Irrigation Water Quality and Long-term Soil Health

Adopting best practices in water quality management can address the challenges of salinity, nutrient imbalances, heavy metal contamination and even water scarcity

The quality of irrigation water is paramount, as it directly influences soil chemistry, plant health and, ultimately, long-term agricultural production. Throughout the United States, where agriculture spans from the arid West to the humid East, this quality varies dramatically. As such, agronomists tasked with optimizing agricultural outputs while safeguarding soil quality must understand how to test, diagnose and correct irrigation water quality. 

In this article we’ll explore not only the impacts of irrigation water on soil quality, but also recent advancements and strategic approaches in managing soil properties and water quality to enhance agricultural outcomes across diverse cropping systems.

The deterioration of irrigation water quality is mainly attributed to geochemical phenomena and human interventions, which significantly affect its agricultural utility. Salinity issues, induced by elevated levels of soluble salts, are a predominant concern, as they can drastically hinder water uptake and yield. This condition is often exacerbated by high concentrations of sodium and chloride ions, which may originate from the dissolution of mineral deposits in certain geological settings or because of human activities such as mining or industrial processes. 

Furthermore, the presence of bicarbonates, resulting from the natural process where carbonate minerals dissolve into carbonic acid, can lead to or worsen soil alkalinity, thereby affecting plant health and nutrient assimilation. Human activities, including agricultural runoff and industrial discharges, compound these problems by introducing minerals and heavy metals into water sources, which may accumulate in soils, posing environmental and health risks. Additionally, the utilization of water from deep wells, inherently rich in various minerals due to percolation through geological strata, can further degrade water quality. 

Collectively, these factors necessitate vigilant management and treatment of irrigation water to ensure the sustainability of agricultural practices and environmental well-being.

Testing for Soil and Water Quality

Understanding the quality of irrigation water and the current state of your soil is essential for effective agricultural management. Here are common tests used to determine soil and water quality issues: 

Irrigation water testing: Testing irrigation water quality is a crucial preliminary step to ensure its suitability for agricultural use. This involves assessing the water’s chemical, physical and even biological attributes, which include nutrient content, electrical conductivity (EC), pH, and the presence of potential contaminants such as heavy metals. Alongside these tests, specific assessments like the Sodium Adsorption Ratio (SAR) and bicarbonate (HCO₃^–) concentration tests are essential. The SAR test evaluates the proportion of sodium to calcium and magnesium in the water, providing insight into the risk of sodicity, which can adversely affect soil structure and permeability. A high SAR value indicates a higher potential for soil dispersion and compaction, making it difficult for roots to penetrate and water to infiltrate, thus necessitating soil amendments or alternative water sources. 

The bicarbonate test, on the other hand, measures the concentration of bicarbonates in the water, which can lead to soil alkalinity problems when in excess. High bicarbonate levels can precipitate calcium and magnesium from the soil solution, further exacerbating sodicity issues. Regularly testing for these parameters ensures that irrigation water quality is within acceptable limits. Table 1 provides a general summary of the discussed water quality parameters and their satisfactory and problematic concentrations in terms of agricultural production. 

Soil pH and Salinity Tests: Soil pH tests measure the acidity or alkalinity of the soil, which affects nutrient availability and microbial activity, using a 1:1 soil to solution mixture. Soil salinity, also called soluble salts, is usually measured at the same time and provides a direct determination of the total soluble salts or EC of the soil. An EC of 4 dS/m is considered saline, although impairments in water availability and reduced yield may be evident between 1 and 2 dS/m. As sodium causes much of the water uptake issues in crop production, diagnostic soil tests to determine sodicity impacts are important. These include the Exchangeable Sodium Percentage (ESP) and a similar Sodium Adsorption Ratio (SAR).

Sodium Adsorption Ratio (SAR) and Saturated Paste Extracts: This test involves saturating a soil sample, roughly 100 grams, with pure water to create a muddy paste (but not oversaturated as to produce standing water), then extracting the liquid for analysis. It’s a reliable method for assessing soluble salts, nutrients and the soil’s electrical conductivity (EC), providing insights into soil salinity levels and nutrient availability. Sodicity issues are determined by the following calculation, where SAR values of >13 are considered sodic: 

Exchangeable Sodium Percentage (ESP): The amount of exchangeable sodium, determined using an ammonium acetate solution at neutral pH, relative to the total cation exchange capacity, is a direct and quick test to diagnose a potentially sodic soil. In this test, the amount of sodium in centimoles per kg is divided by the cation exchange capacity, or CEC, to show how much of the exchange sites are dominated by sodium. ESP values of >15 are considered highly sodic:

Table 2 provides a general summary of the discussed soil parameters as they related to alkalinity, sodicity and salinity in soils.

Impact of Water Quality on Soil Chemistry and Crop Productivity

Irrigation water quality impacts nutrient availability in almost all crops, primarily by increasing salts and pH. This can be significant, particularly for calcium availability to mitigate disorders like bitter pit in Honeycrisp apples. Key tests for water quality, such as the SAR and bicarbonate levels, inform growers on the risk of soil sodicity and potential alkalinity problems, which can affect soil structure and calcium uptake. Regular foliar applications of calcium directly to the fruit have been shown to increase calcium levels, reducing the incidence of calcium-related disorders. This emphasizes the importance of maintaining optimal irrigation water quality through regular testing and targeted calcium supplementation throughout fruit development​​.

Regions receiving low rainfall are often subject to salt buildup or soil salinization, primarily due to irrigation with saline water. Soil amendments, such as gypsum, and the adoption of subsurface drip irrigation systems has been shown to be an effective approach in managing soil salinity levels. By applying gypsum (calcium sulfate), farmers are able to displace sodium ions with calcium ions from gypsum, thereby improving soil permeability and reducing salinity. The use of subsurface drip irrigation minimizes water use and reduces the leaching requirement, conserving water while maintaining crop yields. To reclaim soil to a depth of one foot, gypsum recommendations are as follows: tons of gypsum per acre = 1.7 × (meq Na/100 g) – (CEC × 5%) 

In the case of salt-affected soils that are also highly alkali, applying gypsum with poor irrigation water can further exacerbate soil degradation. In these systems, there is already ample calcium to displace sodium; however, it’s locked up as calcium carbonate precipitates. Soil acidification with elemental sulfur or thiosulfate will dissolve these carbonates through the production of sulfuric acid, freeing up the previously immobilized calcium to then push sodium out of the system. For finer-texture soils, roughly 1,500 to 1,800 pounds per acre of elemental sulfur is required to lower the soil pH one point. Also, adding sulfuric acid directly to water prior to irrigation cuts out the wait time needed to oxidize elemental sulfur in soil. 

In some cases, and unknowingly, heavy-metal-laden groundwater used for irrigation can leave growers dealing with a contamination legacy well into the foreseeable future. Given that soil organic matter acts as a sponge for heavy metals, contaminated groundwater often results in soil concentrations well above EPA limits. In the regions of southern Asia, specifically West Bengal and Bangladesh, crop irrigation water drawn from groundwater naturally enriched in arsenic (up to 2 mg/L) has resulted in soil concentrations as high as 80+ mg/kg. 

Strategies such as the use of biochar and compost have proven effective in mitigating heavy metal impacts in contaminated soil, ensuring the safety and quality of food crops. Biochar, a carbon-rich product derived from the pyrolysis of organic materials, can enhance soil structure, moisture retention and nutrient availability while also immobilizing heavy metals and reducing greenhouse gas emissions from soil. Compost, on the other hand, enriches soil with organic matter and beneficial microorganisms, improving soil fertility and structure. 

Conclusion

The examples and case studies presented here illustrate the critical importance of soil and water quality management in sustaining agricultural productivity and long-term soil quality. From the use of soil amendments and advanced irrigation techniques to innovative water treatment methods and strategic irrigation planning, these approaches highlight the need for comprehensive testing, monitoring and management strategies. By adopting best practices in soil and water quality management, farmers and agronomists can address the challenges of salinity, nutrient imbalances, heavy metal contamination and even water scarcity, ensuring the long-term viability of agricultural systems.