As an essential component of photosynthesis, water helps drive plant growth. Plant roots typically grow downward and outward to access water and dissolved nutrients from the soil. But, soil water availability and quality in the root zone varies by location, soil type, topography, and precipitation. In places where rainfall is low during the growing season, farmers will irrigate using available water sources to promote crop growth. To be successful with water management, though, farmers must consider more than just the amount of applied water.

Testing is Essential

Figure 1: Ag Spectrum Associate sampling water from the pivot

Both soil and irrigation water need testing. ST/MZ-based soil testing performed every two years provides farmers with actionable insights into how irrigation water is affecting soil quality. Similarly, direct testing water before extended use can reveal how irrigation source quality may affect crops and soils when applied at specific rates and times (see Figure 1). Together such testing can point toward how much irrigation water can be safely applied and how to optimize irrigation on a site-specific basis.

Over time, soil testing becomes an essential tool for dialing in proper irrigation rates. In drier soils, evaporation leads to elevated soil salinity (i.e., soils with high concentrations of readily dissolved mineral salts) in the root zone. Once high enough, salts restrict root growth, slow water uptake, and prevent plants from absorbing the proper balance of mineral nutrients from the soil solution. Ag Spectrum’s standard soil tests reveal such problems and if they are associated with regular irrigation. Soil quality issues to watch for include elevated pH (due to application of carbonates), reduced P2:P1 ratios (due to addition of hard water), and increased Na and %Na values (due to sodium in the water).

When properly managed, irrigation water improves yield potential regardless of the quality available. Get in contact with a local Ag Spectrum representative to complete Irrigation Water Quality tests and determine the maximum safe rates to apply to various crops. Those tests indicate if soil or water treatment are cost effective to mitigate water quality problems. Each irrigation water source should be tested multiple times over a three-year period to establish reliable estimates of quality, especially where wells are shallow and/or pumping rates are seasonally restricted. After such baseline ranges are established, one can monitor water quality on a more regular basis with an inexpensive handheld pH and EC meter.

Figure 2: Avoid over-irrigating which can reduce yield potential

Apply the 5Rs to Irrigation

Consider the 5Rs of nutrient stewardship when adding irrigation water.

• Rate: This needs to be sufficient to enter the root zone and support continued photosynthesis or, in the most extreme cases, prevent tissue desiccation. Often, just a quarter- to half-inch of water per acre per day is sufficient. Too often, farmers hurt themselves by overwatering (see Figure 2).
• Time: Irrigation is optimal when temperatures are moderate (< 80 F) and wind speed is low (< 5 mph), typically during the first half of the day (between 2:00 AM and 2:00 PM through most of the summer).
• Placement: This varies with equipment, but it is most often done with overhead sprinklers producing small droplets that can drip below the canopy with little evaporative loss.
• Form: Although quantity, timing, and placement matter most, water quality cannot be ignored. Without sufficient testing and timely adjustments based on data, irrigation can actually destroy yield potential.
• Product: Careful assessment through initial source testing can help farmers determine when irrigation water can be amended, most often with UAN, or if it should be restricted/treated to prevent soil or crop damage. Subsequently, regular spot checking of water using a handheld meter can provide assurance throughout the season that irrigation is providing the expected value.

Figure 3: On-site treatment can be used to improve irrigation water quality

When mitigation of irrigation-associated water quality problems is required, treat soils first and water second. Gypsum can be used to reduce soil sealing caused by excess sodium and/or magnesium deposited by irrigation water. More severe problems associated with increasing soil alkalinity and sodium can also be mitigated by acidifying irrigation water using a sulfur burner (see Figure 3). And high iron levels can be addressed with aerators and filters at the well head. If mitigating water quality issues is not done sufficiently, then irrigation rates may need to be curtailed to keep soils healthy.

Ultimately, making the most of irrigation requires balancing the need to relieve crop water stress while protecting soil quality. Regularly test both water and soil to ensure success. Then apply the right amount of irrigation water, at the right time, and in the right place to increase yield potential.

-provided by Dr. Brian Gardener, Ag Spectrum Technical Director