The University of Georgia College of Agricultural & Environmental Sciences
Cooperative Extension Service

Fertilization

J. Michael Moore and Glendon H. Harris

Nutrient Uptake by Tobacco
Soil Testing
Interpreting Soil Test Recommendations
Liming (pH, Calcium and Magnesium)
Nitrogen (N)
Base Nitrogen Rate
Leaching Adjustments
Phosphorus (P₂O₅) and Potassium (K₂O)
Secondary Nutrients
Micronutrients
Starters
Timing

Nutrient Uptake by Tobacco

As a guide to the fertilizer you should be using, it helps to know how much of each nutrient is taken up by the tobacco plant throughout the season (Table 1). In particular, the amount of P taken up by the plant is very small in relation to the amounts typically applied. A large part of the applied nutrients remain in the soil when the crop is removed. Soil testing is the best way to determine base fertilizer rates.

Soil Testing

The soil test is a valuable diagnostic tool for monitoring soil nutrient status and fertilizer needs. Efficient nutrient use can help reduce fertilizer costs and help protect the farm environment without reducing yield or quality. One of the most important aspects of soil sampling is obtaining a sample that is representative of the soil conditions. Different soil types should be sampled separately. Each soil sample should represent no more than a 10-15 acre area, and should contain soil combined from 15 to 20 separate cores. This combined sample, weighing 1-2 pounds, must adequately represent the soil condition of 20-30 million pounds of soil, so careful sampling is a must. An improperly collected sample will be meaningless, and the corresponding recommendations will be poorly suited for the actual soil conditions.

Interpreting Soil Test Recommendations

Based on the nutrient levels found in the soil sample, recommended rates for each nutrient should supply the amount needed by the crop under average to good growing conditions. Nitrogen is not measured in soil samples. Soil N changes very quickly, and good measurements require special sampling and storage procedures. Recommendations for N (See Nitrogen section below) are based on numerous experiments and the measured requirements of the crop. Soil P and K index values represent the relative availability of these nutrients, not the total P and K in the soil. Recommendations are based on experiments relating response of tobacco to nutrient additions in soils at each soil test level. While a high soil test rating does not rule out a profitable response to fertilizers if all other factors are favorable, the chances of a response are quite low. On the other hand, a soil with a low rating generally will respond to inputs of that nutrient unless prevented by other factors such as adverse climate, poor management, or uncontrolled pest problems. General soil test recommendations are given in Table2.

A soil test is helpful in determining the pH and availability of nutrients. A total fertilizer program involves more than adding the recommended lime and nutrients. You need to consider residual nitrogen, leaching, placement and timing of nutrient applications. Over-fertilization with N, Cl and certain micronutrients can decrease yield and quality of tobacco, increase production costs, and may adversely affect water quality in parts of Georgia. Excessive use of P, K, Mg and S may have little effect on yields or quality but will certainly increase costs. Society may see fit to impose limits on use of fertilizers and chemicals if farm inputs are not wisely managed. Use your soil test results, the information contained below, along with your experience to develop a fertilization program that will work for you.

Liming (pH, Calcium and Magnesium)

The optimum pH for tobacco production is 5.7 to 6.0, although good growth can be obtained at higher and lower values depending on the soil type, disease pressure, and how recently the field has been limed. Never apply lime without a soil test. When lime is applied, always use dolomitic lime which supplies both Ca and Mg. Soils with pH values in the optimum range will generally provide ample Ca and Mg to the plants. On deep sands, additional Mg may be required under conditions of heavy leaching the second or third year after liming. This should be supplied in the mixed fertilizer.

On low, wet soils of the Flatwoods, lime should be used with great caution. Heavy applications of lime on these soils can result in frenching and other problems. At optimum pH, these soils can supply adequate amounts of micronutrients. But as pH increases above 6.3, micronutrients, particularly manganese (Mn), become tightly bound to the soil and deficiencies can result.

Nitrogen (N)

More than any other nutrient, N management affects yield and quality of tobacco. When plants suffer a lack of N, yields are reduced, and the resulting cured leaf is pale and slick with poor texture. While excessive N may slightly increase yields, it also stimulates excessive suckering, delays maturity, and may result in dark colored, unripe cured leaf. Effects on quality are most pronounced in dry years. Total N requirements for tobacco should be based on plant requirements, soil texture, soil depth, residual N levels, tobacco variety characteristics, and adjustments for leaching losses.

Base Nitrogen Rate

Your base N rate should be determined by soil texture and soil depth. Finer-textured soils will hold water and N better and tobacco grown on them will require less applied N than deep sandy soils. For soils with sandy surface horizons, depth to the finer textured horizon is the next most important factor in determining the base N rate. This depth can be measured with a shovel, a soil auger, or even estimated from a published soil survey map. Once the depth to the finer-textured subsoil is known, use Table 3 to estimate your base N rate.

Adjustments to the base rate should include allowance for residual N from the preceding crop. Following a heavily fertilized corn crop (especially if yields were lower than expected), or a legume such as peanuts or soybeans, reduce your total N rate by 10-20 lbs. If the cultivar you plant is known to mature late or cure poorly when overfertilized with N, decrease the base rate by 5-10 lbs/A. Finally, use your experience with tobacco on your particular soils to make any additional adjustments in N rate.

In the past, some growers have used high initial N rates to grow a good stalk of tobacco, then used heavy irrigations to leach excess fertilizer from the soil profile. This practice can seriously affect your water quality, wastes money, depletes natural resources, and simply should not be used.

The best means of avoiding problems due to over-fertilization is to prevent excess application of N in the first place. When over-fertilization does occur, such as in replacing leached fertilizer, negative effects may be reduced by topping several leaves higher than normal, allowing plants to remain in full flower for several days, and/or delaying harvest until tobacco ripens. Leaving plants in full flower increases the risk of plants blowing over during thunderstorms and other periods of high wind. While excess nitrogen may increase yield, it complicates harvesting and curing and generally results in lower quality cured leaf. Excess nitrogen also favors development of several diseases.

Leaching Adjustments

Most Georgia tobacco is grown on soils that have a high rate of water infiltration, and a very low capacity to hold nutrients when large amounts of water move through the soil. Slow, lingering rains can leach N, S, K, and Mg from the root zone of deep sands. N and K losses cause the most problems for tobacco growers. These nutrients are needed in fairly large amounts to produce good quality leaf. Replace leached nutrients, especially N and K, as soon as practical. Do not delay!

Determining the correct amount of replacement fertilizer is one of the more difficult and risky tasks in tobacco production. If uncorrected, N deficiency will cause decreased yields and pale, thin-bodied tobacco. If over-corrected, the many complications of over-fertilized tobacco will be with you throughout the remainder of the production and curing season. Replacement should be based on topsoil depth, age of the crop when leaching occurred and the estimated amount of excess water which moves through the fertilized zone. Topsoil depth determines the ease with which nutrients move from the rooting zone. Since most tobacco roots occur in the upper 12-14 inches of soil, the deeper the clay horizon below this depth, the greater the chance of nutrients moving below the root mass. Age is included because the amount of nutrients to be replaced becomes less as the plants get older. A larger portion of the required nutrients will already be in the plant, rather than in the soil. In addition, older plants will form a canopy which sheds some of the water to the row middles away from the fertilized zones beside the plant.

The most difficult factor to determine is the amount of excess water moving through the soil. Rainfall alone is an unreliable measure, since soil texture, initial soil water content, slope, crusting, and the amount and duration of the rain all influence the amount of leaching. Using past experience, growers can become reasonably good at estimating the amount of excess water and amount of replacement N required. This skill can be refined by keeping up to date records of the amount of rain, and estimates of the amount that actually soaked into the soil. Table 4 suggests the percent of the base fertilizer rate to replace under various soil and leaching conditions. NOTE: If more than the recommended base rate has been applied, the excess N applied should be subtracted from the replacement amount.

Phosphorous (P₂O₅) and Potassium (K₂O)

Standard grades of complete (or mixed) tobacco fertilizers are commonly used in Georgia. The grade is the percentage of N-P₂O₅-K₂O. A 6-6-18 grade contains 6 lb N, 6 lb P₂O₅, and 18 lb K₂O per 100 lb of fertilizer. Most of the grades used in Georgia have three times more K₂O than N, but vary in the ratio of P₂O₅ to N and K₂O. The most common grades have either a 1:0:3, 1:1:3, 1:2:3 or 1:3:3 ratio. With proper selection of the fertilizer ratio, the proper phosphorus rate can be applied without altering the rates of N and potassium.

Many growers waste money and nutrients each year by using higher than required rates of complete fertilizers. Why is this? Phosphorus does not leach very much, even on deep sandy soils. According to the Potash and Phosphate Institute, a 3000 lb crop of tobacco takes up only 26 lbs P₂O₅/A, 11 lbs of which remains in the field after harvest. Many soils used for tobacco production have received several times this rate of P₂O₅ each year they were planted to tobacco. Over 70% of the soil samples submitted to the University of Georgia for tobacco recommendations were rated high and very high inP.

Seven on-farm demonstrations in Georgia during 1990 and 1991 clearly showed that soils with high P levels can produce high yields of high quality tobacco without additional P₂O₅. On-farm experiments in 18 North Carolina locations from 1985-1987 gave similar responses. Based on these results, the University of Georgia lowered P recommendations for tobacco in 1991.

To prevent excess costs, and excessive additions of P₂O₅ and K₂O, consider the primary purpose of complete fertilizers to be the supply of required P₂O₅, a portion of the K₂O, and any additional secondary and micronutrients required by the soil test. A good general rule of thumb is to use only enough complete fertilizer to supply the recommended rate of P₂O₅ and possibly K₂O, but no more than 40 lb N/A. Thus, the recommended P₂O₅ rate should determine which ratio of mixed fertilizer to use. For recommendations of 40 lb P₂O₅ or less, use a 1:1:3 ratio, for 40-80 lbs P₂O₅, use a 1:2:3 ratio, and for greater than 80 lbs P₂O₅ use a 1:3:3 ratio. Additional N (and K₂O if necessary) can be supplied much cheaper as 1:0:0 or 1:0:1 sidedress materials than as complete fertilizers.

The soil test recommendation can also be used to select the proper ratio for a sidedress material. If 120 lbs K₂O or less is recommended, all the K₂O can be supplied with mixed fertilizers. In this situation, use a 1:0:0 sidedress material such as 16-0-0 (sodium nitrate) or 15.5-0-0 (calcium nitrate) to supply the remaining N requirement. When higher rates are required, a 1:0:1 sidedress material such as 15-0-14 can be used. Requirements in excess of 160 lbs K₂O/A would justify use of a 1:0:3 grade sidedress material such as 8-0-24 or 13-0-44). If additional Mg or S is required as sidedressings on deep sands for replacing leached fertilizers, 150 lbs/A of potassium-magnesium-sulfate (Sul-Po-Mag or K-Mag) can be blended with a 1:0:0 sidedress material.

Secondary Nutrients

The secondary nutrients (Ca, Mg, and S) are often included in complete tobacco fertilizers. They may also be added using other readily available nutrient sources such as dolomitic lime (Ca,Mg), gypsum (Ca, S), potassium-magnesium-sulfate (K,Mg,S). Soil testing is strongly recommended to determine the need for lime and supplemental Ca or Mg.

Micronutrients

Responses to micronutrient additions on tobacco seldom occur on Georgia soils that have been in cultivation for several years. Tobacco generally is less sensitive to low levels of micronutrients than other agronomic crops. Problems generally arise on new ground, soils that have been overlimed, or on soils testing very high in soil P. While addition of micronutrients should certainly be used when indicated by low soil test levels or tissue analysis, these materials add considerably to the cost of fertilization and should be used only when needed.

Starters

Starter fertilizers are commonly used on several agronomic crops. But many years of testing show little or no benefit from the use of starter fertilizers on tobacco. It is not uncommon to see the roots of tobacco transplants injured where starters have been used. The lack of response to starters is very likely related to the common practice of banding fertilizer close to the row. As a result of banding fertilizers, nutrient concentrations near the row are relatively high. The combination of fertilizer salts from banded fertilizers and starters can be high enough to damage tender roots.

Timing

Proper fertilization of tobacco requires a working knowledge of the soil, the amounts required by the crop, and when nutrients are in critical demand by the crop. The major goal of a tobacco fertilization program should be to avoid excessive nutrient additions while providing sufficient nutrients to maintain a vigorously growing crop. Georgia tobacco is grown predominately on soils with sandy surface textures. Such soils are subject to leaching losses, low nutrient supplying power and drought. On the other hand, N can be more easily managed to "run out" at the proper time on sandy soils than on heavier textured soils. Sands are also much easier to prepare, shape and cultivate than clays. Just as drought can be handled with a good irrigation program, the negative factors of sandy soils can best be managed using a good soil testing program and well timed, split applications of fertilizers. Using such best management practices, Georgia growers can increase fertilizer efficiency, reduce pollution potential and maximize profits.

When should fertilizers be applied? Until plants recover from the shock of transplanting, little nutrient uptake will occur. Thus, it makes little sense to fertilize the beds several days prior to planting. The first application of fertilizer (all complete fertilizer containing no more than 40 pounds N per acre) should be made at or within a few days after transplanting. This application will insure adequate levels of less leachable nutrients such as P, K, Mg, and any required micronutrients, as well as a portion of the leachable N. The remaining applications, containing only N (and K if required), should be placed no more than six inches from the row. By adding these nutrients in steps, potential for loss of nitrate-N through leaching is greatly reduced. Since most tobacco in Georgia is irrigated, there is little likelihood that late applications of fertilizers will not be moved into the root zone and available for plant uptake. Very rapid vegetative growth occurs from the fourth to the seventh week after transplanting. All fertilizer materials should be applied before rapid growth begins in the third to fourth week after transplanting. Applications after this time may be available in later stages of growth, delaying N depletion, delaying maturity, and adversely affecting quality. When managed properly, soil N levels should decline rapidly as full flowering approaches. After topping, the plants should begin to break from a dark green color to a lighter green color as the N depletion nears completion.

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