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Volume 38, Issue 4 (2023)                   GeoRes 2023, 38(4): 541-548 | Back to browse issues page
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Farajnia A. Temporal Variability of Some Soil Fertility indicators in the Agricultural Lands of East Azerbaijan Province. GeoRes 2023; 38 (4) :541-548
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Authors A. Farajnia *
East Azerbaijan Agriculture and Natural Resources Research and Education Center, Tabriz, Iran
* Corresponding Author Address: East Azerbaijan Agriculture and Natural Resources Research and Education Center, Abbasi Street, Tabriz, Iran. Postal Code: 5255179531 (farajnia1966@yahoo.com)
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Introduction
With the rapid growth of population, the demand for higher food production has become increasingly evident. Given the limited availability of cultivable land, the most effective strategy to achieve this goal is to enhance crop yield per unit area. Among agricultural inputs, the balanced application of chemical fertilizers plays the most significant role in increasing crop productivity. Accordingly, the most rational and scientific method for determining the appropriate fertilizer rate is to base fertilizer recommendations on soil testing [Malakouti et al., 2016].
Soil, as one of the fundamental natural resources, is not isolated from its surrounding environment but rather interacts continuously with it. Climatic factors, air, surface and groundwater resources and their quality, as well as social changes (population growth, urbanization, migration, etc.) and their consequences (land use changes, wastewater disposal, etc.) all illustrate the complex and interdependent relationship of soil with its environment. Interpreting past changes and predicting future trends requires access to relevant data, which constitute essential resources for monitoring soil quality. Therefore, collecting, organizing, and analyzing such data is of critical importance in soil surveillance programs. Nowadays, through the use of computer technologies and Geographic Information Systems (GIS), it is possible to establish soil databases, collect spatially referenced field data, classify and update them, and conduct spatial analyses to obtain valuable information about the spatial distribution of nutrients and their temporal trends, ultimately producing fertility maps [Eastman et al., 1998].
The preparation of soil fertility maps facilitates balanced soil and plant nutrition management. In order to generate these maps, the concentration of essential soil nutrients is measured; in addition to the nutrient contents of different regions, the ratios between nutrients are also determined. Based on these findings, region-specific soil and plant nutrient packages can be designed and recommended. Furthermore, extension services can provide these nutrient packages to farmers at the local level. Fertility maps of agricultural areas thus enable fertilizer recommendations tailored to the spatial variability of essential nutrients across a region [Momeni, 2001]. Applications of soil fertility mapping include estimating the nutrient status of soils, determining crop fertilizer requirements for individual fields or larger regions, and optimizing the use of fertilizers containing both macronutrients and micronutrients by restricting application to deficient areas or applying only nutrients that are specifically lacking for a given crop [Farajnia, 2010].
In Iran, the use of chemical fertilizers began in 1946 with the import of 11 tons of various types of fertilizers, and since 1951 the Ministry of Agriculture assumed responsibility for fertilizer imports. Initially, a relatively balanced use of organic and chemical fertilizers existed; however, this balance was soon disrupted. By the mid-1970s, the use of chemical fertilizers particularly nitrogenous and phosphatic types became widespread, while potassium and organic fertilizers were largely neglected [Karimian, 2012]. Fertilizer consumption in Iran has continued to rise, increasing from 500,000 tons per year to 1.5 million tons in 1989, 2.2 million tons in 1999, and 2.7 million tons in 2018, half of which was supplied through imports [Anonymous, 2020].
The excessive use of fertilizers, however, poses serious risks to public health due to nutrient accumulation in soils and crops, or leaching into groundwater resources and contaminating them. Government subsidies for fertilizer imports and their low market prices further encouraged overuse. As a result, soil analyses conducted across different regions of the country prompted researchers to publish warnings regarding soil and groundwater contamination. Among the scholars who reported such concerns are Karimian, Saleh Rastin, Amin, Farajnia, and Nosratpour and colleagues [Karimian, 1994; Saleh Rastin, 1994; Amin, 1994; Farajnia, 2010; Nosratpour et al., 2010]. Subsequently, the Agricultural Research, Education, and Extension Organization promoted soil test-based fertilizer use by establishing private soil laboratories and expanding research on crop nutrient requirements.
A considerable number of studies on direct evaluation of soil fertility were conducted in Iran between 1996 and 2011. These studies mainly focused on the quantity, source, timing, and method of fertilizer application. Among indirect fertility evaluation methods, soil testing accounted for the largest share of past research. With advancements in computer technologies and GIS, the collection of spatially referenced soil data became possible, enabling classification and spatial analysis. Consequently, useful information on soil factors was produced both as point-based and surface maps. For preparing such maps, both macro- and micronutrient concentrations are measured; beyond their absolute levels, the nutrient ratios are also determined. In this way, soil fertility mapping enables the application of Liebig’s law of the minimum at a regional scale to support large-scale agricultural production. Research findings on critical levels of soil nutrients were compiled in the book Critical Nutrient Levels for Strategic Crops and Appropriate Fertilizer Recommendations in Iran [Malakouti & Gheibi, 1997]. These studies determined critical thresholds of soil nutrients and assessed crop responses to fertilizer application under specific soil and management conditions in different regions.
Nutrients such as nitrogen, phosphorus, and potassium undergo complex microbial transformations before becoming available for plant uptake. Hence, the biochemical properties of soil are essential for plant growth and development [Sun et al., 2003]. Nevertheless, these properties are affected by natural processes such as wind, rainfall, wildfires, and drought, which can reduce soil moisture and permeability, increase surface runoff and erosion, and ultimately lead to nutrient loss and declining soil fertility. In addition, human activities such as fertilization, irrigation, tillage, and grazing disrupt microbial communities and organic matter layers in soil, thereby influencing nutrient concentrations [Godwin & Miller, 2003]. Therefore, soil properties vary spatially and temporally from the field scale to broader landscapes under the influence of intrinsic factors (soil-forming processes, parent materials, organisms, topography, climate, and time) and extrinsic factors (soil management, fertilization, and crop rotation).
Recognizing the inherent heterogeneity of soils, it is essential to monitor and quantify changes in soil properties in order to better understand the long-term impacts of management practices and ultimately promote sustainable agricultural operations [Karlen et al., 2008]. Monitoring requires the selection of representative sites, experimental stations, and periodic sampling. Internationally, diverse approaches have been adopted. In some cases, permanent plots have been used to study nutrient dynamics under different management systems over time; however, Sun et al. [2003] and Huber et al. [2001] argue that such methods are often time-consuming and costly.
In Europe, a coordinated proposal was developed to monitor soil quality across member states, where sites were selected based on landform and land-use characteristics. A network was subsequently designed to record long-term soil data, which, after quality control, were transformed into usable information and made available to soil experts and stakeholders across participating countries. This centralized European soil database led to the development of the EPR software, which provides real-time access to accurate and updated soil information [Armstrong Brown et al., 1998].
In Iran, the Soil and Water Research Institute implemented a soil monitoring program between 2011 and 2016, covering 3,000 selected sites across rain-fed and irrigated lands. Sampling followed a grid-based design with 7 km intervals, and composite soil samples were collected within a 25-meter radius at each site. The physical and chemical properties of these soils were analyzed in laboratories, and the results were published in 2018 [Saadat, 2018]. This study provided estimates of the spatial variability of soil physical and chemical properties in Iranian agricultural soils; however, to evaluate temporal changes, repeated monitoring after several years is required.
The present study was therefore conducted as part of the Comprehensive Soil Fertility and Plant Nutrition Management Program, with the aim of monitoring soil fertility status and assessing the spatial and temporal variations of organic carbon, available phosphorus, and available potassium in the soils of East Azerbaijan Province.
This research specifically addresses the spatial and temporal dynamics of organic carbon, phosphorus, and potassium in agricultural soils of East Azerbaijan Province.


Methodology
The present study was of an applied type and was conducted in East Azerbaijan Province in 2023. This province is located in northwestern Iran, between latitudes 36°45′ to 39°26′ N and longitudes 45°05′ to 48°22′ E. Covering an area of 45,800 km², it accounts for 2.8% of the country’s total area. The province is generally characterized by a cold semi-arid climate. Agricultural land in the province is estimated at approximately 18,000 km², representing 39% of the provincial area and about 9% of the cultivable land of the country.
In this study, soil fertility maps were prepared for two time periods. During the first period (2002–2006), croplands of all counties in the province were gridded at intervals of 1000 × 1000 meters. The geographical coordinates of each point (9,000 stations) were recorded in UTM format and entered into a GPS device to facilitate accurate navigation to the sampling locations. At each site, composite soil samples were collected by mixing five subsamples taken from the 0–30 cm soil depth. About 2 kg of the composite sample was packed, labeled with site information, and transported to the Soil and Water Laboratory. After preparation, soil organic carbon, available phosphorus, and available potassium were measured for all samples [Malakouti & Gheibi, 1997]. The same procedure was repeated about 15 years later (2020–2022) at 1,400 sites.


Findings
The results of the accuracy assessment of different interpolation methods presented that the lowest observed error was obtained using the Inverse Distance Weighting (IDW) method, followed by the Kriging method. Accordingly, based on the IDW method, surface maps of soil organic carbon, available phosphorus, and available potassium were generated for the two study periods.
The spatial distribution map of soil organic carbon indicated considerable changes in both the quantity and distribution of this element during the 16-year period. In lands with poor organic carbon content (0.2–0.5%), a significant reduction occurred; by 2022, the area of such lands had decreased to less than half of their initial extent. Conversely, the area of lands with organic carbon ranging between 1–2% increased from 271,000 ha to 506,000 ha, and those with more than 2% organic carbon rose from 91,000 ha to 165,000 ha.
The temporal changes in available soil phosphorus were more pronounced than those of organic carbon, but in contrast to organic carbon, the amount of available phosphorus decreased considerably. In 2006 only 10.5% of the province’s lands were below the critical phosphorus threshold, whereas by 2022 this figure had increased to more than 85%.
The spatial variation map of available potassium demonstrated only minor changes in the quantity and distribution of this element during the 16-year period. The majority of agricultural lands in the province contained potassium levels above 300 mg/kg, indicating that the available potassium content was well above the critical threshold.


Discussion
The Inverse Distance Weighting (IDW) method outperformed the other evaluated techniques, and the maps of soil organic carbon, available phosphorus, and potassium were classified into different categories of very poor, poor, moderate, high, and very high based on their critical thresholds [Malakouti & Gheibi, 1997]. These findings are consistent with numerous studies [Weber & Englund, 1992; Gotway et al., 1996; Robinson & Metternicht, 2006], which reported that the IDW model has greater efficiency in mapping soil parameters compared to alternative approaches.
The observed increase in soil organic carbon in the counties of Mianeh, Sarab, Charuymaq, and Marand during the past 15 years can be attributed to the growing use of organic fertilizers, particularly humic acid fertilizers, for high-value crops such as rice, potato, tomato, and fruit orchards. For instance, in Mianeh, rice cultivation considered a profitable crop has largely replaced wheat, barley, sorghum, and alfalfa. Farmers in this region have increasingly adopted humic acid fertilizers to enhance rice yields, driven by the significant rise in the crop’s market price over the past five years [Farajnia & Motalebifarrd, 2022].
Soil organic carbon plays a vital role in soil fertility, crop productivity, and overall soil sustainability [Smith et al., 2007]. In addition to providing nutrients, soil organic matter profoundly influences soil chemical, physical, and biological properties. Therefore, monitoring the status and dynamics of soil organic matter is essential, given its multifaceted role in soil quality [Carter, 2002]. Eskandari et al. [2018] report that forest lands, wetlands, and agricultural lands in Marivan had the highest levels of soil organic carbon, respectively. Similarly, Maleki et al. [2014], using geostatistical techniques, have demonstrated that soil organic carbon in Tushan (Golestan province) decreased with elevation and slope steepness.
The sharp decline in phosphorus availability over the past 15 years can be attributed to the increasing price of phosphate fertilizers. During the 1990s and earlier, the affordability of these fertilizers led to their widespread use, particularly in onion, tomato, and potato fields, as well as fruit orchards in East Azerbaijan. In some cases, farmers applied up to 40 bags per hectare, which resulted in soil test values exceeding 80 mg/kg, with over one million hectares having more than 10 mg/kg of available phosphorus by 2006 [Farajnia & Motalebifarrd, 2022]. Since the 1990s, many researchers have warned against the excessive use of chemical fertilizers, highlighting the risks of soil and groundwater pollution [Karimian, 2012; Amin, 1994]. In response, the Soil and Water Research Institute launched the national phosphorus calibration project, recommending that phosphate fertilizers should not be applied where soil available phosphorus exceeds 15 mg/kg [Malakouti & Gheibi, 1997]. Concurrently, the government reduced subsidies and increased fertilizer prices, which led to a drastic decrease in phosphate use. In East Azerbaijan, phosphate fertilizer consumption dropped from about 57,000 tons in 2007 to only 6,000 tons in 2021. Nationally, phosphate consumption had been negligible until 1986 but increased rapidly, reaching 600,000 tons by 1993 [Karimian, 2012]. However, the recent sharp decline in soil phosphorus has forced the government to halve the price of phosphate fertilizers in 2022, while also linking the subsidized supply of urea to the purchase of triple superphosphate.
The use of potassium sulfate fertilizers also declined with rising prices, but the agricultural soils of this region are naturally rich in potassium. Nevertheless, the area of lands with available potassium levels above 300 mg/kg decreased by approximately 300,000 ha. Malakouti et al. [2016] have argued that potassium fertilization improves crop yields even when soil available potassium exceeds critical thresholds, particularly for high-yielding crops such as potato, tomato, and greenhouse crops. Conversely, other researchers such as Falah [2006] reports that soil available potassium decreases during the growing season, suggesting the continued necessity of potassium fertilization due to its beneficial effects on plant growth, drought tolerance, and cold resistance. Likewise, Biradar et al. [2020], in Karnataka, India, prepare nutrient maps and emphasized their utility in helping farmers identify and overcome nutrient-related limitations. Prabhavati et al. [2015] also demonstrate, across three Indian watersheds, that soil fertility maps not only reveal the current status but also provide a basis for monitoring temporal changes and fertility trends.
This study revealed that fertilizer application rates are influenced by multiple factors, with fertilizer prices and crop type being the most decisive. In the 1990s, low fertilizer prices led to excessive applications, well beyond plant requirements. However, rising costs, together with researchers’ warnings regarding nutrient accumulation in soils and crops, led to reduced usage. For crops such as wheat, low market prices further discouraged fertilizer application. To address this issue, it is recommended that agricultural product prices especially wheat be set based on production costs rather than social considerations. The government should instead adjust cash subsidies and expand eligibility for vulnerable households, while allowing wheat and bread prices to be determined by market supply and demand. Excessive reductions in fertilizer consumption over recent years have depleted soil nutrient reserves, lowering not only crop yields but also product quality, with potentially irreversible impacts on public health and child development. It is therefore suggested that the government revise its agricultural policies and involve specialists in determining the diversity and pricing of subsidized fertilizers.

Conclusion
The area of land with soil organic carbon below the critical threshold has decreased substantially, falling to less than half of its former extent, while the area with organic carbon above the critical threshold has increased. In contrast, the amount of available soil phosphorus has significantly declined over this period. Unlike these two elements, however, the level of potassium has shown little change during the same timeframe

Acknowledgments: None reported by the author.
Ethical Approval: None reported by the author.
Conflict of Interest: None reported by the author.
Author Contributions: Farajnia A (First Author), Principal Researcher (100%)
Funding: None reported by the author.
Keywords:

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