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A review on the use of organic fertilizers and amino acids in desalination of agricultural soils

Summary

Soil degradation is one of the significant factors leading to a decline in soil quality and reduced agricultural productivity. Among the primary causes of soil degradation, salinization and soil sodicity pose serious environmental threats worldwide, affecting over 1000 million hectares of land to varying degrees.

In Iran, some of the main reasons for soil salinity include low precipitation, high evaporation rates from the soil surface, low-lying and high-altitude areas, inappropriate water quality for irrigation, and the type of parent rocks. Due to the limitations of saline and sodic soils, it becomes essential to rehabilitate and improve these lands, which have the potential for cultivation and agriculture.

The use of organic fertilizers produced from various organic sources plays a crucial role in enhancing the physical, chemical, and biological structure of the soil. Therefore, the application of soil amendments such as liquid organic fertilizer from Zargreen is necessary to maintain soil stability in such areas.

Introduction
One of the major challenges facing humanity in the 21st century is ensuring food security for the growing population. Soil degradation, including salinization, is a crucial factor contributing to the decline in soil quality and agricultural productivity, adversely affecting food security and the environment. Approximately one billion people in more than 100 countries are impacted by the degradation of land. Salinization occurs through various physical and chemical processes, and its consequences include a negative impact on the ecosystem (Epstein et al., 1980). Soil salinity, which involves the accumulation of salts in the surface and subsurface layers, is a significant limiting factor for agricultural crops. In arid and semi-arid regions, insufficient leaching and excessive accumulation of salts in the soil, along with the use of groundwater rich in sodium carbonate and bicarbonate for irrigation, have led to an increase in soil pH. This has further contributed to soil salinization and sodicity, affecting over 1000 million hectares of land, approximately 7% of the Earth’s land area. In Iran, nearly 77 million hectares of land have been affected by salinity, with 58% of the salinized land attributed to human activities, mainly irrigation. Globally, around 20% of irrigated lands face salinity issues, with some countries like Iran and Egypt reporting even higher percentages, up to 30% (Ghassemi et al., 1995). Given that Iran is located in a dry and semi-dry region, the issue of salinized and sodic soils encompasses a substantial portion of the country’s land. Therefore, optimal management and protection of water and soil resources are crucial during droughts to maintain soil quality (Asadi Kapourchal et al., 1391). The main reasons for soil salinity in Iran include low precipitation, high evaporation from the soil surface, topography, inappropriate water quality for irrigation, and the type of parent rocks. Approximately 32 million hectares (about 20%) of Iran’s soils face varying degrees of salinity, sodicity, alkalinity, or waterlogging issues. Of this area, 7 million hectares are saline marshes in the Lut Desert and the salt desert (Benayi, 1380). Zargreen liquid organic fertilizer is based on plant derivatives and environmentally friendly. It contains various free amino acids (6%), essential plant nutrients like nitrogen (3%), phosphorus (5.2%), potassium (2%), organic matter (30%), and organic carbon (11%) with acidic pH due to the presence of organic acids. This makes Zargreen liquid organic fertilizer one of the organic fertilizers suitable for improving saline soils.
Amino acid and its role in saline soils
Nowadays, foliar spraying with amino acids has become common in most orchards, and its desirable effects on growth, performance, and fruit quality are evident. During the period of rapid fruit growth and development, there is competition between reproductive organs and roots, which reduces nutrient absorption. Foliar spraying can mitigate this competition. Additionally, there are various reports on the role of amino acids in reducing the effects of salinity stress. Typically, the level of free proline in plants irrigated adequately is very low, around 0.2-0.6 milligrams per gram of dry matter. However, this substance increases to 40-50 milligrams per gram of dry matter after water reduction in tissues (Heuer, 1994). Salt-tolerant plants employ various strategies to respond to salinity stress and extract more water from the soil. Some of these strategies include restricting salt entry into the plant through the root system, preventing salt from reaching sensitive organs, transporting ions out of the plant directly through the roots and leaves, diluting ion concentrations by increasing the storage volume through the development of succulent, watery, and thick structures, increasing metabolism, and regulating osmotic pressure through the accumulation of organic and mineral substances in their cells, as well as regulating turgor pressure (Salma et al., 2006). Part of the osmotic regulation is usually due to the increase in the concentration of certain compounds, including amino acids, quaternary amines, and non-organic ions, especially K+. Amino acids, through osmotic regulation, protect intracellular structures and reduce oxidative damage caused by the production of free radicals in response to drought and salinity stress (De Lacerda, 2005). Osmotic regulation (through amino acids such as glycine, proline, alanine, and valine) and detoxification of oxygen species and intracellular pH regulation are some of the roles that amino acids play in plant tolerance to environmental stresses, including drought and salinity stress (Omid et al., 2011). The application of gamma-aminobutyric acid has been studied on spinach (Spinacia oleracea L.) and resulted in a significant increase in plant weight and leaf number under salinity stress conditions. Therefore, gamma-aminobutyric acid can be used as an enhancer for growth and physiological properties of spinach under saline stress (Afzali et al., 2018). The foliar application of amino acids (arginine, tryptophan, and proline) was also studied on tomato plants under salinity stress conditions. As salinity level increased, stem length, leaf number, and node number decreased. However, amino acid supplementation increased the number of leaves, and proline showed a more positive effect on stem length and node number. Salinity had a negative impact on the number of clusters and total chlorophyll content, while it had a positive effect on increasing proline content (Poursoltan et al., 2017).
Usage of nitrogen in saline soils
Despite extensive research in many countries over the past few decades, there is still not a complete understanding of the biochemical and physiological basis of plant salt tolerance. The common response of plants to an increase in the concentration of salts in the root growth region includes osmotic effects, specific ion toxicity, and nutritional imbalances. Agricultural, horticultural, and medicinal plants vary significantly in their tolerance to salts present in the root zone. One of the main effects of salinity stress on plants is disrupting nutrient balance and interfering with nutrient uptake. Research has shown that salinity reduces the uptake of many essential elements by plants. Nitrogen is one of the highly consumed elements crucial for plant growth, and its deficiency leads to severe growth disturbances. However, in saline soils, nitrogen deficiency intensifies due to various reasons. These factors include organic matter deficiency, insufficient root growth, competition between Cl– and NO3– ions for root uptake, leaching of NO3– ions from the root zone, and the lack of suitable conditions for the formation of nitrogen-fixing nodules in legumes in saline soils. Findings have shown that nitrate uptake by plants is negatively influenced by NaCl concentration. Some researchers have investigated the competition between Cl– and NO3– ions for plant uptake. The competition between these two ions has been attributed to the negative potential of root cells and the negative charge of Cl– and NO3– ions and their uptake through the same transporter systems. Consequently, under saline conditions, nitrogen uptake decreases, leading to a reduction in plant growth. The application of nitrogen under salinity stress conditions improves growth parameters such as dry weight, physico-biochemical properties, and performance characteristics (Akhtari et al., 1393).
The effect of organic carbon on soil salinity
The organic compounds that dissolve in water and can pass through a 0.45-micrometer filter are referred to as soluble organic matter. Since carbon is the major constituent of soluble organic matter, it is also called soluble organic carbon. Soluble organic carbon affects the solubility of elements in the soil and their mobility in aquatic systems. It plays a crucial role in various soil processes, such as microbial activity, nutrient transport, and availability. Adding soluble organic carbon to saline soils increases the soil microbial biomass compared to non-saline and non-sodic soils. The use of organic fertilizers, such as chicken and sheep manure, in soils increases cation exchange capacity (CEC) and enhances the absorption of cations such as calcium, magnesium, and potassium compared to sodium. Conversely, the application of soil amendments leads to the leaching of sodium and a reduction in the exchangeable sodium percentage (ESP). The remediation of saline soils, with their impact on plant growth and sodium absorption, has received considerable attention. Remediation strategies include leaching of salts, developing drainage systems, cultivating deep-rooted plants, and using chemical amendments like gypsum, sulfuric acid, and organic amendments (e.g., animal manure, green manure, municipal waste, etc.) (Hasantabar-Shoobi et al., 2018). Based on the conducted research, the sodium absorption ratio in carbon-amended soils decreases. Furthermore, the application of organic amendments increases the concentration of sodium, potassium, calcium, magnesium, and chlorine in the soil solution and helps to replace exchangeable sodium with soil solution, preventing its loss during leaching. Therefore, it results in a reduction of exchangeable sodium percentage (Chaganti et al., 2015). According to the findings of Shoobi et al. (2018), soluble organic carbon increases the concentration of sodium, potassium, calcium, magnesium, and chlorine during the incubation phase, and salt leaching is higher in the presence of soluble organic carbon. Soluble organic carbon reduces electrical conductivity and the sodium absorption ratio, making it a recommended treatment for remediating saline-sodic soils.
The effect of organic matter on soil salinity
Numerous research studies have been conducted on the impact of soil sodicity on its physical properties. The results of these studies indicate that the initial evidence of degradation includes an increase in ESP (exchangeable sodium percentage) and particle dispersion, which reduces the hydraulic conductivity of the soil due to soil aggregates collapsing and large pores being filled (So and Aylmore, 1993). Various methods have been proposed to improve the physical properties of soil, and one of them is the use of organic sources. As the organic matter content of the soil increases, the specific mass of the soil decreases due to the creation of spaces between particles. Organic matter also acts as a bonding agent for primary particles, contributing to the formation and stability of soil aggregates. Soil aggregation and an increase in aggregate size enhance the porosity of the soil, resulting in a decrease in apparent specific mass. Organic matter serves as a cementing agent and is essential for the formation of durable soil aggregates (Puget et al., 2000). In addition to organic matter, gypsum and sulfuric acid are among the amendments used for reclaiming saline-sodic soils. Furthermore, soil remediation for saline-sodic soils has also been carried out through leaching, as investigated by some researchers. Saline-sodic soils, which also contain calcium carbonate, are extensive in dry and semi-arid regions around the world. In such conditions, CaCO3 present in the soil dissolves slowly, providing calcium for the remediation process. Since the solubility of lime is very low in providing calcium, it is usually used in combination with acidic substances or acidifiers (Qadir et al., 1996). Zinc deficiency is commonly observed in soils with high pH, low organic matter content, and salinity-sodicity. High pH and high calcium concentration are the main factors responsible for zinc deficiency in most saline soils (due to competition with zinc uptake). Hence, the bioavailability of zinc is generally low in lime and saline soils. Organic matter is one of the factors influencing the transformation of different forms of zinc in the soil. Adding organic matter to the soil, such as sewage sludge, animal manure, compost, and plant residues, typically leads to the redistribution of zinc in the soil. Long-term addition of organic fertilizers in the absence of chemical zinc fertilizers significantly increases various forms of zinc in the soil, including exchangeable, soluble, organic, and non-crystalline forms of manganese and iron oxides (Boostani et al., 2016).
Remediation of saline soils
Due to the limitations of saline and sodic soils, the reclamation and restoration of these lands, which have the potential for cultivation and farming, seem inevitable. The basis for amending sodic soils is the substitution of exchangeable sodium with calcium. The replaced sodium is washed out of the root zone or soil profile. Calcium, as a common source, either contains calcium itself or dissolves it in the soil solution after application. Therefore, there are two methods for amending such soils, which include adding a calcium-containing source and non-calcareous soils, as well as increasing the solubility of calcium, especially in calcareous soils (Quirk, 2001). Calcium plays a crucial role in completing the cell membrane structure, stabilizing the cell wall, regulating ion transport, and selective membrane absorption. Calcium ions have significant effects on plant physiological processes and improve morphological and biochemical factors in plants under saline stress. Salinity, due to various reasons, including ion toxicity and disruption of plant nutrition relationships, negatively affects plant growth indicators. One of the adverse effects of salinity on plants is disturbing the balance of important nutrients such as potassium, iron, and calcium and their concentrations under the influence of extracellular sodium and calcium levels. Calcium and sodium ions have competitive effects, and their proper regulation significantly impacts the concentration of these elements (Renault, 2005). To increase plant resistance to salinity, efforts have been made using certain chemical materials that reduce sodium uptake and accumulation. Experiments have shown that adding calcium to the environment reduces the damage caused by salinity (Mirzai, 2012). The adequate supply of calcium in saline environments is a crucial factor in controlling the toxicity of specific ions, such as sodium and chlorine, in sensitive plants. Sufficient calcium in the environment contributes to the evolution of plant cell membranes and the preservation of their selectivity, reducing the toxicity of chloride and sodium in plants. The effect of calcium in reducing the detrimental effects of sodium salinity depends on the plant species, calcium concentration, and the source of sodium. Adequate calcium can prevent the transfer of sodium to aerial parts, thus reducing the negative impact of salinity. Under saline stress, sufficient calcium can reduce sodium uptake and improve growth conditions by decreasing the sodium-to-potassium ratio. Sodium ions may compete for calcium binding sites in the membrane; thus, a high level of calcium can preserve the cell membrane from adverse salinity effects (Mozafari, 2005). The selective absorption of ions such as potassium, iron, and zinc in the plant roots under salinity has been studied, and the results show that the presence of an appropriate amount of calcium in the root growth medium improves selective root absorption and, consequently, better absorption of essential nutrients such as potassium. The effect of calcium also depends on time. Salinity creates calcium deficiency in plants, and calcium applications improve the effect of saline stress. In many studies, an increase in calcium in the growth medium has shown weak correlation with plant growth regulators, suggesting that this factor should be controlled based on osmotic conditions (Fahimi, 1996). Soroush Zadeh and Amin Panah (2005) observed that soaking rice seeds in calcium nitrate reduces the effects of salinity. Calcium’s role includes regulating the transport of ions to plant cells, evolving plasma membrane structure, and reducing membrane permeability to chloride and sodium ions and modifying root hydraulic conductivity. The beneficial effect of calcium on ions can be attributed to maintaining cell integrity and exerting plasma membrane control in both roots and aerial parts. Banuls et al. (1991) demonstrated that calcium can act as a corrective and regulatory role in salinity effects. Calcium’s role as an activator of cellular signaling systems and an osmotic regulator has also been observed. Therefore, the proper use of potassium and calcium fertilizers in saline soils can reduce the physiological damages caused by salinity and consequently increase the yield. Considering that many soils in dry and semi-dry regions are calcareous and that lime solubility is low, adding organic matter can increase carbon dioxide pressure in the soil, which increases lime solubility and reduces soil pH. Thus, on the one hand, the intensity of substituting soluble calcium for exchangeable sodium has increased, and on the other hand, due to improved structural conditions and increased soil permeability, sodium is leached faster (Hanay et al., 2004).
Conclusion

Organic nutrition is a global strategy for preserving the natural fertility of the soil by enhancing soil microorganisms. The application of organic fertilizers produced from various organic sources strengthens the physical, chemical, and biological structure of the soil. These actions increase soil porosity, the amount of organic carbon and total nitrogen in the soil’s rhizosphere layer, and help bind soil particles by bonding mineral particles like calcium, magnesium, and potassium to the colloidal form of humus or clay. Considering that Iran is located in a dry and semi-dry region, the amount of organic matter in the soil is very low. Therefore, the use of organic amendments such as Zargreen organic liquid fertilizer is essential to maintain soil stability in these soils. Many soil properties improve as a result of using organic amendments, and organic materials have a significant impact on soil structure. Zargreen organic liquid fertilizer can enhance microbial activity in saline soils and promote growth by increasing the biogeochemical cycling of nutrients in the soil. The use of organic materials in saline soils leads to increased sodium leaching, reduced exchangeable sodium percentage, improved electrical conductivity, and increased permeability. The increase in microbial activity and carbon dioxide pressure in the soil leads to increased lime solubility, resulting in the substitution of soluble calcium for exchangeable sodium, ultimately leading to sodium leaching.

Contributors:
1. Ali Nezhadrangbar
2. Arash Ershadi
3. Mehdi Jafari Asl
4. Mehdi Amini

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