Impact of NPK and CAN fertilizers on tobacco performance and economic returns under three major soil types of Tabora, Tanzania

Introduction

Tobacco (Nicotiana tabacum), a major global cash crop, plays a significant role in the agricultural economies of many countries (Rehman et al. 2019) [23] . The global production of tobacco is concentrated in a few leading countries, with China, Brazil, and India dominating the market (Kuboja et al. 2012; Mavroeidis et al. 2023) [12,20] . China is the largest producer, accounting for approximately over 40% of the world's output, followed by Brazil and India, which contribute significantly to the global supply (Wilkinson et al. 2015; Sporchia et al. 2021) [30, 33] . Africa has seen a notable increase in tobacco cultivation, with countries like Zimbabwe, Malawi, and Tanzania being prominent players (Saloojee, 2004) [24] . In Africa, tobacco is a critical agricultural commodity, providing substantial economic benefits and employment opportunities (Larsen and Gillett, 2017) [13] . Tanzania, in particular, stands out as one of the key tobacco-producing countries in the region (Ilembo et al. 2017) [8] . Several regions in Tanzania known for tobacco cultivation, with the Tabora, Shinyanga, Mbeya, Katavi, kigoma and Ruvuma regions being major hubs. These regions experience suitable climatic conditions and fertile soils, which support extensive tobacco farming, in particular, contribute significantly to the country's tobacco production, which is vital for Tanzania's economy due to its export value. According to Tanzania Tobacco Board (2024), Tabora region contribute 52% of all the tobacco produce in the country. The yield of tobacco per hectare, although relatively high, often falls short of the potential yield due to suboptimal farming practices and insufficient use of modern agricultural inputs (Larsen & Gillett, 2017) [13] .

The average yield in Tanzania ranges from 1,000 to 1,400 kg ha - 1 , whereas the potential yield could exceed 2,000 kg ha -1 with the application of advanced farming techniques and proper fertilizer management (Jew et al. 2017) [9] . Tobacco farming practices in Tanzania typically involve manual labour and traditional methods, However, the adoption of improved production practices, including the use of fertilizers containing nitrogen (N), phosphorus (P), and potassium (K) as NPK and calcium ammonium nitrate (CAN), has shown promise in enhancing tobacco growth and yield. These fertilizers supply essential nutrients that are often deficient in the soil, thereby improving plant health and productivity. The importance of tobacco to Tanzania's economy cannot be overstated. However, optimizing tobacco production to achieve higher economic returns necessitates understanding the effects of different fertilizers on crop performance across various soil types. In Tanzania, tobacco is basal applied with NPK (10:18:24) within 14 days after planting in rates of 500 kg ha -1 ) and topdressed by CAN (27% N) applied at 125 kg ha -1 in 21 days after planting. However, this application does not consider the individual soils types. The productivity of tobacco also determined by suitable weather conditions, like 500-1000 mm annual rainfall and 20-30 o C temperature during its growth period. Tobacco cannot stand if rainfall is more than 1000 mm (Kheawfu et al. 2021) [11] .

The quantity of tobacco produced through various strategies always expected to translate into overall quality of cured leaves for the market value. Tobacco classification, quality evaluation or grading of the flue-cured tobacco leaves are other factors considered, but rely on the judgmental experience of experts, and inevitably limited by personal, physical and environmental factors (Zhang and Zhang, 2011) [31] . Quality in tobacco leaf expressed as the physical appearance of the cured tobacco leaf that is size, colour, leaf body, texture and cleanness (Lisuma, 2020). The cured tobacco leaves are graded according principles, such as position of the leaf in the stalk, percentage of cleanliness, size, and colour (Zhang and Zhang, 2011) [31] .

Materials and Methods

Description of the experimental sites

This study was conducted in Tutuo, Isikizya and Kipera villages found in Sikonge, Uyui districts and Tabora municipal, respectively in Tabora region of Tanzania. These districts lie between latitudes 05 o .47'775" S, 04 o .86'294'' S and 05 o .14'055" S and longitudes of 032 o .65'686" E, 033 o .10'862'' E, and 032 o .68'026'' E respectively (Fig. 1). The region experiences unimodal rainfall patterns, starting from November ends in March.

Fig 1: Map showing study ite

Field experiments and laboratory analysis

A 20 m long and 1.5 m wide nursery seedbed established in each site in September 2022. A 3 g of FCV tobacco seeds (variety K326) sourced from Alliance One Tobacco Tanzania Limited was sown. This is a commonly cultivated variety, adapted in the area and attracts high market prices. At 30 days after sowing, seedlings were re-set onto secondary seedbeds of 10 m long and one wide at a spacing of ~ 3 cm by 3 cm using black-cotton soil potted polythene tubes. At 30 days after re-setting, the seedlings were ready for transplanting. Composite soil samples systematically collected in a zigzag pattern from each experimental site, reaching a depth of 0-30 cm prior to transplantation. Subsequently, these soil samples underwent laboratory analysis to determine their physical and chemical properties, adhering to established protocols.

Experimental design and treatments

Factorial experiments of N10P18K24 and CAN (27% N) combinations were laid down in a randomized complete block design (RCBD) in three replications. There were five treatment combinations making total of 25 treatments running together within a replicate block (Table 1), which constituted 75 experimental plots. An individual experimental plot measured 40 m length and 10 m width, with six ridges each of 37.5 m long and 0.2 m hips. Seedlings transplanted at 1 m between rows (inter-row) and 0.5 m between planting holes within a row (intra-row) in each ridge, with transplanting of tobacco seedlings (Fig 2 (b). The application of N10P18K24 was done 14 days after transplanting, while CAN (27% N) fertilizer was applied 7 days later (21 days after transplanting). Similar experiments were conducted in three sites of Tabora region, characterized by contrasting major soil types, which are Isikizya site in Uyui district (predominantly Mollisols), Tutuo site in Sikonge district (predominantly Alfisols), and Kipera site in Tabora municipal (predominantly Oxisols). Therefore, for three sites did 225 experimental plots all constitute the same research theme.

Data collection

One among four tobacco plants in each plot randomly selected and tagged with laminated tags at each treatment combination. Growth parameters (plant height, leaf length and width and number of leaves) recorded. The data for growth parameters collected by measuring the plant height, number of leaves, and leaf length and width in three growth phases. The first phase was 14 days after planting (before NPK basal fertilization). The second phase recorded in the 42 days after planting (21 days after top-dressing with CAN). The final growth data taken on 84 days after planting (6 days before harvesting commenced).

The leaves full ripened at 90 days after transplanting, harvesting done by handpicking. The leaves were graded, labelled and tied in the sticks by using jute twine ropes. The sticks were loaded in the barns and hanged in the barn rakes, then fire was set and temperature controlled within the curing barns by using thermometer. The tobacco leaves left in the barns for 9 days until well dried. The vents of the curing barns opened to allow the dried leaves to cool by natural air. After cooling, the leaves were grouped (graded) based on their similarities (size, colour, leaf body, texture, cleanness and other factors). After grading, the authorized Tobacco Classifier from Tanzania Tobacco Board invited to assign grade names for each similar leaves. The dry leaves measured using digital weighing balance to determine the total produce (kg ha -1 ) for each experimental plot.

Economic benefits

The partial budget analysis calculated to determine the benefits derived from costs associated with tobacco production using NPK and CAN fertilizers, following steps described by Shaaban and Kisetu (2014) [25] and Tlatlaa et al. (2024) [29] . Shaaban and Kisetu (2014) [25] indicated that the benefit to cost ratio (BCR) simplifies decision-making in economic profitability. Tlatlaa et al. (2024) [29] indicated that gross benefit alone does not provide concluding results if benefit to cost ratio is not considered. The costs for a 50 kg capacity bag of N10P18K24 and CAN (27% N) fertilizers during the 2022/23 cropping season were USD 70.72 and USD 59.99, respectively. In this experiment, one bag of each fertilizer type was sufficient for the entire experiment. However, specific amounts of each fertilizer used in a site adjusted accordingly to reflect their actual market values for the quantities applied. Therefore, these costs converted into 1 kg of fertilizer to give USD 1.41 and USD 1.20 for NPK and CAN, respectively. Assuming that all other variable costs uniformly distributed across experimental sites, only costs for fertilizers and person-days for fertilizer applications considered (Tlatlaa et al., 2024) [29] . After converting the total average yield per hectare of each fertilizer combination, the produce subjected to the 2022/23 market price matrix with an average price of USD 2 per kilogram of cured tobacco, at an exchange rate of one USD being TZS. 2344 on April 01, 2023. The gross benefit (GB) was determined by using Equation 1.

Where Avp is average price; Y = Yield (kg ha -1 )

The marginal return (MR) was determined by subtracting the total variable costs (TVC) from the GB. The benefit to cost ratio (BCR) was determined by dividing GB by TVC.

Statistical data analysis

The data on growth and yield parameters subjected to a two-way analysis of variance (ANOVA) with experimental sites forming the whole plot, fertilizer combinations forming the sub-plot and replicate blocks treated as random effect. The sites considered under whole plot during statistical analysis since the main interest, as a contributing factor is the fertilizer combination, tested across contrasting soil types. The significant treatment means among sites were compared through the least significant difference (LSD) while the significant differences due to NPK and CAN fertilizers' combinations and/or their interactions with the sites were compared using standard errors of differences of means (S.E.D.) at a 5% threshold by Tukey's Honest post hoc.

Descriptive statistics engaged for data on leaf quality to compute the mean percentage quality for each fertilizer combination. On the other hand, the data on economics subjected to a one-way analysis of variance with each experimental site treated independently and only fertilizer combinations considered a factor and replications as random effect. The significant means compared through the standard errors of differences of means (S.E.D.) at a 5% threshold by Tukey's Honest post hoc. In addition, simple linear regression analysis was performed between fertilizer combinations and economic indicators (gross benefits, marginal return/net benefit, and benefit to cost ratio) to unveil the significantly highest economic return from fertilizer combinations as described by Tlatlaa et al. (2024) [29] .

Results

Physical and chemical properties of the soils in the study areas

The physical and chemical soil properties for Uyui, Sikonge, and Tabora sites presented in Table 2 and Table 3.

Effects of NPK and CAN fertilizer application on tobacco performance

Plant height measurements showed that Uyui achieved the highest recorded height of 119.7 cm when applying 700 kg NPK ha -1 and 175 kg CAN ha -1 , Sikonge followed closely with a plant height of 109.9 cm under the same fertilizer application, while Tabora reached 109.0 cm with 600 kg NPK ha -1 and 150 kg CAN ha -1 . In terms of leaf length, Uyui exhibited the longest leaves at 76.0 cm with 700 kg NPK ha -1 and 175 kg CAN ha -1 , followed by Sikonge with 65.4 cm using 600 kg NPK ha -1 and 125 kg CAN ha -1 and Tabora at 63.3 cm with 700 kg NPK ha -1 and 175 kg CAN ha -1 . Leaf width measurements showed Uyui with the widest leaves at 36.3 cm under 700 kg NPK ha -1 and 175 kg CAN ha -1 , Sikonge with 28.7 cm using 600 kg NPK ha -1 and 125 kg CAN ha -1 , and Tabora with 24.7 cm under 700 kg NPK ha -1 and 175 kg CAN ha -1 . The highest number of leaves per plant observed in Uyui (19) under application of 700 kg NPK ha -1 and 175 kg CAN ha -1 , followed by Sikonge (18) and Tabora (16) with the same fertilizer combinations Figure 3. presents the means for tobacco barn dry weight (kg), demonstrating the effects of fertilizer treatments and sites on yield, the results demonstrated Uyui achieving the highest leaf barn dry weight (kg) of 2201 kg ha -1 with 700 kg NPK ha -1 and 175 kg CAN ha -1 , followed by Sikonge with 1916 kg ha -1 under the same fertilizer regime, and Tabora with 1753 kg ha -1 following the same application. The lowest yields were recorded in the absolute control, with 217 kg ha -1 , 163 kg ha -1 , and 96 kg ha -1 for Sikonge, Uyui, and Tabora, respectively.

Economic feedbacks from NPK and CAN fertilization on tobacco

The data in Table 5, tells the economic benefits of various fertilizer combinations on tobacco crop across Sikonge, Tabora, and Uyui. Economic analysis revealed that the gross benefit (GB) increased significantly with varying rates of NPK and CAN application. For instance, in Sikonge, the GB ranged from $434 with no fertilizer to $3,588 with the combination of 600 kg ha -1 NPK and 175 kg ha -1 CAN. Marginal returns (MR) and BCR for Sikonge were $2529.4 and 3.4, respectively at the same level of fertilizer combinations. In Tabora, the GB and MR increased from $192 with no fertilizer to $2,642 and $1785, respectively while the BCR was 3.1 with the combination of 500 kg ha -1 NPK and 125 kg ha -1 CAN fertilizers. In Uyui, the GB and MR increased from $326 with no fertilizer to $4,407 and $3202, respectively while the BCR was 3.7 with the combination of 700 kg ha -1 NPK and 175 kg ha -1 CAN fertilizers.

The benefit-cost ratios observed in varying NPK and CAN doses, demonstrating that more intensive fertilizer use can significantly improve economic outcomes. However, the optimal fertilizer combination varied by location, with Uyui showing the highest profit from the highest fertilizer combinations. 001 Key: TnIn = Fertilizer combinations; GB=gross benefit; MR=marginal return; BCR = benefit cost ratio. *Means along the same column in a specific factor category bearing different letter(s) differ significantly; otherwise, they are similar according to Turkey's HSD test.

Discussion

The soil characteristics assessed in this study offer critical insights into the challenges and suitability of tobacco production across different regions. Soil pH emerged as a significant factor influencing nutrient availability. Across all study sites, pH levels found to be below 5.5, indicating acidic soils. This acidity can limit the availability of essential nutrients such as nitrogen, phosphorus, potassium, and micronutrients crucial for tobacco growth and development (Karuma et al. 2015) [10] . Bulk density, another crucial parameter, varied among the sites. Lower bulk density values in Sikonge and Tabora suggest looser soil structures favorable for root penetration and aeration, which are vital for optimal plant growth (Zhang et al. 2016) [32] . In contrast, higher bulk density observed in Uyui indicates compacted soils that may hinder root growth and water infiltration, potentially compromising tobacco yields if not addressed through appropriate soil management practices (Cochavi et al. 2020) [4] . Nutrient levels in the soils assessed, revealing deficiencies in key elements such as calcium, magnesium, and potassium across different sites. Calcium is essential for cell wall formation and disease resistance in tobacco plants (Naiji and Souri, 2018) [21] , while magnesium plays a crucial role in chlorophyll synthesis and enzyme activation (Li et al. 2023) [14] . Potassium is vital for root development and nutrient transport within plants (Subhashini 2016) [27] . Deficiencies in these nutrients highlight the need for targeted fertilization strategies tailored to specific soil conditions to optimize tobacco crop performance. Excessive soil nitrogen will negatively affect tobacco leaf curing by producing dark brown to black color, dry and chaffy and have a strong and pungent smoke (Drake et al. 2015) [5] . Micronutrient levels, including zinc, copper, iron, manganese, and sulfur evaluated. Excessive levels of micronutrients like copper and iron can lead to toxicity, adversely affecting plant health and productivity. Conversely, deficiencies in essential micronutrients can limit crop growth and development (Sillanpää, 1982; Fageria et al. 2002) [26,6] . These findings underscore the importance of balanced fertilization practices and soil testing to ensure adequate nutrient supply while minimizing environmental impact. Spectrum Analytic Inc., (2010) reported that high amount of S has negative effects on tobacco aroma, nicotine, combustibility, ash colour and smoking characteristics. The application of fertilizers had a profound impact on tobacco growth, leaf quality, and economic returns across the study sites. Different fertilizer combinations rich in nitrogen, phosphorus, potassium, and other essential nutrients were tested to address soil nutrient deficiencies identified in the soil analyses. In Sikonge, where soils exhibited acidic pH and deficiencies in key nutrients, the combination of 600 kg ha -1 of NPK and 175 kg ha - 1 of CAN proved highly effective. This treatment significantly improved tobacco yield and quality, enhancing economic returns for farmers. The fertilization strategy tailored to Sikonge's specific soil conditions contributed to robust plant growth and optimal leaf production, reflecting the importance of addressing soil acidity and nutrient deficiencies in maximizing crop performance. Similarly, in Tabora, where soil conditions required a different approach, the application of 500 kg ha -1 of NPK and 125 kg ha -1 of CAN resulted in improved nutrient uptake and enhanced tobacco yield. This combination effectively supplemented soil fertility, supporting better root development and nutrient utilization by tobacco plants. The results underscored the importance of adapting fertilizer applications to meet specific soil nutrient demands, thereby optimizing crop productivity and economic benefits. In Uyui, characterized by high nutrient demand and compacted soils, the highest yield and economic returns reached at the application of 700 kg ha -1 of NPK and 175 kg ha -1 of CAN. This intensive fertilization strategy compensated for nutrient deficiencies and improved soil structure, promoting vigorous plant growth and maximizing tobacco production. The findings highlight the critical role of balanced fertilization practices and soil management in overcoming soil challenges and enhancing agricultural sustainability.

Conclusion

This study recommends tailoring fertilizer applications to address specific soil nutrient deficiencies identified through comprehensive soil testing. Optimal fertilizer combinations (NPK and CAN) be applied to maximize nutrient uptake efficiency and minimize environmental impacts. The study also advocates for promoting sustainable agricultural practices that conserve soil biodiversity, reduce nutrient runoff, and mitigate climate impacts. Adopt integrated nutrient management strategies to improve soil health and long-term agricultural productivity. Additionally, the study recommends establishing monitoring programs to assess the long-term impacts of fertilization practices on soil health, water quality, and biodiversity. Fertilizer strategies continuously be evaluated and adjusted based on crop response and environmental sustainability criteria

References

1. Bray RH, Kurtz LT. Determination of total, organic, and available forms of phosphorus in soils. Soil Sci. 1945;59(1):39-46.
2. Cochavi A, Cohen IH, Rachmilevitch S. The role of different root orders in nutrient uptake. Environ Exp Bot. 2020;179:104-212.
3. Drake MP, Vann MC, Fisher LR. Influence of nitrogen application rate on the yield, quality, and the chemical components of flue-cured tobacco. [Internet] 2015 [cited 2024 Mar 20]. Available from: https://DOI.org/10.3381/14043
4. Fageria NK, Baligar VC, Clark RB. Micronutrients in crop production. Adv Agron. 2002;77:185-268.
5. Gee GW, Bauder JW. Particle-size analysis. In: Klute A, editor. Methods of soil analysis: part 1-physical and mineralogical methods. Agronomy Monograph No. 9. ASA and SSSA; 1986. p. 383-411.
6. Ilembo BM, Kuzilwa JA, Larsen MN. Evolving governance structures and contract farming in the tobacco value chain in Tanzania. In: Contract farming and the development of smallholder agricultural businesses. Routledge; 2017. p. 5167.
7. Jew KK, Dougill AJ, Sallu SM. Tobacco cultivation as a driver of land use change and degradation in the miombo woodlands of south-west Tanzania. Land Degrad Dev. 2017;28(8):2636-2645. doi:10.1002/ldr.2827.
8. Karuma AN, Gachene CK, Msanya BM, Mtakwa PW, Amuri N, Gicheru PT. Soil morphology, physico-chemical properties and classification of typical soils of Mwala District, Kenya. Int J Plant Soil Sci. 2015;4(2):156-70.
9. Kheawfu K, Kaewpinta A, Chanmahasathien W, Rachtanapun P, Jantrawut P. Extraction of nicotine from tobacco leaves and development of fast dissolving nicotine extract film. Membranes. 2021;11(6):403.
10. Kuboja NM, Kalala AM, Lisuma JB, Mrutu M. Tobacco production baseline survey in Serengeti, Tarime and Rorya districts, Mara Region. J Agric Sci Technol. 2012;2(4):471.
11. Li J, Muneer MA, Sun A, Guo Q, Wang Y, Huang Z, Li W, Zheng C. Magnesium application improves the morphology, nutrients uptake, photosynthetic traits, and quality of tobacco (Nicotiana tabacum L.) under cold stress. Front Plant Sci. 2023. p. 1-14.
12. Lindsay WL, Norvell WA. Development of a DTPA soil test for zinc, iron, manganese, and copper. Soil Sci Soc Am J. 1978;42(3):421-428.
13. Lisuma JB, Mbwambo AF. The effects of blended N10P18K24 fertilizer on agronomic performance of tobacco (Nicotiana tabacum L.) plants and quality of flue-cured tobacco leaf. Tanzania J Agric Sci. 2023;22(2):300-308.
14. Lisuma JB, Semoka JM, Mbwambo AF. Effect of timing fertilizer application on leaf yield and quality of tobacco. Heliyon. 2023, 9(9).
15. Lisuma J, Mbega E, Ndakidemi P. Influence of tobacco plant on macronutrient levels in sandy soils. Agronomy. 2020;10(3):418.
16. Mavroeidis A, Stavropoulos P, Papadopoulos G, Tsela A, Roussis I, Kakabouki I. Alternative crops for the European tobacco industry: A systematic review. Plants. 2023;13(2):236.
17. Naiji M, Souri MK. Nutritional value and mineral concentrations of sweet basil under organic compared to chemical fertilization. Acta Sci Pol Hortorum Cultus. 2018;17(2):167-175.
18. Rehman MZU, Rizwan M, Sohail MI, Ali S, Waris AA, Khalid H, et al. Opportunities and challenges in the remediation of metal-contaminated soils by using tobacco (Nicotiana tabacum L.): A critical review. Environ Sci Pollut Res. 2019;26:18053-18070.
19. Shaaban H, Kisetu E. Response of Irish potato to NPK fertilizer application and its economic return when grown on an Ultisol of Morogoro, Tanzania. J Agric Crop Res. 2014;2(9):188-196.
20. Subhashini DV. Effect of NPK fertilizers and co-inoculation with phosphate-solubilizing arbuscular mycorrhizal fungus and potassium-mobilizing bacteria on growth, yield, nutrient acquisition, and quality of tobacco (Nicotiana tabacum L.). Commun Soil Sci Plant Anal. 2016;47(3):328337.
21. Tlatlaa JS, Tryphone GM, Nassary EK. Impact of sowing date and level of phosphorus application on economic returns in cotton. Front Plant Sci. 2024;15:1402731.
22. Wilkinson J, Junior VW, Lopane ARM. Brazil, the Southern Cone, and China: the agribusiness connection. BRICS Initiative for Critical Agrarian Studies (BICAS) Working Paper. 2015, 16.
23. Zhang F, Zhang X. Classification and quality evaluation of tobacco leaves based on image processing and fuzzy comprehensive evaluation. Sensors. 2011;11(3):2369-2384.
24. Zhang J, Zhang Z, Shen G, Wang R, Gao L, Kong F, Zhang J. Growth performance, nutrient absorption of tobacco and soil fertility after straw biochar application. Int J Agric Biol. 2016, 18(5).
25. Sporchia F, Thomsen M, Caro D. Drivers and trade-offs of multiple environmental stressors from global rice. Sustainable production and consumption. 2021 Apr 1;26:16-32.