilizers are strained and properly banded and covered with soil as recommended.


Table 5-10. Effects of Liquid Nitrogen Sidedresser Fertilizers on Tobacco Yield and Grade Index in 13 Tests, 1996-99a

Sidedress Fertilizer
Yield (lb/a)
Grade Index

16-0-0 (dry) 2,572 67
30% Liquid N 2,566 67
24 or 25% Liquid N + S 2,614 66

aTests conducted on research stations near Whiteville, Kinston, and Oxford.



As an example, for each 10 pounds of nitrogen needed per acre from these fertilizers, only 3.1 gallons of 30 percent liquid nitrogen or 3.9 gallons of 24 percent liquid nitrogen would be required per acre. The liquid nitrogen sources will need to be diluted with water if the liquid application equipment will not uniformly apply the 7 to 14 gallons per acre that most fields would need. However, because no adverse stand or growth effects were observed with the undiluted liquids, it should be sufficient to use only enough water to ensure uniform application. If 10 gallons of volume per acre were used, for example, a 200-gallon liquid applicator would sidedress about 20 acres of tobacco between refills. The disadvantages of using liquid nitrogen fertilizers compared to traditional dry sidedress fertilizers are the requirement for liquid banding equipment and a slight reduction in soil pH in the row (0.1 to 0.2 units in 1997 tests). However, a number of growers have constructed their own inexpensive liquid applicators in the last year or two, and the cost of the extra dolomitic lime needed to correct the pH reduction is only about one dollar per acre.

Another possible advantage of liquid nitrogen sidedressers is that they might be tank mixed with certain soil pesticides and both applied in one trip over the field. Numerous tests conducted on research stations have shown no adverse effects on early growth, yield, or quality from adding 1 pint per acre of Ridomil-Gold EC with the liquid nitrogen sources. In two additional on-farm tests conducted in cooperation with NC State plant pathologist Tom Melton, tank mixing Ridomil-Gold EC with 30 percent liquid nitrogen did not reduce black shank control. In most of these tests, a compatibility agent was included because Ridomil-Gold EC tended to float on the water, particularly when mixed with the sulfur-containing liquid nitrogen sources. Therefore, a "jar" test is suggested to evaluate compatibility before tank mixing Ridomil-Gold EC with any liquid nitrogen source. Growers are also reminded that tank mixing Ridomil-Gold EC with liquid fertilizers is not specified on the Ridomil-Gold EC label, so Syngenta may not assume responsibility for poor black shank control if its product is used in this manner.

Cooperative research with NC State entomologist Clyde Sorenson is also underway to evaluate tank mixing of Admire 2F or Platinum 2SC (a Syngenta insecticide likely to be approved for 2001) with the liquid nitrogen sources. While agronomic and aphid control results obtained in 2000 were encouraging, tank mixing these products with liquid nitrogen sources is not yet labeled for grower use.

Secondary Nutrients

The secondary nutrients of concern for tobacco are calcium (Ca), magnesium (Mg), and sulfur (S). These nutrients are called secondary because they are usually needed by most crops in smaller amounts than the primary nutrients. However, they must be available in adequate amounts for good yields and quality.

Calcium and Magnesium (Dolomitic Lime)

If soil pH is kept within the desirable range of 5.8 to 6.0 with dolomitic limestone, the available levels of calcium and magnesium will usually be high enough to meet the needs of the crop. Otherwise, 40 to 50 pounds of calcium (Ca) and 15 to 20 pounds of magnesium (Mg) per acre are needed from the N-P-K fertilizer. Even with proper liming, some magnesium deficiency may occur on deep, sandy soils (more than 15 inches to clay) under severe leaching conditions. In these instances, supplying 15 to 20 pounds of magnesium per acre in the fertilizer may be desirable in the second and third seasons after lime application. However, using N-P-K fertilizers containing calcium and magnesium will not substitute for using dolomitic lime if soil pH is too low. Growers should be especially aware of low soil pH. The state's latest soil test summaries show that 29 percent of the tobacco fields tested in 2000 had a pH lower than 5.5, and piedmont soils generally were more acid than those in the coastal plain.

Low pH causes greater solubility of soil aluminum (and manganese in some piedmont soils), which reduces root growth and development. Therefore, liming to promote healthy root systems improves drought tolerance and nutrient absorption, sometimes resulting in better yields (see Table 5-11). In this test, limed plots produced higher yields than nonlimed plots regardless of nitrogen rate. Also, note that the yield of nonlimed plots that received 15 pounds per acre of extra nitrogen was no higher than that of limed plots that received 15 pounds per acre less than suggested nitrogen. These data indicate:

  • Extra nitrogen cannot overcome the adverse effects of low soil pH.

  • Lower nitrogen rates are possible when acid soils are limed according to soil test suggestions.



Table 5-11. Effects of Lime and Nitrogen on Tobacco Yield

N Rate
(lb/a)
Lime Used

No Yes


Yield, lb/a
Suggested - 15 2,272 2,497
Suggested 2,434 2,688
Suggested+15 2,405 2,516




Sulfur (S)

Sulfur deficiencies are most likely on deep, sandy soils (over 15 inches to clay) that are low in humic matter (less than 0.5 percent). Because sulfur leaches, deficiencies are more likely in these soils following heavy rainfall in the winter and spring, especially if sulfur is omitted from the fertilizer of the next tobacco crop.

Symptoms of sulfur deficiency are very similar to and often mistaken for those of nitrogen deficiency. When a plant is low in nitrogen, the lower leaves are paler than the upper leaves and "burn up" prematurely. However, sulfur deficiency begins as yellowing in the buds; the leaves gradually pale from top to bottom, and the lower leaves do not "burn up" prematurely unless nitrogen is also deficient. Because sulfur is required for nitrogen use in the plant, adding high rates of nitrogen to sulfur-deficient crops will not turn the crops green, and can, in fact, reduce leaf quality. Therefore, accurate diagnosis of the deficiency is very important and often requires tissue analysis.

Soil tests for sulfur are sometimes unreliable. Therefore, to reduce the chance of sulfur deficiency on deep, sandy soils, add 20 to 30 pounds of sulfur (S) per acre from the N-P-K fertilizer every year. Sulfur deficiency occurring before lay-by can be corrected by banding 100 to 150 pounds of Sul-Po-Mag or potassium sulfate (0-0-50) as soon after the deficiency is identified as possible. However, sulfur deficiency on soils less than about 12 inches to clay is often temporary, even when no extra sulfur is applied, because adequate sulfur is usually contained in subsoils (Table 5-5) and will be absorbed as roots reach this depth.

Minor Nutrients (Micronutrients)

The soil test report for tobacco shows a $ symbol in the "Suggested Treatment" block for copper (Cu) and zinc (Zn), and a $pH symbol for manganese (Mn) if the availability index for one of these micronutrients is low. The $ symbol indicates that corrective treatment may be beneficial, but it is uncertain that tobacco will respond to application of copper or zinc. The $pH symbol appears on the report when soil pH is greater than 6.1 and the manganese availability index is less than 26 (low or very low). The symbols also call attention to an enclosed note, also identified by a $ symbol, that provides information on suggested rates, sources, and application methods for these three micronutrients.

Crops differ in their response to micronutrients, and tobacco is considered less sensitive to low soil levels than crops such as corn, soybeans, and small grains. Micronutrients are also somewhat expensive, depending on kind and source. Therefore, their application for tobacco is not likely to be beneficial unless indicated by soil or tissue analyses. When in question, tissue analysis or strip testing on several rows may be needed to confirm a micronutrient need.

Copper (Cu) and Zinc (Zn)

Known deficiencies of copper or zinc are extremely rare for tobacco. Rates suggested on the soil test report will be sufficient for several years, and the time and need for reapplication should be based on soil testing.

Manganese (Mn)

Manganese deficiency begins on the lower leaves as flecks very similar to those caused by high ozone concentrations in the air (commonly called weather fleck). While weather fleck can occur anywhere in the state, manganese deficiency occurs primarily on low-manganese, overlimed soils in the coastal plain. Using too much lime causes soil pH to increase, which reduces manganese availability to plant roots. Most tobacco crops that develop confirmed manganese deficiency are grown on soils with a pH of 6.2 or higher and low levels of soil-test manganese (availability index less than 26). Tobacco performs well when soil pH stays in the 5.8 to 6.0 range. Other major crops, such as soybeans, corn, and small grains, also perform well in this pH range if soil phosphorus is high. Therefore, when these crops are in rotation with tobacco, they usually should not be limed at rates higher than those suggested by the soil test for tobacco.

Tissue analysis of flecked leaves, along with a soil test, is the best way to distinguish between manganese deficiency and weather fleck. However, it is important to submit leaf and soil samples as soon as flecking occurs because several days are required to complete analyses. If the problem is manganese deficiency, a corrective treatment should be made as soon as possible. If it is weather fleck, only cooler, drier weather will help.

Manganese deficiency can be corrected by soil or foliar application of several manganese sources. Manganese sulfate is a relatively soluble, inexpensive source that can be used for soil or foliar treatment. The more expensive chelated sources generally perform satisfactorily as foliar sprays but are not superior to sulfates when applied to the soil. For soil applications, mixing the manganese source with acid-forming fertilizers increases its effectiveness, and banding is usually better than broadcasting. Do not apply manganese broadcast on soils with a pH greater than 6.1 because it will be converted to a less available form. For band application, special blends may be required because premium fertilizers usually do not contain enough manganese to correct a deficiency. General recommendations for manganese application in North Carolina are about 3 pounds per acre banded, 10 pounds per acre broadcast, or 0.5 pound per acre as a foliar spray. Foliar application of manganese is an efficient way of correcting an unexpected deficiency because lower rates are often as effective as much higher rates of soil-applied manganese.

Chloride (Cl)

There is no suitable soil test for chloride, but this nutrient is included in most N-P-K tobacco fertilizers. You will apply sufficient chloride when you use N-P-K fertilizers guaranteeing chloride at rates suggested in Table 5-6. Suggested rates of most fumigants also supply adequate amounts of chloride; when Telone C-17 or Chlor-o-Pic is used, it is not necessary that the N-P-K fertilizer contain chloride. Otherwise, sufficient chloride should be included in the fertilizer to provide a maximum of 20 to 30 pounds per acre. Higher rates will not improve yield but can reduce quality. Chloride may not be included in some fertilizers, particularly blends or liquids, unless requested by the grower.

Excessive rates or improper application of some micronutrients can cause toxicity. Contact your county Cooperative Extension agent if you suspect you had a micronutrient problem in 2000 or if your soil test indicates that a problem might occur in 2001. Your agent can help you decide whether treatment is advisable and, if so, which sources, rates, and application methods are most effective.

Other Economic Considerations

  • Premium fertilizers with small amounts of several micronutrients and 20 to 30 percent water-insoluble nitrogen usually cost $10 to $20 more per ton than regular-grade fertilizers, but the yield or quality increases sometimes claimed for premium fertilizers cannot be confirmed in field tests.

  • Blended fertilizers generally cost several dollars less per ton than manufactured fertilizers of the same grade; they also perform as well as manufactured fertilizers of similar nutrient content if properly blended.

  • You can save an additional several dollars per ton if you use bulk rather than bagged fertilizers.

  • Prescription blends and liquids prepared according to soil test specifications are often less expensive than dry, standard-grade fertilizers, but they may not be the best choice if broadcast application is the only option (see "Dry Versus Liquid N-P-K Fertilizers" discussed previously in this chapter and "Time and Method of Application" below). Also, growers with deep, sandy soils should make sure that the liquid or blended fertilizer purchased will supply sulfur and possibly chloride at rates suggested above.

Time and Method of Application

Proper placement and timing of fertilizer application provide maximum return for each dollar spent on fertilizers. Fertilizers should be applied at the proper time and with the proper method to maximize nutrient use by the crop while minimizing leaching losses and fertilizer salts injury to roots. Four methods of fertilizer application have been evaluated in on-farm tests under a wide range of soil and climatic conditions. Results varied among locations, primarily because of differences in soil moisture at and following transplanting:

  • If soil moisture was adequate but not excessive, the "bands at transplanting" and "bands within 10 days after transplanting" methods yielded moderately better than the "broadcast" or "one band deep" methods.

  • If early leaching conditions occurred, best results were obtained with the "bands within 10 days after transplanting" method, with "bands at transplanting" being a close second, and the "broadcast" method giving the poorest results.

  • When the soil was dry, which contributed to fertilizer injury, the "bands within 10 days after transplanting" method gave the best results, and the "one band deep" method the poorest results.

  • Overall, the "bands at transplanting" and "bands within 10 days after transplanting" methods produced better yields more consistently than the "broadcast" and "one band deep" methods. These methods are also more environmentally sound than pretransplant methods because nutrient uptake is more efficient and leaching losses are reduced.

The "broadcast" and "one band deep" methods may contribute to fertilizer salts injury, particularly on sandy soils in dry seasons. Also, because the fertilizer is sometimes applied several weeks before transplanting with these methods, there is more time for leaching to occur. The "bands at transplanting" and "bands within 10 days after transplanting" methods virtually eliminate fertilizer injury, resulting in a more uniform crop, and they may reduce leaching losses because the fertilizer is exposed to leaching conditions for a shorter time early in the season when leaching is most likely to occur.

A problem with the "bands within 10 days after transplanting" method is that prolonged rains after transplanting may delay fertilizer application for more than 10 to 14 days. Because this delay may reduce early growth and possibly yields, many growers apply the N-P-K fertilizer as soon as the plants regain turgor or they apply some nitrogen (and sometimes potassium) at transplanting with equipment ("Soda-Flo") attached to the rear of the transplanter. This latter practice may be especially beneficial on poorly drained fields when the "bands within 10 days after transplanting" application method is planned for the N-P-K fertilizer. However, you should reduce the rate of regular sidedress nitrogen accordingly, if more nitrogen is needed at all, to prevent ripening and curing problems due to excess nitrogen. On soils where leaching is not a problem, some growers use "Soda-Flo" as the only sidedress application, regardless of how the N-P-K fertilizer is applied. This eliminates a trip over the field or reduces the time and labor needed later for normal cultivation.

Fertilization Summary

  1. Have a soil sample tested to determine nutrient and lime needs. Use dolomitic lime, if needed, to adjust pH and supply magnesium as well as calcium. Do not overlime!

  2. Use a base nitrogen rate of 50 to 80 pounds per acre (total nitrogen from N-P-K and sidedress fertilizers); your portion of the rate range will depend on topsoil depth and texture, previous crop grown, variety to be grown, and personal experience.

  3. Select a high-analysis ratio of N-P-K fertilizer that will supply all the phosphorus suggested by the soil test when used at a rate that will supply no more than 40 pounds of nitrogen per acre. For many tobacco soils in North Carolina, this rate of a 1:1:3 or 1:0:3 ratio fertilizer will also supply adequate potassium.

  4. Consider applying 20 pounds of sulfur per acre or possibly chloride on deep, sandy soils. There are no suitable soil tests for these nutrients, and some fertilizers do not contain them in adequate amounts.

  5. Select a sidedress nitrogen fertilizer that contains potassium only if the N-P-K fertilizer in step 3 will not supply all the potassium suggested by the soil test. Only nitrogen from 1:0:0-ratio sidedressers will be sufficient for many soils. Additional potassium, if needed, can be obtained economically from 1:0:1-ratio sidedressers. Potassium levels in North Carolina tobacco soils are rarely low enough to require a 1:0:3-ratio sidedresser if 100 to 120 pounds of potassium per acre are applied from the N-P-K fertilizer. In addition, recent soil test results indicate that available potassium in subsoils of many tobacco fields is high enough that only 1:0:0 ratio sidedress fertilizers are needed in many instances, even when topsoil analyses suggest more than 120 pounds per acre of potassium. However, application of a sidedress fertilizer containing both nitrogen and potassium on partially drowned tobacco may be beneficial.

  6. Determine and make leaching adjustments for nitrogen losses with caution only after leaching has occurred, not on the assumption that it will. Using the leaching adjustment procedure on drowned tobacco often results in overapplication of nitrogen.

  7. Use a method of N-P-K fertilizer application that maximizes nutrient uptake efficiency but minimizes fertilizer salts injury and early-season leaching losses, such as the "bands at transplanting" or "bands within 10 days after transplanting" methods. The latter method is more risky on poorly drained soils because frequent rains after transplanting could delay fertilizer application for more than 10 days.
 
 
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