A review of the role of nitrogen, zinc, and boron on the growth and performance characteristics of plants
The origin of animal and human food comes from plants. To achieve long-term food security in human societies, the amount of production in agricultural lands, or in other words, crop yield per unit of agricultural land, must increase. Providing the necessary elements for plant growth is one of the most important factors related to achieving suitable crop production, which also helps maintain soil fertility and prevent its degradation in the long term. Nowadays, fertilizers are used as a tool to maximize production per unit area. However, these fertilizers should not only increase production but also enhance the quality of agricultural products. The undeniable effect of sulfur on plant growth has been well established. Multiple reports have demonstrated the positive impact of sulfur as an inexpensive and abundant substance in increasing the solubility and absorption of essential nutrients required by plants in the soil, thereby improving plant nutrition. Research findings have shown that sulfur consumption is crucial not only as a necessary nutrient for plants but also, more importantly, due to its beneficial effects in locally acidifying the soil and increasing the availability of essential elements such as phosphorus, manganese, iron, zinc, and copper. Zargreen organic liquid sulfur fertilizer, being derived from plant tissues along with non-mineral compounds, is effective in all products and alkaline soils, positively impacting the improvement of both the quality and quantity of agricultural products.
Keywords: Organic, Nutrients, Sulfur, Calcareous Soil, Zargreen.
Throughout history, humans have faced multiple famines caused by agricultural crop shortages. The fear of famine has exerted strong political pressure, making self-sufficiency in food production a significant political goal in many countries. Despite intermittent famines in some regions of the world, the supply and provision of food have, until now, managed to meet the growing needs of the global population. Part of this success has been achieved through cultivating new lands, but since the 1950s, the major increase in food production has resulted from three main avenues:
- Advancements in science and technology, such as understanding how plants absorb nutrients from the soil for growth.
- Progress in producing and using these essential nutrients.
- Improvements in crop management, conservation, irrigation, and plant breeding.
Just like various organs in the human body suffer disorders and diseases due to deficiencies or shortages of certain nutrients, plants also need to receive and convert their required nutrients and elements into energy, cells, and tissues. The scarcity or deficiency of any element or nutrient directly impacts fertility, productivity, health, and resistance to diseases and pests in plants. Additionally, a proper balance and ratio of nutrients and elements play a vital role in plant nutrition. It should be noted that, just as the human body acquires essential nutrients from different foods, plants absorb their required nutrients and elements from the soil solution and various fertilizers applied to the soil. Sulfur, with the chemical symbol S and Latin name Sulfur, is the fifth most abundant element in the Earth’s crust, ranging from 0.06% to 1.0%. It is present in the structure of amino acids such as cysteine, methionine, and taurine, as well as in some enzymes. Sulfur is utilized to improve soil quality. Chemical fertilizers containing sulfur include superphosphate, ammonium phosphate, ammonium sulfate, ammonium thiosulfate, diammonium sulfate, etc. Sulfur is mainly absorbed by plants through the roots in the form of sulfate. The primary absorption site for sulfate is the region of root hairs. The maximum sulfate absorption occurs at pH 4 and decreases with increasing pH.
Sulfur deficiency is observed in soils that are heavily leached or subject to intense washing. To address this deficiency, sulfur needs to be added to the soil as a fertilizer (19).
Sulfur is recognized as the fourth essential element for plants after nitrogen, phosphorus, and potassium (Jamal et al., 2010). The primary absorbable form of sulfur by plants is sulfate, which is taken up through the roots. A small portion of the plant’s sulfur needs can be met through foliar absorption of SO2, but high concentrations of SO2 in the atmosphere can be toxic to plants. The mechanism of sulfate uptake is similar to other anions, especially phosphate. Sulfate absorption consists of two stages: active and passive absorption, with passive sulfate uptake being more common. In passive uptake, sulfate anions are exchanged with bicarbonate ions on the root surface, and they are absorbed by the root itself. However, the rate of this process is lower and comparable to the uptake of nitrate and chloride but higher than that of phosphate (3, 2). The sulfur concentration in plants varies significantly depending on the plant species, plant part (seeds or stems), and phenological stage. Maleki et al. (4) and Delouche (6) reported the optimal average sulfur concentration in plants to be in the range of 0.1% to 0.5% of the dry weight of the plant.
Sulfur plays various roles in plants, including:
- Participating in the synthesis of amino acids (cysteine and methionine), which are essential components of proteins. Therefore, sulfur deficiency in plants is associated with an accumulation of nitrogen in the form of NH2 and NO3 in the leaves.
- Being a significant role player in the formation of disulfide bonds between polypeptide chains, which play a crucial role in protein structure and characteristics.
- Being essential for the synthesis of coenzyme A, which has an active role in oxidation, fatty acid, and amino acid synthesis.
- Effectively contributing to the formation of plant chlorophyll.
- Being a fundamental part of ferredoxin, which plays an active role in nitrate and sulfate reduction.
- Having a substantial impact on the quantity and quality of oils and proteins.
The undeniable effect of sulfur on plant growth has been demonstrated in various studies. For instance, Sexton et al. (20) showed that sulfur deficiency in soybeans limited protein production and reduced the growth of new leaves. Insufficient growth of new leaves restricts the production of photosynthetic substances and ultimately reduces ribisco enzyme synthesis. Reduced photosynthesis and limited resources lead to decreased growth and cell division. Hassan et al. (10) reported that the height of sunflower plants increases with an increase in sulfur levels. The number of branches in the plant is also affected by sulfur, where the highest number of branches was observed when the highest sulfur application of 90 kilograms per hectare was used for canola plants, while the lowest number of branches was observed when no sulfur was applied (12)
Sulfur is used as an element to amend sodic soils and remove surface-absorbed sodium, replacing it with calcium. The oxidation of sulfur compounds, especially elemental sulfur, is a critical reaction in ecosystems because the end product of this process, sulfate, leads to soil acidification. The oxidation of elemental sulfur and its conversion to sulfuric acid, particularly in calcareous soils, is beneficial for reducing soil acidity, providing sulfate, and increasing the availability of low-consumption nutrients such as phosphorus, iron, zinc, and others, thus improving soil quality (18). Sulfur possesses the necessary potential for reducing soil acidity due to its capacity for oxidation and production of sulfuric acid, especially on a small scale. Therefore, it can be effective, particularly in the rhizosphere, in dissolving insoluble nutrient compounds and releasing essential elements. The application of sulfur in alkaline soils increases the dissolution of anions and cations, enhancing the leaching of these salts. It also leads to an increase in electrical conductivity and a decrease in pH compared to soils without sulfur application (9).
In addition to its importance in plant nutrition, sulfur is also the oldest-known fungicide used by humans. The use of sulfates under in vitro conditions to create resistance against various pathogenic fungi has proven useful, but its effectiveness against pathogenic bacteria is generally considered ineffective. The effect of gaseous sulfur in the form of hydrogen sulfide (H2S) on pathogenic agents is still a subject of debate (13). With its fungicidal properties and ability to enhance the plant’s defense system, sulfur can be utilized in organic farming and the production of healthy crops. Sulfur-containing defensive compounds are vital for the survival of plants under biotic and abiotic stress. These compounds include elemental sulfur, H2S, glutathione, phytochelatins, various secondary metabolites (such as glucosinolates in the Brassicaceae family), and sulfur-rich proteins. When a plant is exposed to different stressors, the production of sulfur-containing defensive compounds increases in response to jasmonic acid and other signaling molecules, suggesting that these compounds may have a significant impact on the plant’s defense potential. Glutathione protects the cytosol and other cellular components against reactive oxygen species produced under stress conditions (16).
Many studies and applications of various sulfur-containing fertilizers have shown that sulfur-containing fertilizers can significantly improve product quality and disease resistance. They can also have positive effects on the uptake of nitrogen, phosphorus, and potassium fertilizers. Tangasemi et al. (22) reported that the use of sulfur leads to increased garlic production without compromising its nutritional quality. Although the total absorption of sulfur, pyruvic acid, and total phenol increased at high sulfur doses, this increase did not result in higher yields. Chavdari et al. (5) investigated the effects of sulfur fertilizer on Aloe vera and found that different levels of sulfur had a significant effect on leaf growth and performance. They concluded that the effect of sulfur on growth parameters and their significant positive correlation with performance indicates the importance of sulfur in Aloe vera growth and quality.
In a study by Shivaie et al. (21) on rice, they reported a positive response to sulfur fertilizer, with the plant responding favorably up to the highest level tested (45 kg per hectare). Sulfur fertilization resulted in increased crude protein and sulfur concentrations in rice grains. Therefore, sulfur fertilization increased both grain yield and sulfur concentration in rice grains. Randall et al. (15) also observed a good response of rice to sulfur fertilization. They also reported a positive correlation between grain yield and sulfur concentration in rice grains resulting from sulfur fertilization. The increase in wheat grain yield due to sulfur fertilization has also been reported by several researchers (14, 11, 7). Galjones et al. (8) also reported an increase in sulfur concentration due to sulfur fertilization, although a significant increase in performance was not observed.
In many soils of Iran, due to the high amount of calcium carbonate and high pH, the soluble and absorbable form of many elements is often lower than the amount necessary for proper plant growth and development. Soil pH is one of the most important chemical properties of soil, which affects the solubility and transfer of ions. In dry and semi-dry regions, organic materials and acid-producing compounds like sulfur-containing fertilizers are added to the soil to reduce pH.
Zargreen Organic Sulfur Liquid Fertilizer, a product of Zarnameh Farhikhtegan Industrial Group in the field of food and agriculture, is one of the leading companies in this industry. Zargreen Refinery is the largest exclusive grain refinery complex, processing thousands of tons of grains daily, and is dedicated to supplying the needs of food and pharmaceutical industries and addressing agricultural industry concerns. This organic sulfur fertilizer is produced using sophisticated and innovative processes, utilizing plant-based bases and the latest global knowledge, in addition to sulfate and thiosulfate compounds. With 15% sulfur, 6% nitrogen, a significant amount of 14% organic carbon, 3% free-form amino acids, and a pH range of 4 to 5, it provides an excellent combination for the growth of all plants.
Research on sulfur changes in soil indicates that sulfur, along with nitrogen, and in the form of sulfate, is more efficiently supplied to the soil and taken up by plants. Considering the importance of the N/S ratio in protein production, enzymatic activities, plant growth, and the significance of having an optimal ratio of these two elements, the use of sulfur-containing fertilizers with nitrogen can have overall positive effects in balancing this ratio. Zar Green Organic Sulfur Liquid Fertilizer has high solubility and can be used through irrigation water and foliar spraying. This fertilizer contains both sulfur in the form of sulfate and elemental sulfur. Therefore, when adding Zar Green Organic Sulfur Liquid Fertilizer to the soil, its sulfate content becomes immediately available, while its elemental sulfur will be slowly converted to sulfate by bacterial oxidation and will be gradually available to the plant in the long term. This fertilizer significantly affects reducing soil pH. Moreover, due to the presence of organic matter in this fertilizer, it leads to long-term improvements in the physical, chemical, and biological properties of the soil.
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