Abstract

Acid soils significantly challenge the rapidly growing production of black pepper (Piper nigrum L.) in Vietnam. The perennial vines suffer from malnutrition, which gradually leads to plant deterioration, susceptibility to various diseases, and consequent reduction in yield and quality. While farmers already practice frequent fertilizer application, different types of fertilizers are required in order to further improve nutrient availability and to broaden nutrient range in the soil. Polyhalite is a natural mineral consisting of K₂O, SO₃, MgO, and CaO at 14, 48, 6, and 17%, respectively, and has potential as a slow-release multi-nutrient fertilizer. For this study, polyhalite was examined in combination with MOP (KCl), in equal proportions, to provide doses of 120, 240, and 360 kg K₂O ha^-1 yr^-1, split into six applications during the year. These treatments were compared to doses of zero (control), 120, and 270 (farmers’ practice) kg K₂O ha^-1 applied solely as MOP.

The present study demonstrates the pivotal role of K application in black pepper production on acid soils. Splitting the K dose into bimonthly applications brought leaf K contents to the required range. Polyhalite application can partially replace MOP as the K source and, furthermore, polyhalite provides the crop with other essential nutrients such as Ca, Mg, and S. The supplemental nutrients strengthened the black pepper vines against mealybug attacks, supported better crop performance, and significantly improved yield and produce quality, which resulted in higher profits. The combination of 120/120 kg K₂O ha^-1 of MOP/polyhalite, respectively, gave rise to the best crop performance and to the highest yield, produce quality, and profit. However, the exact contribution of polyhalite to the soil nutrient status requires further research.

Keywords: Acid soil; calcium; magnesium; Piper nigrum L.; polyhalite; potassium; Pseudococcus citri; sulfur.

Introduction

Black pepper (Piper nigrum L., Piperaceae), the ‘king of spices’, originated in the tropical evergreen forests of the Western Ghats of India (Sivaraman et al., 1999), and is one of the oldest spices known to humankind. Global black pepper production is led by Vietnam, with 216,432 Mg yr^-1, followed by Indonesia, India, Brazil, and China, with 82,167; 55,000; 54,425; and 34,587 Mg yr^-1, respectively (FAOSTAT, 2016). In Vietnam, pepper production is concentrated on the red soils of Central Highlands and in Phu Quoc Island. The increasing prices in recent years have led to further expansion of pepper cultivation to other regions in Vietnam.

Black pepper grows successfully between 20° north to 20° south of the equator and from 0 to 1,500 m above sea level. It is a plant of the humid tropics, requiring 1,250-2,000 mm of rainfall, tropical temperatures and high relative humidity with little variation in day length throughout the year (Sivaraman et al., 1999). Black pepper grows well on soils ranging from heavy clay to light sandy clays rich in humus with porous friable nature, well drained, but still with ample water retention. Soils with near neutral pH, high organic matter and high base saturation with calcium (Ca) and magnesium (Mg) were found to enhance black pepper productivity (Mathew et al., 1995).

Nutrient removal and composition of black pepper vines varies with variety, age, season, soil type and management. Sim (1971) estimated the macronutrient removal by black pepper as 233, 39, 207, 30, and 105 kg ha^-1 of nitrogen (N), phosphorus pentoxide (P₂O₅), potassium oxide (K₂O), magnesium oxide (MgO), and calcium oxide (CaO), respectively, and later estimates did not differ significantly (Sivaraman et al., 1999). The critical stages of nutrient requirement for black pepper are during initiation of flower primordia and flower emergence, and during berry formation and development (Raj, 1978). Nybe et al. (1989) reported that phosphorus (P) and potassium (K) had greater importance than N in enhancing black pepper yields. Leaf macronutrient concentration ranges required for normal pepper development were estimated to be 3.1-3.40%; 0.16-0.18%; and 3.4-4.3% for N; P; and K, respectively. The suitable leaf concentration ranges of sulfur (S), Ca, and Mg should be 0.09-0.29%, 1.42-3.33%, and 0.40-0.69%, respectively (de Waard, 1969; Phan Huu Trinh et al., 1988).

Black pepper is a surface feeder; feeding roots are concentrated in the top 50-60 cm layer of the soil. In the past, black pepper, as well as coffee and tea plantations were established on virgin forests after clearing the vegetation (de Geus, 1973; Chiem and Nhan, 1974; D’haeze et al., 2005). However, owing to heavy rains and unsustainable soil management practices, soils became poor in fertility and balanced manuring of crops became essential (de Waard, 1969). Over the course of time, sustainable traditional manuring practices were replaced with manufactured chemical fertilizers and, consequently, dieback of branches, foliar disorders, low yields and considerable reduction in life span of vines were observed (de Waard, 1969; Raj, 1978; Sivaraman et al., 1999; Zu et al., 2014).

Soil acidity is an acute problem in the humid tropics, where annual precipitation exceeds 2,000 mm or frequent heavy rainfall events take place. Under such environmental circumstances, soil acidification is a natural process; appreciable quantities of exchangeable bases (Ca²⁺, Mg²⁺, and K⁺) are leached from the soil’s surface layer. Subsequently, the rising relative concentrations of exchangeable hydrogen (H⁺) and exchangeable aluminum (Al³⁺) reduce soil pH, and hence, are responsible for soil acidification (Coulter, 1969; Pavan, 1983). The content of mobile Al in soils with pH below 5.5 is rather high, which leads to increased uptake of toxic Al by plants, root growth retardation and dysfunction (Ryan et al., 1993; Zu et al., 2014), and to consequent diminishing nutrient uptake (Duchanfour and Souchier, 1980). Where soil pH declines below 5.5, the availability of plant nutrients, particularly N, P, K, Ca, Mg, S, molybdenum (Mo), and boron (B), decrease significantly (Zu et al., 2014; Aloka, 2016).

Overcoming the direct and indirect effects of acid soils on crop performance requires complex simultaneous solutions. Repeated liming is useful in many cases as a practice aimed to reconstruct soil pH (Fageria and Baligar, 2008). However, liming has not always been successful due to its low solubility in water, very slow effect, unsuitable methods of application, and high cost (Liu and Hue, 2001). Gypsum (CaSO₄·2H₂O) was proposed as an effective amendment for subsoil acidity (Shainberg et al., 1989) and, in a recent study, demonstrated significant enhancement of black pepper crop performance (Aloka, 2016). Nevertheless, along with efforts to reduce detrimental effects of acid soils, consistent nutrient availability throughout the year must be taken care of. In this respect, the microflora of black pepper rhizosphere has been recently explored (Xiong et al., 2015; Li et al., 2016), in order to influence beneficial chemical processes in the soil. However, in the absence of adequate soil fertility, and under a frequent precipitation regime, any kind of external nutrient supply should address this point. Splitting the fertilizer dose, where practical, is one promising solution. Slow-release fertilizers provide another solution; fertilizer efficiency to supply N and P significantly improved when slow-release ‘nimin’ (nitrification inhibitor) coated urea (Sadanandan and Hamza, 1993) and mussoorie rock phosphate (Sadanandan, 1986), respectively, were applied to black pepper. Still, more stable K fertilizers are needed, as well as long-lasting sources of Ca and Mg.

Polysulphate™ (produced by Cleveland Potash Ltd., UK) is the trade mark of the natural mineral ‘polyhalite’, which occurs in sedimentary marine evaporates, and consists of a hydrated sulfate of K, Ca, and Mg with the formula: K₂Ca₂Mg(SO₄)₄·2(H₂O). The deposits found in Yorkshire, in the UK, typically consist of K₂O: 14%, SO₃: 48%, MgO: 6%, CaO: 17%. As a fertilizer providing four key plant nutrients - S, K, Mg, and Ca - polyhalite may offer attractive solutions to crop nutrition. In addition, polyhalite releases the nutrients considerably slower than other K-containing fertilizers, which may also be significant for soil K availability. Once a proper application is established, polyhalite may not only provide a significant part of crop K requirements, but also supply secondary macronutrients that are essential under the present cropping environment of black pepper in Vietnam.

The objectives of the present study were to evaluate the effectiveness of polyhalite as a supplementary fertilizer on black pepper performance, yield, quality, and economic efficiency, and to offer new alternatives for black pepper fertilization under the conditions of the Central Highlands of Vietnam.

Materials and methods

The experiment was located in the Nguyễn Văn Tứ household, H’Lốp commune, Chư Sê district, Gia Lai province of the Central Highlands of Vietnam (Map 1), and took place from January 2016 to December 2017, in a black pepper garden (cultivar Loc Ninh), planted in 2012.

The experiment was conducted on an acidic (pH_KCl: 4.5-4.6) reddish brown soil (Rhodic Ferralsols). Organic matter (OM) and total N contents were relatively high (OM: 3.50-3.62%; N: 0.18%). Total P was high, 0.21-0.23%, however, available P was poor (42.5 mg P₂O₅ kg^-1). Available K and S were moderate (102.6-103.1 and 21.9-22.5 mg kg^-1 K₂O and SO₄, respectively). Alkaline cations, Ca²⁺ and Mg²⁺ were very low, 1.89 and 1.59 cmol kg^-1, respectively.

The experiment consisted of six treatments with four replications in a randomized complete block design (RCBD). Each plot included 30 pepper plants (180 m²). A detailed description of the fertilization regime and treatments is given in Tables 1 and 2. Treatments included farmers’ practice (FP) as the first control (T₁), and a second control (T₂), which received only the standard N and P fertilizers. Treatments T₃-T₆ were applied with the standard N and P fertilizers, but differed in the rate and combination of MOP and polyhalite, to provide a consistent rate of K₂O supply. Thus, T₃ and T₄ received a yearly dose of 120 kg K₂O ha^-1: T₃ - MOP, exclusively; and T₄ - MOP and polyhalite, 60 kg K₂O ha^-1 each, but 100 and 429 kg fertilizer ha^-1, respectively. In treatments T₅ and T₆, K rates increased to 240 and 360 kg K₂O ha^-1, respectively, equally divided between MOP and polyhalite (Table 1). FMP (fused magnesium phosphate) was applied twice a year, during May-June and November-December. Urea, MOP, and polyhalite were applied every two months, as shown in Table 2.

Five plants per plot were monitored each year for vegetative growth at the beginning and end of the rainy season. In each plant, the length of four branches of the first order were measured and their elongation during the rainy season was calculated. Similarly, the number of lateral (second order) branches added during the rainy season to four tagged branches was counted.

Diagnostic leaves were sampled twice in July, before and 20 days after fertilizer application. The leaves (eight leaves from each of three trees plot^-1, from four different directions around the tree) were collected from non-bearing internodes of fruit-bearing branches. Leaves were heated for an hour at 105-110°C and then dried at 80°C for 8-12 hours, until a constant weight was achieved. The dry leaves were milled to fine powder, which was kept in desiccators until nutrient analyses were carried out. Leaf N content was determined using the Kjehldahl method. To determine leaf total P and K contents, the powder was extracted using H₂SO₄+ HClO₄, and then measured using a spectrophotometer and a flame-photometer, respectively. Leaf Ca and Mg were determined using an atomic absorption spectrometer. Leaf S content was determined using the turbidity comparison method (Tabatabai and Bremner, 1970).

Pest examinations (yellow leaf disease and mealibugs) were carried out monthly and the rate of infested plants was determined. Additionally, young fruit were counted on first order branches before the rainy season and, again towards harvest, giving rise to the fruit abscission rates. At harvest, the total yield was determined (Mg ha^-1). Black pepper quality traits, such as fresh/dry weight ratio, weight and volume of 1,000 acorns, and fruit density were determined. Piperine content in fruit was extracted and determined following Raman and Gaikar (2002). The evaluation of the economic efficiency included: total income (calculated according to yield); quality; current produce price in Million VND; total cost (including fertilizers); absolute profit; and, profit rate (%).

Results and discussion

Leaf nutrient concentrations were monitored during the year, prior to and 20 days after each fertilizer application. Nitrogen and P leaf contents prior to fertilizer application in July were 2.85-2.91% and 0.13-0.14%, respectively - under the minimum threshold. Twenty days after fertilization, leaf N and P contents increased significantly to 3.16-3.21% and 0.17-0.18%, respectively, within or above the optimum range, in all treatments. Thus, the N and P fertilizer levels and the application regime practiced in the present study (300 kg N, 100 kg P₂O₅), as well as the farmers’ practice (345 kg N, 90 kg P₂O₅), are quite suitable for black pepper cropping on reddish brown soil in Chu Se district, Gia Lai province.

The annual K dose was split into six applications during the year (Table 2). The effects of fertilizer treatments on leaf K contents were very similar in both years (Fig. 1). Pre-application leaf K contents were significantly lower than the minimum threshold of 3.4%. Post-application measurements revealed an obvious recovery in leaf K content in all treatments but the control, which continued to decline.

The provision of 120 kg K₂O ha^-1 in treatments 3 and 4 considerably increased leaf K content; nevertheless, values remained below the minimum threshold for pepper. At this low K dose, no significant advantage for the 1:1 combination of KCL and polyhalite was observed. The higher K doses applied in T₁, T₅, and T₆, ranging from 240-360 kg K₂O ha^-1, brought post-application leaf K contents into the optimum range. Yet, even with the significant positive response to the rising K dose, leaf K contents remained substantially lower than the upper threshold of 4.3%.

The fluctuations in leaf K content, and particularly its decline below the optimum range, suggests that under the environmental circumstances in the Central Highlands of Vietnam, plants’ opportunities for K uptake are too short to refill the nutrient holding capacity. This may also indicate that the agronomic efficiency of the K application regime can be further improved.

In addition to K, polyhalite is a source of S, Ca, and Mg. Prior to fertilizer application, leaf contents of these nutrients were below the minimum thresholds 0.09, 1.43, and 0.4% for S, Ca, and Mg, respectively, for all treatments (Fig. 2). Following the bimonthly fertilizer application in July, nutrient levels continued to decrease in treatments supplied solely with MOP and the unfertilized control (T₁-T₃). Whereas in plants supplied with combined MOP and polyhalite (T₄-T₆), leaf S, Ca, and Mg increased significantly, reaching but not exceeding the optimum range. Generally, the greater the polyhalite dose, the higher the corresponding nutrient content in the leaves. However, similar to the case of K, the effect seems transient and the plants possibly experience considerable fluctuations in leaf nutrient contents during the year.

Two major disorders quite often infest black pepper crops: the yellow pepper leaf (de Waard, 1986), and mealybugs (Tang Ton and Buu, 2011). The yellow leaf disease - named after its most noticeable symptom - is a multi-pathogen disease, which begins with a nematode (Meloidogyne incognita) attack that injures the roots, and continues with various fungi soil-borne opportunist pathogens such as Fusarium spp., Phytophthora spp., Pythium spp., etc. that cause root rot diseases. Once infected, the old leaves’ veins turn yellow, a symptom which gradually expands to the whole pepper leaf. Consequently, infected plants shed leaves and stems, their canopy becomes scattered, and they die one to three years after infection. The disease has substantial effects on crop yield and quality as flowering, and fruit set and development, are significantly damaged. Mealybugs (Pseudococcus citri) attack weak plants and impact on their carbon and energy balance, and hence, reduce pepper fruit yield and quality. In addition, mealybugs are known as vectors of various plant virus diseases that negatively affect crop performance (Selvarajan et al., 2016). A well-balanced crop nutrition is very efficient at preventing pests and diseases (Tang Ton and Buu, 2011).

In the present study, in both years, the basic rates of yellow pepper leaf disease were very low, leaving no room for any improvement by the fertilizer treatments (data not shown). In contrast, mealybug infestation rate in the control was 8-10% (Fig. 3). Potassium applications significantly reduced mealybug infestation rates in a dose-dependent manner. Furthermore, the results of treatments T₃ and T₄ may indicate that polyhalite application is more efficient than MOP in strengthening black pepper plants against mealybugs attack (Fig. 3). This indication is supported by recent findings demonstrating that S fertilization increases glucosinolate production in plant leaves (Bohinc et al., 2012; Santos et al., 2018) and subsequently, the plants’ effectiveness against generalist insect pathogens rises significantly (Kos et al., 2012).

Potassium applications significantly enhanced the vegetative growth of black pepper plants (Fig. 4). While the elongation of primary branches at the high K dose increased by 30%, compared to the unfertilized control, the number of secondary branches more than doubled, providing a substantially larger and stronger platform for the reproductive phase and consequently, for higher yield and better quality. Premature fruit abscission was dramatically reduced from 35% at the unfertilized control to 20% at the farmers’ practice, which received 270 kg K₂O ha^-1 yr^-1. Treatments T₃ and T₄, with a significantly lower K dose (120 kg K₂O ha^-1 yr^-1), displayed higher fruit abscission rates, while at the higher K doses (240 and 360 kg K₂O ha^-1 yr^-1), fruit shedding rates were further reduced to 16.9 and 16.2%, respectively (Table 3).

Black pepper yields of the unfertilized control in 2016 and 2017 were poor, 1.84 and 1.55 Mg ha^-1, respectively, significantly lower than in the other treatments (Fig. 5). MOP application at 120 kg K₂O ha^-1 yr^-1 gave rise to significant yield increases of 56 and 66% more than the control, respectively. A similar dose, equally divided between MOP and polyhalite, had no further significant effects on yields. Farmers’ practice, with a higher K dose of 270 kg K₂O ha^-1 yr^-1 applied as MOP, produced a much higher yield, averaging 3.45 Mg ha^-1. The greatest yields, ranging from 3.7 to 4.1 Mg ha^-1, were obtained at K doses of 240 and 360 kg K₂O ha^-1 yr^-1, which were equally split between MOP and polyhalite (Fig. 5). There was no significant difference in yields between these two doses, suggesting that, at least under the circumstances of the present study, a yearly dose of 240 kg K₂O ha^-1 would be sufficient. When compared to the farmers’ practice, the advantage of polyhalite becomes obvious: a smaller K input of 240 kg K₂O ha^-1, 50% of which originated from polyhalite, resulted in 8-14% more yield than obtained at 270 kg K₂O ha^-1 applied with MOP.

Conclusions

The results of this study demonstrate the pivotal role of K application in black pepper cultivation on acid soils. Splitting the K dose into bimonthly applications brought leaf K contents to the required range. Polyhalite application can partially replace MOP as the K source and, furthermore, polyhalite provides the crop with other essential nutrients such as Ca, Mg, and S. The supplemental nutrients strengthened the black pepper vines against mealybug attacks, supported better crop performance, and significantly improved yield and produce quality, resulting in higher profits. However, the exact contribution of polyhalite to soil nutrient status requires further research.

Acknowledgements

This research was supported by the International Potash Institute (IPI). We thank Mr. Gershon Kalyan, IPI Coordinator for Southeast Asia for his support and advice to the research.

References

• Aloka, Y.G. 2016. Gypsum as a Soil Ameliorant for Black Pepper (Piper nigrum L.) in Acid soils of Wayanad (doctoral dissertation, College of Agriculture, Padannakkad).
• Bernardi, A.C.C., G.B. de Souza, and F. Vale. 2018. Polyhalite Compared to KCl and Gypsum in Alfalfa Fertilization. International Potash Institute e-ifc 52:3-9.
• Bohinc, T., S.G. Ban, D. Ban, and T. Stanislav. 2012. Glucosinolates in Plant Protection Strategies: A Review.Archives of Biological Sciences 64(3):821.
• Chiem, N.T., and D.T. Nhan. 1974. Change of Some Soil Chemical and Physical Properties in Basaltic Soil Cultivated with Coffee and Rubber in Phu Quy. Soil and Fertilizer Research 4:3-26.
• Coulter, B.S. 1969. The Chemistry of Hydrogen and Al Ions in Soils, Clay Minerals and Resins. Soils Fertil. 32:215-253.
• De Geus, J.G. 1973. Fertilizer Guide for Tropicals and Subtropicals. 2^nd edition, Centre d’Etude de I’ Azote Zurich. p. 440-471.
• de Waard, P.W.F. 1969. Foliar Diagnosis, Nutrition and Yield Stability of Black Pepper (Piper nigrum L.) in Sarawak (doctoral dissertation, Koninklijk Instituut voor de Tropen).
• de Waard P.W.F. 1986. Current State and Prospective Trends of Black Pepper (Piper nigrum L.) Production. Outlook Agric. 15(4):186-195.
• D’haeze, D., J. Deckers, D. Raes, T.A. Phong, and H.V. Loi. 2005. Environmental and Socio-Economic Impacts of Institutional Reforms on the Agricultural Sector of Vietnam: Land Suitability Assessment for Robusta coffee in the Dak Gan Region. Agriculture, Ecosystems and Environment 105:59-76.
• Duchanfour, P., and B. Souchier. 1980. pH and Lime Requirement. Comptes Rendus des Seances de l’Academie d’Agriculture de France 66(4):391-399.
• Fageria, N.K., and V.C. Baligar. 2008. Ameliorating Soil Acidity of Tropical Oxisols by Liming for Sustainable Crop Production. Adv. Agron. 99:345-399.
• FAOSTAT. 2016.
• Kos, M., B. Houshyani, R. Wietsma, P. Kabouw, L.E. Vet, J.J. van Loon, and M. Dicke. 2012. Effects of Glucosinolates on a Generalist and Specialist Leaf-Chewing Herbivore and an Associated Parasitoid. Phytochemistry 77:162-170.
• Li, Z., Zu, C., C. Wang, J. Yang, H. Yu, and H. Wu. 2016. Different Responses of Rhizosphere and Non-Rhizosphere Soil Microbial Communities to Consecutive Piper nigrum L. Monoculture. Scientific Reports 6:35825.
• Liu, J., and N.W. Hue. 2001. Amending Subsoil Acidity by Surface Application of Gypsum, Lime and Compost. Commun. Soil Sci. Pl. Anal. 32:2117-2132.
• Mathew, P.G., P.A. Wahid, and G. Sreekandan Nair. 1995. Soil Fertility and Nutrient Requirement in Relation to Productivity in Black Pepper (Piper nigrum L.). J. Plantation Crops 23:109-115.
• Nybe, E.V., P.C.S Nair, and P.A. Wahid. 1989. Relationship of Foliar Nutrient Levels with Yield in Black Pepper (Piper nigrum L.). Tropical Agric. (Trinidad) 66:345-349.
• Pavan, M.A. 1983. The Relationship of Non-Exchangeable, Exchangeable and Soluble Al with pH, CEC, Al Saturation Percentage and Organic Matter in Acid Soils of Parana State, Brazil. Rev. Bras. Cien. Solo. 7:39-46.
• Phan Huu Trinh, Tran Thi Mai, Vu Dinh Thang, and Bui Dac Tuan. 1988. Pepper Cultivation Techniques, Agriculture Publishing House.
• PVFCCo (Petrovietnam Fertilizer and Chemicals Corporation). 2016a. Polyhalite Application Improves Tea (Camillia sinensis) Yield and Quality in Vietnam. International Potash Institute e-ifc 46:22-29.
• PVFCCo (Petrovietnam Fertilizer and Chemicals Corporation). 2016b. Polyhalite Application Improves Coffee (Coffea robusta) Yield and Quality in Vietnam. International Potash Institute e-ifc 47:12-19.
• Raj, H.G. 1978. A Comparison of the System of Cultivation of Black Pepper - Piper nigrum L. in Malaysia and Indonesia. In: Silver Jubilee Souvenir, Pepper Research Station, Panniyur. p. 65-74. Kerala Agricultural University, Trichur.
• Raman, G., and V.G. Gaikar. 2002. Microwave-Assisted Extraction of Piperine from Piper nigrum. Industrial and Engineering Chemistry Research 41(10):2521-2528.
• Ryan, P.R., D.J.M. Tomaso, and L.V. Kochian,. 1993. Aluminum Toxicity in Roots: An Investigation of Spatial Sensitivity and the Role of Root Cap. J. Exp. Bot. 44:437-446.
• Sadanandan, A.K. 1986. Efficiency of P Sources for Pepper. In: First Workshop on the Role of Slow Release Fertilizers in Plantation Crops. p. 45. Central Plantation Crop Research Institute, Kasaragod.
• Sadanandan, A.K., and S. Hamza. 1993. Comparative Efficiency of Slow Release N Fertilizers on Transformation of Nitrogen and Yield Response of Black Pepper in an Oxisol. J. Plantation Crops 21(Suppl.):58-66.
• Santos, N.A., N.C. Teixeira, J.O.S. Valim, E.F.A. Almeida, M.G.A. Oliveira, and W.G. Campos. 2018. Sulfur Fertilization Increases Defense Metabolites and Nitrogen but Decreases Plant Resistance against a Host-Specific Insect. Bulletin of Entomological Research 108(4):479-486.
• Selvarajan, R., V. Balasubramanian, and B. Padmanaban. 2016. Mealybugs as Vectors. In: Mealybugs and their Management in Agricultural and Horticultural Crops. p. 123-130. Springer, New Delhi.
• Shainberg, I., M.E. Sumner, W.P. Miller, M.P.W. Farina, M.A. Pavan, M.V. Fey. 1989. Use of Gypsum on Soils: A Review. Adv. Soil Sci. 9:1-11.
• Sim, E.S. 1971. Dry Matter Production and Major Nutrient Contents of Black Pepper (Piper nigrum L.) in Sarawak. Malaysian Agric. J. 48:73-93.
• Sivaraman, K., K. Kandiannan, K.V. Peter, and C.K. Thankamani. 1999. Agronomy of Black Pepper (Piper nigrum L.) - A Review. J. Spices and Aromatic Crops 8(1):01-18.
• Tabatabai, M.A., and J.M. Bremner. 1970. A Simple Turbidimetric Method of Determining Total Sulfur in Plant Materials 1. Agron. J. 62(6):805-806.
• Tam, H.M., D.M. Manh, T.T. Thuan, H.H. Cuong, and P.V. Bao. 2016. Agronomic Efficiency of Polyhalite Application on Peanut Yield and Quality in Vietnam. International Potash Institute e-ifc 47:3-11.
• Tang Ton, N., and B.C. Buu. 2011. How to Prevent the Most Serious Diseases of Black Pepper (Piper nigrum L.) - A Case Study of Vietnam. PC Annual Meeting in Lombok, Indonesia 2011 www.ipcnet.org.
• Vale, F., and D.R. Sério. 2017. Introducing Polyhalite to Brazil: First Steps of a New Fertilizer. International Potash Institute e-ifc 48:3-11.
• Xiong, W., Z. Li, H. Liu, C. Xue, R. Zhang, H. Wu, R. Li, and Q. Shen. 2015. The Effect of Long-Term Continuous Cropping of Black Pepper on Soil Bacterial Communities as Determined by 454 Pyrosequencing. PloS One 10(8):e0136946.
• Zu, C., Z. Li, J. Yang, H. Yu, Y. Sun, H. Tang, R. Yost, and H. Wu. 2014. Acid Soil is Associated with Reduced Yield, Root Growth and Nutrient Uptake in Black Pepper (Piper nigrum L.). Agricultural Sciences 5(05):466.