Effect of Calcareous Soil Amendments on Quantitative and Qualitative Yields of Black Seed (Nigella sativa L.)

Phosphorus deficiency due to the chemical function of calcareous compounds is considered as one of the most important problems in the sustainable nutrition of black seed (Nigella sativa L.), especially in arid and semi-arid regions of Iran. Furthermore, low soil organic matter content in these areas can adversely affect phosphorus availability and thus challenge the sustainable production of black seed. Accordingly, biochemical modification of calcareous soils based on ecological approaches including biological sulfur oxidation or increased phosphorus solubility by organic fertilizers, such as vermicompost, may alleviate these problems.

In order to some approaches for increasing the soil soluble phosphorus and its effects on quantitative and qualitative yields of black seed in a calcareous soil, a field experiment was conducted at Faculty of Agriculture, Ferdowsi University of Mashhad, Iran, in the growing years of 2010-2011. A complete randomized block design based on the factorial arrangement with three replications and 12 treatments were used. The resources of soil amendment (1- ‍Control, 2- Vermicompost + Thiobacillus, 3- Sulfur + Thiobacillus and 4- Vermicompost + sulfur + Thiobacillus) and three levels of phosphorus (0, 30 and 60 kg.ha-1) were the first and second experimental factors, respectively. In order to evaluate the experimental treatments, plots were designed with 3 m long and 2 m width, 0.5 m apart from each other. To eliminate all influence of lateral effects, 1 m alley was kept between blocks. Vermicompost (pH: 7.59, organic carbon: 34.76%, total nitrogen: 2.31%, and total phosphorus: 1.78%), sulfur (along with Thiobacillus) and diammonium phosphate were incorporated into the soil before seed sowing. The analysis of variance and the least significant difference test (LSD: 0.05) were performed using SAS 9.3 software (SAS, 2011).

According to the results, the interaction effect of soil amendment × phosphorus rate on plant height, leaf area index and plant dry weight of black seed was significant. Biological and grain yields of black seed notably affected by the interaction between soil amendment × phosphorus rate. Based on our results, plant height significantly increased with increasing levels of phosphorus fertilizer. In addition, phosphorus solubility due to sulfur + Thiobacillus application considerably improved this index compared to control. From the results, grain and biological yields of black seed by applying the sulfur + Thiobacillus treatment were considerably higher than phosphorus 30 or 60 kg.ha-1 treatments. The highest biological and grain yields of black seed were observed in vermicompost + Thiobacillus or vermicompost + sulfur + Thiobacillus treatments. With applying the mentioned treatments, grain yield of black seed were significantly increased (by 63.3 and 84.4%, respectively) compared to control treatment. In general, optimal growth and yield of crops are dependent on balanced nutrient uptake. Given the positive correlation between sulfur and nitrogen, as well as between phosphorus and nitrogen, the specific role of biological sulfur oxidation combined with vermicompost can be due to increased phosphorus solubility in the soil and ultimately facilitated uptake nitrogen.

In alkali soils, biological sulfur oxidation individually or in combination with vermicompost treatment can be a suitable approach to increase the soil soluble phosphorus and quantitative and qualitative yields of black seed. The results also suggested that the application of phosphorus fertilizers in soils dominated by calcareous compounds might have limited efficacy. As a result, the biochemical reactivity of Nigella plant to phosphorus uptake is improved only in conditions where the rhizosphere can be balanced in terms of soil pH and phosphorus solubility. Acknowledgements The authors acknowledge the financial support of the project by Vice President for Research and Technology, Ferdowsi University of Mashhad, Iran.