The effect of sulfur fertilizers in intensifying or modulating the stressful effects of NPK fertilizer on the ornamental-medicinal plant Euphorbia tirucalli L. in potted conditions

The present study investigates the effect of sulfur fertilizer on Euphorbia tirucalli L. in pot culture conditions. Plants were cultivated under 10 NPK treatments with sulfur fertilizers (granular or bentonite, with or without thiobacillus and ammonium sulfate each in two concentrations) and one NPK treatment without sulfur, compared with the control in randomized complete blocks for 4 months. Growth parameters including dry and fresh weight of plants, roots, shoots and the ratio of fresh or dry weight of roots to plants as well as protein, chlorophyll, potassium, phosphorus and sulfur content in roots or shoots were measured. NPK fertilizer did not increase phosphorus and potassium uptake but decreased root growth. Addition of sulfur fertilizers except for bentonite sulfur with a concentration of 0.8 g per 80 cm2 of soil with Thiobacillus commercial liquid fertilizer, 250 ml / l, had no effect on recovering the negative effects of NPK and even in most treatments, shoot dry weight and fresh weight decreased. Ammonium sulfate showed the most negative effect on growth parameters. Also, some sulfur treatments had a positive effect on increasing the amount of shoot protein. Chlorophyll a, b and total levels did not show a significant difference between treatments. The effect of sulfur fertilizer on soil pH was different and higher concentration of granular sulfur fertilizer containing Thiobacillus caused a more severe drop. None of the sulfur treatments increased the phosphorus of the shoot and even some treatments had a negative effect on the phosphorus content of the roots as well as the potassium content of the roots and shoots. Only in the treatment of granular sulfur fertilizer with Thiobacillus at lower concentrations, root potassium content increased. Negative effects of sulfur fertilizers were not due to increased sulfur uptake, because the sulfur content changes between treatments were not significantly different.