The Influence of Soil Application of Organic Fertilizers and Foliar Application of Growth Stimulants on the Growth and Physiological Indices of Lepidium sativum L.

Lepidium sativum is a widely consumed vegetable worldwide, and plays an essential role in ensuring human health by enhancing the body's resistance against free radicals. The agricultural management practices depend on the using fertilizers to increase crops yield. A proper plant nutrition is a crucial factor to improve vegetables quality. Organic fertilizers are essential factor for modulating pH, increasing cation exchange capacity, increasing the activity of microorganisms, and improving soil's physical and chemical properties. Using seaweeds extract in agricultural systems ensures a sufficient supply of nutrients, and increases the efficacy of sustainable farm systems. Plants with high vegetative growth accumulate excess nitrogen in their tissues. Nitrate has a detrimental effect on human health due to its conversion to nitrite which is a carcinogenic compound in combination with amines. Due to adverse effects of chemical fertilizers application and nitrate accumulation in vegetables; using organic fertilizers and seaweed extract could be a reliable solution to improve crop qualitative parameters. The high levels of soil pH in the arid and semi-arid regions of Iran lead to considerable reduction in nutrients absorption rate, subsequently leading to lower crops yield. Therefore, it seems necessary to use these organic fertilizers during crops cultivation.           

To investigate the influence of soil application of organic fertilizers (vermicompost and poultry manure) and foliar application of some growth stimulants (Ascophyllum nodosum seaweed extract and Dobogen bio-stimulant) on some physiological traits of sativum L., a factorial experiment was performed based on a randomized complete blocks design with three replications at Research Farm of Azarbaijan Shahid Madani University, Iran. Plowing was done in autumn. At early spring, watercress seeds were directly sown in experimental plots after adding organic fertilizers (0 (control), 5 and 10 ton ha-1 of vermicompost and poultry manure). Each plot was 1×1 m. Seeds were sown at depth of 0.2 cm and a distance of 20 cm between rows. First time foliar application of seaweed extract and Dobogen solution (0 (control), 2 and 4 ml L-1) was carried out at three-leaf stage. The second time foliar application was done two weeks later. Dobogen contained salicylic acid (10%), soluble boron (5%) and soluble molybdenum (0.005%). Seaweed extract contained magnesium (0.1%), boron (16%), calcium (0.06%), iron (25%), cytokinin (0.01%), nitrogen (0.1%), phosphoric acid (0.05%) and soluble potassium (4.2%). Weeding was done mechanically. Plants were harvested to evaluate the characteristics, 35 d after seed sowing.

Total nitrogen, nitrate, total soluble solids, ascorbic acid, flavonoids, and total phenolic compounds were affected by the experimental treatments. The highest accumulation rate of nitrogen and nitrate was recorded in poultry manure 10 ton ha-1 × Dobogen 2 and 4 ml L-1. Vermicompost 10 ton ha-1 × seaweed extract 2 and 4 ml L-1 increased the soluble solids and total phenolic compounds. Ascorbic acid was responsive to vermicompost 10 ton ha-1 × seaweed extract 4 ml L-1. The experimental treatments affected the yield, height, and chlorophyll content as well. Vermicompost and poultry manure 10 ton ha-1 increased plant dry weight, chlorophyll, and calcium content. The potassium content was the highest with the application of vermicompost 10 ton ha-1. Moreover, phosphorus and magnesium content increased by the application of 5 and 10 ton ha-1 The highest plant height was recorded by applying poultry manure 10 ton ha-1. Chlorophyll, phosphorus, and magnesium contents were increased by the foliar application of 2 and 4 ml L-1 seaweed extract. Seaweed extract 4 ml L-1 increased plant height. It seems that manures application improves growth and yield performance of watercress plants mainly by affecting soil physico-chemical characteristics. Also, seaweed extract has noticeable amounts of minerals, proteins, carbohydrates and plant growth regulators such as ascorbic acid and cytokinins which can positively affect plants vegetative growth and qualitative characteristics.

The use of chemical fertilizers has increased due to the large-scale production of vegetables. Increasing the application of chemical fertilizers causes the pollution of water and soil resources, and leads to the accumulation of nitrate in vegetables tissues. Replacing organic fertilizers and seaweeds extract with chemical fertilizers is an excellent way to increase the crops yield with the minimal fertilizer residues. Results of the current study indicated that the combined application of vermicompost and poultry manure and, the foliar application of seaweed extract had a promising effect on improving the physiological traits of watercress and reducing the nitrate accumulation rate.