Potassium (K) and sulfur (S) play key roles in plant growth and development of peppers. Deficiencies of any of these elements may result in reduced plant growth leading to poor yields and/or fruit quality. In addition, soil salinity may further affect bell pepper yield.
Bell pepper is a major vegetable crop produced worldwide with China accounting for more than 50% of global production, followed by Mexico, Turkey, Indonesia, and Spain (USDA, 2013). Most bell peppers are grown in open field, although protected agriculture is becoming an increasing trend because of high yields under this system.
Cultivars of bell pepper vary by market needs with different growth habits (determinate or indeterminate), growing environments, colors, and shapes. Potassium fertilization helps maintaining water balance, sugar transport, fruit set, and development in the plant but also improves the resistance to pest and disease injuries. Fertilizer requirements for the crop vary widely from 60 to 250 kg/ha of K2O due to differences in soil types and crop yield potential. Potassium deficiency results in reduced root growth and fruit set due to inadequate carbohydrate translocation within the plant, already apparent before visible symptoms occur.
Deficiency symptoms appear first in older leaves as yellowing of leaf edge, later turning into necrotic tissue. Additionally, plant stunting, reduced fruit set and thinner fruit cell walls could be observed. As visual symptoms of K deficiency, even observed early during the growing season, can only be partially corrected by a foliar application, it is advised that adequate K is applied to the soil before planting in the form of SOP.
Figure 1. Potassium deficiency symptoms on bell pepper (source: ARRIS, Australia).
Bell pepper is also sensitive to saline conditions, where plant germination, growth and yields due to reduced nutrient uptake and root damage, are likely to occur due to high sodium concentrations in the soils. A study conducted by Sa et al., (2017) showed bell pepper emergence was reduced from 55% to 20% at high salinity conditions. In another study, Navarro et al., (2002) compared the effects of Cl- and SO42- ions under saline conditions in bell pepper grown in a hydroponic system. Bell pepper yield and quality decreased as increasing salinity in solution. However, detrimental effects were steeper in Cl- containing treatments. Similarly, Wahb-Allah et al. (2013) showed that increasing salinity levels to 3.5 dS/m resulted in up to 50% fruit yield compared to the control. Therefore, using low-salt fertilizer index fertilizer, such as SOP, helps promoting growth and yield while avoiding potential salinity damage of soil salinity build-up.
With regard to the optimum K rate for bell pepper, a study was conducted to determine the effects of K doses in pepper fruit yields. Doses of K ranged from 0 to 450 kg K2O/ha and were applied using SOP as K source. Doses showed significant differences with increasing fruit yields as increasing K doses (Figure 2). Additionally, quality parameters such as dry matter content and vitamin C increased as result of K application with the highest values when using either 135 or 270 kg K2O/ha (data not shown).
Figure 2. Effects of K doses on bell pepper fruit yields (Modified from Ni, 2002).
Soil testing early in the season, conducted before planting, helps to determine the right amount of K to be applied. Tissue K levels during the season should range between 4% and 6% (Silva and Uchida, 2000). Potassium tissue levels below 2% are considered deficient. Monitoring plant tissue nutrient levels during the season may help to identify and alleviate deficiencies by foliar application, especially under conditions, when supply through roots is inhibited, e.g. by limited soil moisture during dry spells. It is recommended to apply sufficient K, e.g., granular SOP as preplant to the soil and to cover peak demands during the growth period to apply additional soluble SOP through foliar applications to supply fast growing plants with optimum levels of K and S.
ARRIS, Australia. (2017). Capsicum nutrient deficiencies and toxicities. Available at: http://www.growingcapsicums.com.au/pdf/1_plant_nutrition/nutrient_deficiency_and_tox_in_caps.pdf
Navarro, J. M., Garrido, C., Carvajal, M., and Martinez, V. (2002). Yield and fruit quality of pepper plants under sulphate and chloride salinity. The Journal of Horticultural Science and Biotechnology, 77(1), 52-57.
United States Departments of Agriculture. (2003). Commodity highlight: Bell peppers. Available at: https://www.ers.usda.gov/webdocs/publications/39535/41021_vgs353sa1.pdf?v=42125
Sá, F. V. D. S., Souto, L. S., de Paiva, E. P., Araújo, E. B., de Oliveira, F. A., de Mesquita, E. F., Ferreira-Neto, M. and Dantas, J. S. (2017). Initial Development and Tolerance of Bell Pepper (Capsicum annuum) Cultivars under Salt Stress. Journal of Agricultural Science, 9(11), 181.
Silva, J. and Uchida, R. (eds). (2000). Plant nutrient management in Hawaii’s Soils. Approaches for Tropical and Subtropical agriculture. College of Tropical Agriculture and Human Resources, University of Hawaii at Manoa. Available at: https://www.ctahr.hawaii.edu/oc/freepubs/pdf/pnm4.pdf
Smith, H. (2014). Booting your harvest with potassium supplements. Maximum yield newsletter. Available at: https://www.maximumyield.com/boosting-your-harvest-with-potassium-supplements/2/1251
Wahb-Allah, M. A. (2013). Responses of some bell-pepper (Capsicum annuum L.) cultivars to salt stress under greenhouse conditions. Journal of Agricultural & Environmental Sciences of Damanhour University, 12(1).
Ni, W. (2002). Yield and quality of fruits of solanaceous crops as affected by potassium fertilization. Available at: http://www.ipni.net/publication/bci.nsf/0/DAE4BFAF7DCF638185257BBA0065CC64/$FILE/Better%20Crops%20International%202002-1%20p06.pdf