Abstract Potassium (K) and nitrogen (N) are two major nutrients in crop production. A deficiency of either one or both of these nutrients causes yield loss. In the specific case of tobacco production, both nutrients play a key role in controlling important quality parameters such as leaf color, texture, hygroscopic properties, combustibility, sugar and alkaloid contents. Monitoring N applications thoroughly for form, quantity, and timing of application is a prerequisite in modern agriculture. As in other field crops, balanced N-K fertilization enhances tobacco growth and improves the uptake of both nutrients, which in turn reduces nitrate losses during and after the cropping season. The importance of K on mineral nutrition in tobacco production in terms of yield and quality is presented. The role of K in determining the chemical composition of tobacco leaf is now well established. K content of dry matter must reach 2 to 2.5 percent and chloride (Cl) content must remain below 1 to 1.5 percent in order to ensure good maturation, perfect combustibility and a good taste. The paper reports several pot and field experiments carried out in France, China and Cuba, in which various factors affecting yield and quality of the tobacco crop were studied including dose, source, and timing of potash applications. The effect of the combination of various potash forms (potassium sulphate, potassium chloride, potassium nitrate, potassium bicarbonate) with two N sources (nitrate –N from potassium nitrate, and ureic nitrogen from urea) was investigated. The following measurements were recorded: yield parameters; mineral nutrients; sugar; the alkaloid contents of tobacco leaves at three stalk-levels; and water-soluble alkalinity which is an indicator of combustibility. In an experiment on tobacco for cigar production, the effect of sources of K for fertigation was investigated showing the benefit of potassium sulphate on quality of tobacco when applied over a longer period. Finally, an experiment with foliar applications of potassium sulphate in China is presented, illustrating the positive effect of foliar applied K on K content in the tobacco leaf.

Keywords: Tobacco, mineral nutrition, potassium, leaf composition, fertigation, foliar application.

Introduction The term "burning qualities" as applied to smoking tobacco, usually includes several different elements. Of these, according to GARNER (9, P. 8), the fire holding capacity, the evenness and completeness of combustion, and the character of the ash is of most importance. Knowledge of the factors influencing the burning qualities of cigar-leaf tobacco is of considerable importance, both from the standpoint of the producer and the manufacturer, As the value of tobacco for cigar manufacture is based almost wholly upon these qualities. For this reason, therefore, considerable investigational work has been conducted upon this subject, and, as a result, it is now known that several factors, especially those of a chemical nature, influence the "burn" to a considerable extent. Growth and general physiology of crop plants can be significantly altered by varying cultural practices. In tobacco, the effects of spacing, topping, suckering, and harvesting all directly influence total yield as well as the proportion of the different classes of leaf, and their chemistry.
Standard tobacco production requires 130-150 kg N ha-1, 30-40 kg P2O5 ha-1 and 230-240 kg K2O ha-1 which is closely dependent on mineral nutrient supply. Fertilization also plays a key role in influencing the most important quality parameters such as leaf color, texture, hygroscopic properties, combustibility, sugar and alkaloid contents. Leaf-burn, or combustibility, is one of the key criteria taken into account by the tobacco industry for assessing quality. Many studies have shown that organic acids, associated with K, enhance combustibility, whereas excessive N, especially in the ammonium form, is detrimental in this respect. The detrimental role of Cl in inducing leaf burn and poor smoke taste is well documented in contrast to the role of other nutrients, such as sulphur (S), where their influence is less known. Sugar and alkaloid content are also important parameters of quality, which are partly controlled by mineral nutrition, and especially by the quantity and form of N fertilizer applied. This paper presents the results of pot and field experiments carried out over recent years in France, Cuba and China to improve knowledge of the influence of fertilizer application on the yield and quality of the tobacco crop. The K fertilizer sources used in these experiments were potassium nitrate (KNO3, NOP), potassium sulphate
(K2SO4, SOP), potassium chloride (KCl, MOP) and potassium bicarbonate (K2CO3, Kbic).

REVIEW OF LITERATURE SCHLOSING was one of the first investigators to study the chemical factors affecting the burning qualities of tobacco. He treated tobacco leaves with various salt solutions and found that potassium salts of various organic acids were decidedly beneficial. He attributed their effects to the tendency of these salts to swell on heating, thus exposing more surface of combustible material. By impregnating filter-paper with solutions of various salts, NESSLER found that the sulphate and carbonate of potassium increased the fire-holding capacity to a marked degree. He also observed that potassium acetate promoted combustibility in a manner similar to the carbonate, although this salt does not possess the power to swell when heated, at least to a marked degree, therefore, SCIuO6SING'S interpretation of the catalytic effect of potassium salts of organic acids was probably not correct. In analyzing the ash of good-burning tobacco, VAN BEMMETEN always found a greater quantity of potassium present as the carbonate and lesser quantities as the chloride and sulphate. MAYER studied the influence of organic and inorganic substances on the capacity of filter-paper to glow or burn with a flame. The results obtained with inorganic compounds were similar to those obtained by NESSLER. In addition, he found that nearly all organic substances promote burning with a flame. It has been suggested by BARTH, that the burn and characteristic glow of a good tobacco is due to a reduction of the alkali salts to oxides and small quantities of free metal, and that such a reduction would increase the efficiency of these metals as oxidizing catalysts. BEHRENS , as a result of his investigational work on the burning qualities of tobacco, states that a high content of potassium in combination with citric, malic and oxalic acids largely determines the burning quality. GARNER, is also of the opinion that potassium in such combinations is highly desirable, and further suggests that the favorable action of the carbonate obtained from the combustion of these substances may be due to its effect of functioning as a catalyst in combustion by taking up carbon dioxide and forming the acid carbonate at the most favorable moment and losing it later on. KRAYBILL, however, obtained results which seem to disprove this theory. RIDGWAY found a striking relationship existing between the fire holding capacity and the degree of aggregation of various salts normally present in tobacco leaves. Under certain conditions of curing, it appears that there is a noticeable tendency for calcium and magnesium to crystallize as malates, citrates and oxalates. As these bases do not appear to promote the fire-holding capacity to any considerable degree, such a process is desirable, as relatively larger quantities of the tissues are thus left free to burn. He found that where such crystallization did not take place, the burning quality was always poor. According to LOEW, the more oxidizable material, and the more oxidation going on within the tobacco leaf, the more will the crystals or "grain" develop in the curing and sweating processes and hence it will in many cases, although by no means in all, confirm the idea of some tobacco manufacturers that a well-developed grain is a good sign of the quality of the tobacco. That chlorine exerts a deleterious effect on the burning quality of cigar leaf tobacco is quite definitely established. MAYER, BEHRENS, NESSLER, FESCA, GARNER, AMES and BOLTZ, OLSON, JENKINS, and others have reported on the poor quality of cigar-leaf tobacco fertilized by chlorine-containing material. JENKINS also states that while some chlorine is absolutely essential for the development of the tobacco plant, a large excess may prove deleterious to the burn. Some work has been done to show a relation between the water-, ether-, and alcohol-soluble constituents of the leaf to its burning quality. GARNER, made water extractions and found that the extracted leaf lost its glowing capacity. On examining the extract, he found that it contained the chloride, sulphate, nitrate, malate and citrate of potassium, ammonium and nicotine, and small quantities of lime and magnesium. Extracts from both good-burning and poor-burning leaves contained about the same quantities of potassium but the inferior leaf contained a much higher percentage of mineral acids. Hence he concluded that the potassium salts of organic acids such as malic and citric are the chief factors controlling the burn. Extraction with alcohol had little or no effect on the burn. It has also been shown by GARNER that some of the weaker organic acids, on the order of tannic, in combination with nicotine, are deleterious to the burning quality. GRAHAM and CARR made petroleum ether, sulphuric ether, alcohol and hot water extractions of tobacco and showed a relation between poor burning quality and a high extractive content. They showed that tobacco fertilized with sodium nitrate or muriate of potash yielded a high amount of extractives.

DISCUSSION
France - pot experiments (1995) Two experiments were carried out in which the pots were irrigated daily, with leached water being recycled for the next irrigation. The first experiment was conducted on dark air-cured tobacco (ITB 1000) from December 1994 till May 1995 in a closed greenhouse under controlled conditions, and the second on flue-cured tobacco (ITB 32) grown from June till September 1995 in an open greenhouse with adjustable roof protection. The same soil was used for both experiments: a loamy soil, slightly acidic, poor in phosphorus, and very poor in exchangeable K (Table 1). All fertilizers were mixed with the soil before filling the pots. The quantity of K applied to Virginia flue-cured tobacco was fixed at 270 kg K2O ha-1, corresponding to the concentration of NOP (13-0-44) on the basis of 80 kg of N, in accordance with K and N requirements under French conditions. NOP, SOP, KCl and Kbic were applied with urea on the same basis for N and K quantities. Dark air-cured tobacco grown under French conditions requires 200 kg ha-1 and 400 kg K2O ha-1 on the basis of 38,000 plants ha-1. In this case, the additional N required in the NOP treatment was made up with urea in order to have the same rate of N application in all treatments. Each treatment was replicated six times and randomized within each block. The equivalent of 75 kg P2O5 ha-1 and 20 kg MgO ha-1 was applied uniformly as triple super phosphate and as magnesium carbonate. The threshold of two percent necessary for adequate quality of tobacco was easily achieved in both experiments. It is worth remembering, however, that this value is not so easily reached under field conditions. All potash forms gave similar results in terms of K concentration in tobacco leaves. The K content decreased from lower to upper leaves, which is in agreement with previous observations. Although the air-cured tobacco received a much higher application of potash (400 kg K2O ha-1) than the flue-cured tobacco (270 kg K2O ha-1) as occurs in practice in contrast to N, the leaves of the air-cured did not contain more K except in the upper leaves. In both experiments the Cl content was very high in the MOP treatment. It is generally accepted in the tobacco industry that a very good grade of tobacco requires less than one percent Cl and when it exceeds two percent Cl, the tobacco has no commercial value. There is a highly significant relationship between leaf-burn and water-soluble ash alkalinity (WSA). An even closer relationship is obtained if WSA is replaced by the ratio of WSA to total N, expressed as percent of the DM. Potentially, the higher this ratio, the better the combustibility. In both experiments, the lowest ratios, and consequently the lowest potential regarding combustibility, were obtained in the MOP treatment which relates to the high Cl content. WSA values were similar for NOP, SOP, and Kbic treatments in both experiments. Highest WSA:N ratios were found for Kbic in the flue-cured experiment because the total N content was lowest in this treatment. The ratios for NOP and SOP were similar. Consequently, S content has probably only a marginal effect on combustibility compared to K and Cl.
France - field experiments (2001) An experiment was carried out in 2001 at Pôle d’Aspach, a research station in the eastern part of France. This station is located in the second largest tobacco cropping area in the country and the aim of the experiment was once again to examine the effect of N and K application in relation to timing. The three treatments maintained the same nutritional levels as 156 kg ha-1 N, 100 kg ha-1 P2O5, and 400 kg ha-1 K2O. The fertilizers applied were ammonium nitrate (AN), NOP, SOP and a soluble form of K sulphate: T1: conventional fertilization at planting (AN + SOP, 466 + 800 kg ha-1), using drippers for irrigation only (control).
T2: conventional fertilization at planting (AN + SOP, 299 + 400 kg ha-1) and fertigation every week with AN + soluble SOP (56 + 40 kg ha-1) for three weeks followed by fertigation with soluble SOP (40 kg ha-1) for seven weeks. T3: conventional fertilization at planting (AN + SOP, 299 + 400 kg ha-1) and fertigation every week with NOP (43.5 kg ha-1) for ten weeks. In the two field experiments, both leaf production and quality parameters (based on tobacco industry criteria) were analyzed. Table 3 reports leaf production and shows the advantage of an early N application followed by a later application of K in response to plant requirement. Table 4 indicates that quality is also in line with yield production. Treatment 2 gave the lowest N and Cl contents, and adequate K based on cigarette factory requirements. In addition, S values did not exceed the threshold of 0.65 percent (considered as the maximum for a good combustibility). For both varieties, the grade index shows there is a benefit from the application of N in the first three weeks after planting. On Virginia, the effect was much more pronounced and the fraction of class A leaf was close to 60 percent.

Cuba - field experiments (1997/2001) In order to define the most suitable timing and number of applications of fertilizer on shade-grown, dark tobacco (variety “Criollo 98”) for cigar production, fertigation research was carried out at the Tobacco Experimental Station in San Juan y Martinez, Pinar del Río province where five treatments were studied. The soil at this station was classified as an Ultisol, with ferrallitic quartz tic yellow and lixiviated characteristics according to the latest classification of Soil Taxonomy. The experiment was conducted following a protocol using long plots with five treatments and four replications. The fertilizers applied were ammonium nitrate (AN), NOP and a soluble form of potassium sulphate. For all treatments, levels of N, P2O5, and K2O were 125, 51 and 188 kg ha-1 respectively, but applied at different times and methods. The variety used was “Criollo 98” developed at the Tobacco Experimental Station. For this variety the size of the largest leaf ranges between 33 and 36 cm in maximum width and between 53 and 58 cm in length. The variety has a potential mean yield of 2,250 kg ha-1, and is resistant to the main diseases. Five plants were selected and identified at random in the calculation area in each plot between 20 and 25 days after the plantation establishment. Measurements of the length, width and dry mass of the middle leaf were made according to Torrecilla et al. (1980). Combustibility was measured by the procedure proposed by Guardiola (1992). In the cultivation of the crop, all normal agricultural procedures were carried out as required in wrapper tobacco production. The crop was planted during the second ten days of November and harvested at the beginning of February. The mean temperature, relative humidity and precipitation during the three-year experiment did not show any significant difference to the mean value over 25 years for any of the months in which the field experiments were carried out. In the soil tests for each experimental plot, conducted before and after concluding each season, only small variations in the values of pH, OM, and P and K concentrations were observed. It should be noted that the nutrient ratios K/Ca, Ca/Mg, and pH were based on the recommendations in Cuba according to Morejón (1988).
Table 8 indicates that in all treatments, the increase in length, width, dry mass, and combustibility of the leaf were superior to the control (farmers’ practice). Combustibility was considered “excellent” in all treatments but the control. The combination of formulations used in T2 (basal dose + NK fertigation, using SOP and limiting N application to 21 DAT) achieved the highest dry matter yield, while that of T4 (basal dose + NK fertigation, using
NOP and NK fertigation till 28 DAT) achieved the highest combustibility value. Table 9 presents the yield and its fractions. Highest yield of wrapper leaves for export, a very significant factor for the economics of tobacco growing, was achieved with T2. This treatment was superior in all fractions, demonstrating the value of SOP used with shortened time for N fertigation to reduce its negative effects (up to 21 DAT), and long for K fertigation (up to 28 DAT). In general, under these Cuban conditions, fertigation between 7 and 28 days after transplanting with four application timings, markedly increased the quality and total yield of cigar wrappers for export.
As a consequence of the results obtained in the experiment, the recommendation to tobacco growers is to use fertigation techniques and to limit N application to the first stage of the vegetative cycle, whilst taking advantage of the fact that K can be successfully applied later.
China - field experiments (1997) As already observed on many crops, foliar application of soluble SOP has a positive effect on production and increases the K content of the leaf. Additionally, because of the lack of foliar-applied N, leaf N content decreases which is favorable for tobacco maturation. Experiments were conducted in China and France. In the experiment presented below, carried out in Guangdong province, the same basal dressing was applied to each plot (120 kg N ha-1, 60 kg P2O5 ha-1 and 200 kg K2O ha-1). Three foliar sprays were applied 45, 55 and 65 days after transplanting. Treatments were designed to determine the optimal K concentration in the foliar spray. The results confirmed that increased K concentration in the foliar spray solution up to six percent affects and increases K concentration in the leaves, which in turn increase yields. Maximum yield (3,062 kg ha-1) was achieved with four percent of soluble SOP in the foliar spray. Foliar spray with six percent soluble SOP further increased K concentration in the leaves (2.25 percent), but with no further yield response. Foliar spray with four percent of soluble SOP achieved the highest tobacco quality (data not shown). A high response was observed as the K content increased from 1.66 percent up to 2.25 percent as a consequence of increasing K content in the foliar spray. In comparison with the control, a slight effect on yield in upper and middle leaves was also observed. In relation to the foliar application, the small amount of sulphate of potash sprayed cannot in itself explain the K increase in the tobacco leaves. A tentative explanation to account for this beneficial effect may be as follows: spraying K salts on the leaves of plants of low K status but adequately supplied with other nutrients would be expected to promote sucrose (and K) transport in the phloem from shoot to roots. Energy would thus be provided to further root growth thereby enhancing K acquisition from the soil in a K pump-like action. Both foliar applied and any enhanced K uptake would also favor growth by improving water status, photosynthetic activity, protein synthesis etc.
CONCLUSION
From this series of experiments in different regions, and the experimental conditions under which they were carried out, the following conclusions may be drawn: • K fertilization is a prerequisite to quality production of tobacco in order to improve curing and combustibility. • Application of K fertilizers separately from N application can be beneficial because it allows the adjustment of N and K, both in supply and timing of supply, which in many cases can improve quality aspects of tobacco. • MOP significantly reduces the quality of tobacco, which includes altering N metabolism and drastically decreasing combustibility. • As a general rule, split applications of potassium are more efficient when water is not a limiting factor as shown in the experiments in France. In drip irrigation cropping systems, SOP and Kbic can be applied when K uptake is maximized and N is not required. NOP should be used only if application of N is limited at the early stage of tobacco growth, as demonstrated in the experiment from Cuba. Foliar application of K is a very effective way of supplying K under severe K deficiency or when K is restricted by soil characteristics. It enhances K uptake by the roots and consequently improves K leaf content which is an important quality criteria for the cigarette factories.
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