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 a consequent reduction in yield and quality. While farmers already practice frequent fertilizer application, different types of fertilizers are required to further improve nutrient availability and to broaden nutrient range in the soil. Polyhalite is a natural mineral consisting of potassium oxide (K₂O), sulfur trioxide (SO₃), magnesium oxide (MgO), and calcium oxide (CaO) at 14, 48, 6, and 17%, respectively, and has potential as a prolonged-release multi-nutrient fertilizer. For this study, polyhalite was examined in combination with potassium chloride (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 KCl. The present study demonstrates the pivotal role of potassium (K) application in black pepper production on acid soils. Splitting the K dose into bimonthly applications brought leaf K contents to the optimal range. Polyhalite application can partially replace KCl as the K source and, furthermore, polyhalite provides the crop with other essential nutrients such as calcium (Ca), magnesium (Mg), and sulfur (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 KCl/polyhalite, respectively, gave rise to the best crop performance and to the highest yield, produce quality, and profit.

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 262,658 tonnes yr^–1, followed by Indonesia, India, Brazil, and China; however, average yield levels in Vietnam are low, 2.44 Mg ha^–1, considerably below the yield levels of some other producing countries (FAOSTAT, 2018). In Vietnam, pepper production is concentrated in Phu Quoc Island and on the red soils of the Central Highlands. 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° N to 20° S 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 a 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; 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 3.1-3.4%, 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).

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 (Fig. 1A), and took place from January 2016 to December 2018 in a black pepper garden (cultivar Loc Ninh) planted in 2012. The site has typical humid tropic climate with a relatively cool and dry season from November-April, and a warmer rainy season from May-October (Fig. 1B), with an average yearly precipitation of 2,400 mm.

The experiment was conducted on an acidic (pHKCl: 4.5-4.6) reddish-brown soil (Rhodic Ferralsols). Soil samples were collected twice, before the first fertilization (Feb 2016) and at the end of the experiment (Nov 2018). Soil was sampled from 0-30 cm depth at five scattered locations in each experiment plot, mixed, and examined. Soil pH was measured using the KCl 1N solution method. Soil organic matter was determined using the method of Walkley and Black (1934). Nitrogen was determined using the Kjeldahl method (1884). Total P and K were determined by soil digestion in H₂SO₄ + HClO₄ and measurements using spectrophotometer and flame-photometer, respectively. Available P was determined using the Bray II method (Bray and Kurtz, 1945); available K was extracted in H₂SO₄ 0.1N solution and measured using a flame-photometer. Calcium and Mg cation exchange was measured using an atomic absorption spectrophotometer.

The experiment consisted of six treatments with four replications in a randomized complete block design (RCBD). Each 180 m² plot included 30 pepper plants. 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 KCl and polyhalite. Thus, T3 and T4 received a yearly dose of 120 kg K₂O ha^–1: T3 – KCl, exclusively; and T4 – KCl 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, KCl, and polyhalite were applied every two months, as shown in Table 2.

Five plants per plot were monitored per 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 each from 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 to exterminate yeasts, 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 stored in desiccators until nutrient analyses were carried out. Leaf N content was determined using the Kjeldahl method. To determine leaf total P and K contents, the powder was extracted using sulfuric acid (H₂SO₄) + perchloric acid (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 mealybugs) 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 drop 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 corns, 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; profit and return on investment (ROI) rates (%).

Statistical analyses were carried out between treatments within years, between different years, and over the whole 3-year experiment using ANOVA and IRRISTAT software.

Results

The experiment was conducted on an acidic reddish brown soil (Rhodic Ferralsols). Initial soil pHKCl was 4.5-4.6, and did not change during the 3-year experiment (Table 3). Soil organic matter, as well as available P increased very slightly, with no differences between treatments. Available K, which initially varied from 102.6-103.1 mg K₂O kg^–1, decreased during the experiment in treatments T2 (unfertilized control), T3, and T4, but increased in T1 (FP), T5 and T6 (Table 3). Soil available S, Ca, and Mg consistently decreased in treatments T1-T3, and increased during the experiment in treatments T4-T6 proportionally to the polyhalite dose (Table 3).

Nutrient content of diagnostic leaves was extremely sensitive to fertilizer applications. Leaf N and P concentrations before application were 2.9 and 0.03%, respectively, consistently below the recommended range. However, 20 days after fertilizer application, N and P values dramatically increased to 3.3 and 0.17%, respectively, reaching the desired range (data not shown).

Mean pre-application leaf Ca, Mg, and S concentrations were consistently below the minimum thresholds of 1.43, 0.4, and 0.09%, respectively, regardless of the fertilizer treatment. Expectedly, treatments with no polyhalite application displayed no significant response to fertilizer application events, remaining at sub-optimal levels (Fig. 3). In contrast, polyhalite applications brought about significant increases in leaf Ca, Mg, and S, all of which reached the optimum range. Increase in leaf nutrient concentrations were proportional to the polyhalite application dose (Fig. 3).

Fertilizer treatments did not have any significant influence on the infestation rate of black pepper plants to yellow leaf disease (Table 4). Treatments did have significant effects on mealybug infestation rates; the higher the K application rate the lower the infestation rate. Nevertheless, a direct influence of polyhalite on the mealybug infestation rate could not be discerned under the circumstances of the present study (Table 4).

Fertilizer treatments had an obvious effect on young fruit drop rate (Table 4). Fruit drop rate was much greater, 36.1% of the initial fruit number, in the control treatment, which did not include any K fertilizer. Fruit drop rate declined significantly in direct correlation with K application dose. No clear influence on fruit drop was observed for polyhalite application (Table 4).

Plant vegetative growth and development were clearly affected by the fertilizer treatments (Fig. 4). Elongation of first order branches, which was significantly smaller in control plants (17.6 cm), gradually increased with the rising K application dose, reaching a maximum of 24 cm at 240 and 360 kg K₂O ha^–1, significantly greater than at 120 kg K₂O ha^–1. No advantage was observed for the combined KCl with polyhalite at 120 kg K₂O ha^–1; however, branches grew longer under 240 kg K₂O ha^–1 with polyhalite (120:120 KCl:polyhalite) than under 270 kg K₂O ha^–1 with KCl as the sole K donor (Fig. 4A).

The number of second order branches also increased with the rising K application dose (Fig. 4B). There were only 2.2 branches per first order branch in control plants, which more than doubled at the higher K dose levels. Under similar, or close, K doses, the combination of KCl and polyhalite tended to result in greater numbers of second order branches than with KCl alone.

Fertilizer treatments had significant effects on all black pepper fruit quality parameters tested (Table 5). Fruit fresh/dry weight ratio, which was very high (4.52) in control fruit, declined consistently with the increasing K application dose up to 240 kg K₂O ha^–1, obtaining values around 2.75 that did not change in response to a further rise in K dose. Fruit weight, which was significantly smaller at the control (42.4 g 1,000^–1 corns), gradually increased to about 60 g 1,000^–1 corns under 240 kg K₂O ha^–1, but not further. With regard to fresh/dry weight ratio and fruit weight, the combination of KCl and polyhalite did not show any significant advantage over KCl as the sole K donor (Table 5). Fruit volume, which was much smaller in control plants, increased significantly with each rise in K dose up to a maximum of 240 kg K₂O ha^–1. Under similar or close K doses, the combination of KCl with polyhalite resulted in significantly higher fruit volumes (Table 5). Similarly, fruit density gradually increased with the rising K dose but the tendency of polyhalite to enhance fruit density was not statistically significant. Fruit piperine concentration was hardly influenced by the fertilizer treatments, although it was significantly higher at 360 compared to 120 kg K₂O ha^–1, and did not show any special influence of polyhalite (Table 5).

Black pepper fruit yield increased significantly in response to the rising K application dose, from 1.67 up to 4.01 Mg ha^–1, under zero (control) and 360 kg K₂O ha^–1, respectively (Fig. 5A). Under similar or close K application levels, the KCl:polyhalite combinations tended to result in higher yields, however, this trend was significant only under doses of 240-270 kg K₂O ha^–1. When K dose was raised from 240 to 360 kg K₂O ha^–1, yield increase was negligible (Fig. 5A).

The mean agronomic K efficiency (AKE) was highest at the lower dose of 120 kg K₂O ha^–1, and significantly declined with each rising step in K dose. Interestingly, mean AKE was significantly higher at the combined KCl and polyhalite applications, compared to KCl application alone: it was 10.4 vs. 7.9 kg kg^–1 K₂O, and 9.0 vs. 6.5 kg kg^–1 K₂O, under 120, and 240-270 kg K₂O ha^–1, respectively (Fig. 5B). The marginal AKE, which quantifies the contribution of each rising step in K application dose, ranged from 5.4-10.4 kg kg^–1 K₂O at K doses from 120-240 K₂O ha^–1, but it dropped dramatically when K dose was raised to 360 kg K₂O ha^–1 (Fig. 5C).

Cost analysis of the fertilizer treatments showed that the common farmer (FP) usually invests 142 million VND ha^–1, 30 million VND more than without any K fertilization. The use of combined KCl and polyhalite at 240 or 360 kg K₂O ha^–1 would increase farmers’ costs by 10-17 million VND, or 6.9-11.8%, respectively (Table 6). The common farmer’s revenue was 297 million VND, twice as much as without any K fertilizer application. While K dose reduction from 270 to 120 kg K₂O ha^–1 considerably cut farmer’s revenue, irrespective of the fertilizer composition, a slight decrease in K dose to 240 kg K₂O ha^–1 of combined KCl and polyhalite increased farmer’s revenue by 35 million VND ha^–1, or 11.7%. Increasing K dose to 360 kg K₂O ha^–1 increased the total revenue by about 51 million VND, compared to the common farmer (Table 6). The common farmer’s profit (net income) was 154.5 million VND ha^–1 yr^–1, about five-fold higher than without K fertilizer. The reduction of K dose from the farmers’ practice to 240 kg K₂O ha^–1, equally divided between KCl and polyhalite (T5), gave a profit increase of 25 million VND ha^–1 yr^–1, or 16.1%. A further rise in K dose to 360 kg K₂O ha^–1 added less than 10 million VND to the farmer’s profit (Table 6). The ROI, which was 108% for the common farmer, reached 118% at T5, but did not increase further at T6 (Table 6).

Discussion

Accelerating soil acidity is among the most serious agricultural challenges in humid tropic regions (Pavan, 1983; Zu et al., 2014). In acid soils, the high proton (H⁺) concentration in the soil solution rapidly weathers the fine structure of the soil particles, releasing plant-toxic Al³⁺ ions (Coulter, 1969; Ryan et al., 1993). Furthermore, the protons compete with essential nutrient ions such as K⁺, Ca²⁺, and Mg²⁺ on the cation exchange capacity of the soil particle surface, and consequently, nutrient availability for plants is significantly reduced (Fageria and Baligar, 2008; Aloka, 2016). In the present study, however, soil acidity remained very low (pH 4.5-4.7) but quite constant during the 3-year experiment and seemed unaffected by the various fertilizer treatments (Table 3).

Under the precipitation regime typical to the rainy season in the region (Fig. 1B), crop K nutrition might be severely challenged. Potassium is highly soluble and, therefore, very mobile in the soil profile (Zörb et al., 2014). Any amount of K fertilizer applied in a given moment might be leached within a few days, carried by the large water quantities passing through the rhizosphere. Therefore, K application practices in the tropics have been undergoing principal changes, the first of which is splitting the seasonal dose into several consecutive applications, as demonstrated in Table 2. Under this practice, a seasonal dose of 240 kg K₂O ha^–1 appears sufficient to maintain a stable soil K status over long periods. Higher doses slightly increased soil K availability, whereas smaller doses brought about K degradation (Table 3).

Supplementary polyhalite application gave rise to considerably higher soil Ca, Mg, and S concentrations at the end of the experiment, compared to a clear lessening of these nutrients where no supplementary fertilizer was applied (Table 3). Soil enrichment with Ca is pivotal to the buildup and maintenance of more suitable soil structure and texture, particularly on acid soils (Lie and Hue, 2001; Shainberg et al., 1989).

The approach of splitting K fertilizer dose to several applications, together with K dose increase, had an immediate effect on K uptake by black pepper plants, as indicated by the significant upsurge in leaf K concentration soon after application, which was dose-proportional (Fig. 2). The central role of K nutrition in black pepper crop performance was demonstrated by the enhanced vegetative growth (Fig. 4), the substantial reduction in fruit drop rates (Table 4), the improvement of most fruit quality parameters (Table 5), and the significant yield rise (Fig. 5). Nevertheless, the rapid reduction of leaf K to sub-optimal levels prior to each fertilizer application during the season (Fig. 2) implies a transient impact of K fertilization. Further improvement of K application practices is required to achieve more reasonable nutrient use efficiency and additional enhancements of crop performance.

Beyond soil amendment, the supplementary application of Ca, Mg, and S through polyhalite tended to improve most crop performance parameters. Although not always significant compared to similar K treatments (Fig. 4; Tables 4 and 5), the contribution of polyhalite to each parameter was augmented and clearly manifested in significantly higher yields (Fig. 5). Interestingly, polyhalite application enhanced the significant positive effect of K nutrition, strengthening black pepper plants against mealybug attack (Table 4). 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). Fortunately, throughout the present study, the basic rates of yellow pepper leaf disease were very low, leaving no room, however, for any improvement by the fertilizer treatments (Table 4).

Economic analyses unequivocally demonstrate the significance of adequate and timely K application and the benefits of supplementary polyhalite. This analysis also determines the upper fertilizer dose (240 kg K₂O ha^–1, 1:1 KCl:polyhalite), above which the profit parameters tend to diminish (Table 6). These results confirm recent results obtained with other crops grown on acid soils in Vietnam (PVFCCo, 2016a; PVFCCo, 2016b; Tam et al., 2016), and in Brazil (Vale and Sério, 2017; Bernardi et al., 2018) that have demonstrated the agricultural and economic advantages of using polyhalite as a source of K, Ca, Mg, and S.

Acknowledgement

We thank Dr. Nguyen Van Bo and Mr. Gershon Kalyan for giving their advice and assistance for the study; thank Mr Nguyễn Văn Tứ and staffs at the Central Highland Soils and Fertilizers Research Centre for taking care the field experiment. This study was funded by the International Potash Institute via the research project “Investigation of the Agronomic Efficiency of Polysulphate on Yield and Quality of Some Crops in Vietnam”.

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).
• Barbarick, K.A., 1991. Polyhalite Application to Sorghum-Sudangrass and Leaching in Soil Columns. Soil Science 151(2):159-166.
• Bernardi, A.C.C., G.B. de Souza, and F. Vale. 2018. Polyhalite Compared to KCl and Gypsum in Alfalfa Fertilization. International Potash Institute (IPI) 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.
• Bray, R.H., and L.T. Kurtz. 1945. Determination of Total, Organic, and Available Forms of Phosphorus in Soils. Soil Sci. 59:39-45.
• 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. 2nd edition, Centre d’Etude de l’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. 2018. http://www.fao.org/faostat/en/#data/QC
• Kjeldahl, J. 1883. A New Method for the Determination of Nitrogen in Organic Matter. Z Anal Chem 22:366-382.
• 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., C. Zu, 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.). Journal of 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, Ho Chi Minh City.
• PVFCCo (Petrovietnam Fertilizer and Chemicals Corporation). 2016a. Polyhalite Application Improves Tea (Camillia sinensis) Yield and Quality in Vietnam. International Potash Institute (IPI) 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 (IPI) 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. Kerala Agricultural University, Trichur. p. 65-74.
• Raman, G., and V.G. Gaikar. 2002. Microwave-Assisted Extraction of Piperine from Piper nigrum. Industrial & 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. Central Plantation Crop Research Institute, Kasaragod. p.45.
• 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. Springer, New Delhi. p. 123-130.
• Shainberg, I., M.E. Sumner, W.P. Miller, M.P.W. Farina, M.A. Pavan, and 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. Journal of Spices and Aromatic Crops 8(1):1-18.
• Tabatabai, M.A., and J.M. Bremner. 1970. A Simple Turbidimetric Method of Determining Total Sulfur in Plant Materials. Agronomy Journal, 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 (IPI) 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. IPC Annual Meeting in Lombok, Indonesia 2011. http://www.ipcnet.org/admin/data/ses/1329362855thumb.pdf
• Vale, F., and D.R. Sério. 2017. Introducing Polyhalite to Brazil: First Steps of a New Fertilizer. International Potash Institute (IPI) e-ifc 48:3-11.
• Walkley, A., and I.A. Black. 1934. An Examination of the Degtjareff Method for Determining Soil Organic Matter, and a Proposed Modification of the Chromic Acid Titration Method. Soil Sci. 37:29-38.
• Xiong, W., Z. Li, H. Liu, C. Xue, R. Zhang, H. Wu, 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.
• Yermiyahu, U., I. Zipori, I. Faingold, L. Yusopov, N. Faust, and A. Bar-Tal. 2017. Polyhalite as a Multi Nutrient Fertilizer – Potassium, Magnesium, Calcium and Sulfate. Israel Journal of Plant Sciences 64(3-4):145-157.
• Yermiyahu, U., I. Zipori, C. Omer, and Y. Beer, 2019. Solubility of Granular Polyhalite under Laboratory and Field Conditions. International Potash Institute (IPI) e-ifc 58:3-9.
• Zörb, C., M. Senbayram, and E. Peiter. 2014. Potassium in Agriculture–Status and Perspectives. Journal of Plant Physiology 171(9):656-669.
• Zu, C., Z. Li, J. Yang, H. Yu, Y. Sun, H. Tang, 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.