Impacts of Different Irrigation Methods on Soil Chemical Properties in Desert Agriculture
Reducing agricultural water use is a pressing issue for growers and policymakers in the US Southwest. As more irrigation water cutbacks loom, a shift in irrigation systems is evident. However, little is known about how different irrigation systems may impact soil health and sustain soil fertility for optimum crop production. We compared three major irrigation methods in the Arizona agroecosystem: drip, sprinkler, and furrow irrigation. Key soil properties were measured in multiple Arizona crops in rotation (cantaloupe and broccoli). We also tested a clay-based soil amendment, Liquid Natural Clay (LNC), manufactured by Desert Control® that can potentially improve soil water-holding capacity and contribute towards a reduction in agricultural water consumption. Our initial results highlighted an accumulation of excess nitrate-nitrogen and soluble salts in the soils of the drip-irrigated plots. Compared with furrow irrigation, drip irrigation uses less water but cannot flush soluble salts below the root zone. Although nitrogen recommendations are available for major Arizona crops, relatively little guidance is available on how recommended rates should be adjusted for fertigation via subsurface drip versus through furrow irrigation. As a result, a higher nitrogen fertilization rate is often used in subsurface drip-irrigated systems, leading to overapplication of nitrogen fertilizers. Therefore, a revised nitrogen recommendation is needed for drip-irrigated crops based on their nitrogen requirements. This study helps to establish a fundamental understanding of soil fertility management under different irrigation systems. Outcomes from this study should seed future research in precision fertility management for drip and sprinkler irrigation in crop production systems of Arizona and similar arid environments.
Introduction
Growers in the Desert Southwest are seeking new ways to reduce agricultural water use without sacrificing crop yield as they face water cuts. Arizona’s decades of water management research provides an important foundation for developing new strategies to conserve water while supporting the long-term sustainability of the state’s agricultural sector. To strengthen our commitment to becoming more water-aware and efficient users of precious surface and groundwater, the University of Arizona led state programs such as the Water Irrigation Efficiency Project (The University of Arizona Cooperative Extension, n.d.). This program offered financial incentives for growers to adopt the latest available technologies, such as sprinklers, surface and subsurface drip irrigation, and furrow irrigation (Orr et al., 2023). However, little information was available on how these potentially water-efficient irrigation systems impact soil properties that support plant growth and crop production. We therefore designed a study to compare furrow irrigation with an overhead sprinkler system and a subsurface drip system for their impacts on agriculturally essential soil properties
Description of different irrigation systems included in study
Furrow irrigation
Furrow irrigation is the dominant irrigation method in Arizona due to its simplicity and low cost. It is often argued that a large volume of water is used in furrow irrigation compared to drip or sprinkler irrigation, but this high volume can also help leach soluble salts below the root zone. Maintaining lower salinity in the root zone can promote seed germination, root water uptake, and nutrient acquisition throughout the growing season.
Drip irrigation
Drip irrigation has become much more prevalent in the 21st century. This method requires plastic tubing to be run down the length of each bed in a field so that water can be slowly released through small perforations, called emitters, into the root zone of each plant. These plastic tubes can be run along the surface or underneath. In this study, subsurface (6-8 inches below the surface) driplines were used and are referred to as “subsurface drip.” Because water is released beneath the surface and in smaller volumes, less is lost to runoff and evaporation compared to the furrow. Disadvantages include high installation and maintenance costs if the drip tape tears. Additionally, many studies have raised concerns about salinity buildup in drip-irrigated fields (Ball et al., 2023) and the need for ‘skilled’ labor to maintain and operate the system. Skilled labor shortages in the US Southwest have increased in recent years.
Center pivot (CP)
Center pivot (CP) is a type of sprinkler irrigation system that uses an overhead metal pipe (usually aluminum) connected to supports with wheels that pivot around a center fulcrum. Sprinklers are attached to the overhead pipe that emit differing rates of water flow depending upon position along the pipe, because travel speed is greater at the outside end of the center pivot system. This irrigation method mimics precipitation and is often considered highly efficient. However, like furrow irrigation, it is susceptible to evaporative water loss and, like drip irrigation, has a high installation cost.
Although furrow irrigation is traditionally the choice for growers in Arizona due to its simplicity and contribution towards salinity mitigation, drip and center pivot techniques have been gaining traction in recent years in the face of looming water shortages.
Study objectives
This study compares these three irrigation methods at two irrigation rates (100% ET and 80% ET) by measuring crop productivity as well as soil biogeochemical changes. We also investigated the impact of adding Liquid Natural Clay (LNC), a clay-rich soil amendment intended to increase water retention by increasing microporosity and water holding capacity of light textured soils (Desert Control, n.d.). This project is a commitment of the University of Arizona Cooperative Extension team to apply a ‘system’s approach,' and through convergence science, multiple scientific disciplines investigate new technology before it gets into the hands of our stakeholders.
Methods
Figure 1. Plot layout of the drip and furrow field (bottom) and center pivot field (top) at the Maricopa Agricultural Center, Arizona.
Experimental layout
This experiment was conducted in a field just under 13.5 acres in size, split up between two irrigation regimes: on the east side, a 9-acre section irrigated by center pivot, and on the west side, a 3.8-acre section irrigated by a mix of drip and furrow (Figure 1). The west section had 3 replications of 4 treatment combinations: drip at 100% evapotranspiration (ET), drip at 80% ET, furrow at 100% ET, and furrow at 80% ET. Additionally, within each 6-bed plot was a 6-meter subplot that had the soil amendment applied. Over two seasons, two crops were planted: cantaloupe (Cucumis melo) and broccoli (Brassica oleracea var. italica) in Summer 2024 and Winter 2024, respectively, to observe the effects of these variations in irrigation approach and soil treatment (Figure 2).
Sample collection
Soil samples were collected with the crop present in the field at a depth of 0-12 inches (0-30 cm). Within each plot, soil was randomly sampled from the central region, away from the edges, to reduce potential edge effects (Figure 2). Samples were taken from the edge of the bed. Multiple scoops were collected and mixed in a bucket to form a composite. Subsamples were sent to Ward Laboratories for routine nutrient analyses.
Irrigation and fertilization
The most water was used in furrow 100%, where 19.7 acre-inches per acre were applied to the field. Furrow 80% followed behind at 16.8 acre-inches per acre, with drip 100%, drip 80%, CP 100%, and CP 80% receiving 15.8, 13.3, 16.4, and 14.0 acre-inches per acre, respectively. Variable irrigation rates were implemented after each crop’s stand establishment; however, for this manuscript, we will only discuss the full irrigation rate. Urea Ammonium Nitrate (UAN-32, 32% Nitrogen) was applied to these crops via fertigation during mid-season irrigation, targeting 75 lbs. of N per acre. A similar nitrogen fertilizer rate was used across all treatment combinations, meaning the same amount of nitrogen (by mass) was applied regardless of the amount of water used or the area and soil volume affected by fertigation.
Soil properties
Although we measured a suite of soil properties, we have selected a subset for inclusion in this publication. Electrical conductivity (often termed as ‘soluble salts’) measures the total soluble salts present in a 1:1 soil-to-water extract. This is a major indicator of soil salinity, and a value above 1 mmhos/cm is indicative of soil salinity and may restrict germination and crop growth. A value above 2 mmhos/cm indicates a moderately saline soil, while a value above 4 mmhos/cm is considered highly saline, according to the classification system presented by Gruttadaurio et al. (2013). Sodium adsorption ratio (SAR) expresses the relationship between the concentration of sodium (Na) and the concentrations of magnesium (Mg) and calcium (Ca) (Zhang, 2017). While the former (Na) disperses soil particles, the latter two (Ca and Mg) cause clay particles to flocculate (meaning ‘clump together’), resulting in better aggregation and improved soil structure. An SAR value above 5 means there’s a possibility that the soil may show a sodicity problem, and a value above 15 confirms sodicity (Soil Survey Staff, 2023). Soil nitrate-nitrogen is the primary plant-available form of nitrogen and serves as a key macronutrient supporting plant growth and development (Zhang et al., 2018). The term nitrate-N refers specifically to the nitrogen portion of the nitrate molecule’s total mass. Using nitrate-N values, the corresponding amount of nitrate present in the soil can be determined. Although nitrate is naturally occurring in soils (Natural Resources Conservation Service [NRCS], 2014), it is usually supplemented by the addition of chemical fertilizer, here UAN-32. Its high solubility allows it to be extracted with a potassium chloride solution and analyzed via flow injection analysis (Ward Laboratories, 2021).
Figure 2A. Broccoli field as part of field trial at the University of Arizona Maricopa Agricultural Center in Maricopa, Arizona.
Chaz Stackpole
Figure 2B. Soil sample collection as part of the field trial at the University of Arizona Maricopa Agricultural Center in Maricopa, Arizona.
Diaa Eldin Elshikha
Figure 3. Drip irrigated plots under the broccoli crop.
Chaz Stackpole
Results
In both of our trials, furrow-irrigated plots reported lower EC values than drip-irrigated plots, indicating a salinity buildup under drip irrigation (Figure 4). Soil salinity was higher during the first cantaloupe trial, but was reduced in the second, possibly due to crop uptake of nutrients and addition of more water over time. It is to be noted that fertilizers are soluble salts, and too much residual fertilizer will contribute to soil salinity (Flynn and Ulery, 2011).
Sodicity was prevalent in drip-irrigated plots as indicated by SAR values (Figure 5). In the broccoli trial, furrow treatments had the lowest SAR values, regardless of LNC amendment application. Similarly, in the cantaloupe trial, furrow irrigated plots had lower SAR values than LNC-amended drip and non-amended CP plots.
Furrow also produced lower concentrations of nitrate-N than drip (Figure 6). In broccoli, the difference was in the realm of one order of magnitude (i.e. 2.60 ppm N for Furrow, NA vs. 74.6 ppm N for Drip, NA), while in cantaloupe, the gap was two orders of magnitude (4.07 ppm N vs. 206 ppm).
Crop yields data across treatments was similar for cantaloupe and broccoli see Elshikha et al. (2025a,b). For both, the highest-yielding treatment was D100A, despite the treatment’s relatively high SAR and EC value.
Figure 4. Electrical conductivity (EC) values for 5 treatment combinations. Notably, drip-irrigated plots had higher values than furrow irrigated plots. The high standard deviation in the drip treatments is due to the uneven salt distribution that occurs as water evaporates from small pockets of soil and leaves behind soluble salts. Values for “CP, A” were not included because of insufficient data points on a limited amended area under the Center Pivot.
Figure 5. Sodium Adsorption Ratio values for 5 treatment combinations. Drip values are consistently lower than furrow values, with cantaloupe’s “Drip, NA” being the sole exception. The high standard deviation in the drip treatments may be due to a relatively higher sodium concentration in the irrigation water than in the calcium and magnesium concentrations. Values for “CP, A” were not included because of insufficient data points on a limited amended area under the Center Pivot. CP: Central Pivot, A: with LNC amendment, NA: without LNC amendment.
Figure 6. Nitrate-N values for 5 treatment combinations. Furrow nitrate-N concentrations are, on average, lower than drip. The high standard deviation in the drip treatments is due to the localized evaporation of soil water, which creates uneven nitrate distribution, as the dissolved ions are concentrated near the edges of wetted zones. Values for “CP, A” were not included due to insufficient data points on a limited amended area under the Center Pivot. CP: Central Pivot, A: with LNC amendment, NA: without LNC amendment.
Discussion
Furrow irrigated plots maintained lower SAR and EC values due to the leaching of salts below the active root zone of the plants by the additional water pressure exerted during irrigation (Food and Agriculture Organization of the United Nations, 1985). In other words, the salts were 'flushed out.' However, if optimal irrigation frequency (or interval) is not respected, there is a risk of salts resurfacing via capillary processes. By comparison, irrigation through center pivot (CP) can take anywhere from half to a full day, with a smaller volume of water infiltrating the soil at once. Similarly, drip irrigation can run for 6-8 hours a day and is applied more frequently than furrow irrigation.
Drip irrigation is also a very localized form of irrigation, which can result in pockets of high salinity around the root zone as water evaporates and leaves behind its salt content (Figure 8; Liu et al., 2020). Under drip irrigation, salts are leached from localized areas of water saturation near the emitters. At the edge of the wetted area, dissolved soluble salts are left behind as water evaporates. Salts are leached downward via gravity and laterally via capillary action (Guan et al., 2022). Consequently, the interior of the wetted zone is low in salinity, while its boundary exhibits higher salt concentrations (Nagaz et al., 2017).
Figure 8. Diagram demonstrating water infiltrating the root zone (left) and leaving behind salt as it evaporates (right).
Created with BioRender.com.
The shape of the wetted zone is influenced by emitter spacing and discharge rate, soil texture, and the depth at which the drip system is installed. The least amount of water and the most salt are found between emitters (Hanson et al., 2011). Drip-irrigated sandy soils have a narrow wetted zone that extends deep into the soil; in contrast, clay soils have a wetted zone that extends farther laterally while covering a shorter vertical distance (Food and Agriculture Organization of the United Nations, 1997; Guan et al., 2022). Whether the drip system is installed on the soil surface or a few inches below it also affects the wetted zone. Similar to the shape of the wetted zone in clay soils, subsurface drip creates a zone that expands further horizontally, whereas surface drip results in a deeper, narrower zone (Hanson & May, 2011). Uneven salt distribution in drip plots may have contributed to the higher salinity readings observed, since soil samples were collected from the bed edges to avoid puncturing the drip tape. Sampling closer to the root zone might have yielded lower EC values.
High levels of salt in soil can cause plants to undergo osmotic stress, as water diffuses from areas of low salt concentration to areas of high salt concentration. If the soil is more saline relative to the plant, water will be lost by the plant rather than absorbed (Elshikha et al., 2025b). Metabolic activities such as photosynthesis slow down due to water stress, resulting in decreased plant growth, and therefore, decreased yield (Chaves et al., 2009; Sanusi et al., 2025).
Lower salinity values in the furrow system could be further diminished by irrigating every other row. This method, known as alternate row irrigation, helps to reduce salt concentration in the root zone, in addition to using less water than standard furrow. As water flows from the irrigated furrow across the adjacent beds, it carries soil salts with it and evaporates on the dry side of the beds, leaving the salts behind (Silvertooth, 2001). Roots grow toward the irrigated row due to its higher water saturation and, therefore, higher osmotic potential, which facilitates water absorption by the roots as water naturally moves from high to low osmotic potential (OpenStax, 2018).
Available research from Arizona and New Mexico suggests that soil nitrate levels should be maintained between 10-20 ppm for both broccoli and cantaloupe (Desert Ag Solutions, n.d.; Bevacqua & Cardenas, 2002). Under drip irrigation, a buildup of soil nitrate levels between 58 and 206 ppm indicates excess fertilization of both crops (Figure 6). The negative effects of nitrate levels exceeding crop needs include decreased yield and lower crop quality (Albornoz, 2016). This excess nitrate mainly came from following fertilizer application guidelines for furrow irrigation, which are not tailored to the specific requirements of a drip-irrigated field. Additionally, studies indicate that Central Arizona groundwater has nitrate-N levels ranging from 0.2 to 39 mg/L, which should be considered when determining whether the crop needs additional nutrients (Desert Ag Solutions, n.d.). To avoid excess nitrate buildup in future trials, nitrate strips could be used to monitor soil nitrate concentrations before applying nitrogen fertilizers, and N application to drip-irrigated plots could be reduced or skipped to maintain uniform nitrate levels across all plots within irrigation treatments. It should be noted whether such a modified nitrogen application would lower yields in drip-irrigated plots and if there is a reason to maintain higher soil nitrate levels relative to furrow- and sprinkler-irrigated plots after all.
Conclusion
This study emphasizes the positive effect of flood irrigation on salt levels. Pre-plant flooding is often recommended for drip and sprinkler-irrigated plots to leach excess salts. However, pre-irrigation often requires a large amount of water, which may narrow the difference in water consumption among different irrigation methods (drip vs. sprinkler vs. furrow) for a specific crop. Flood irrigation of alternating rows could offer a compromise between the water-saving advantages of drip irrigation and the salt-leaching benefits of traditional furrow irrigation. Another option is a hybrid approach, where occasional surface irrigation is applied in conjunction with drip irrigation. Additionally, it was found that the current nitrogen fertilizer application guidelines are not broadly applicable to drip-irrigated crops and should be revised. Alternatively, close monitoring of nitrate levels is necessary to avoid over-application of fertilizer and the buildup of salinity in drip-irrigated crops.
Acknowledgement
The authors would like to thank Dilshani Aswin, Aaron Dodge, Kaitlin Fahlgren, Katherine Jakubowski, Mercedes Martinez, Eli Mata, Kaelyn Mayo, Alvin Onyango, Pavan Thodeti, and Makayla Walizer for their help in collecting soil samples and/or running tests in the lab. This work was supported by the University of Arizona Cooperative Extension Water Irrigation Efficiency Program, which is funded by the Arizona State Legislature.
This publication provides an objective summary of an irrigation experiment and does not endorse or promote any particular brand, product, or trademark. Any references to product names, trademarks, or companies are included for informational purposes only.
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