Cooperative Extension
Colorado State University

FROM THE GROUND UP

JUNE 1998
VOLUME 12
ISSUE 6

In This Issue

Salinity Issues in Colorado

Managing Irrigation to Control Salinity

Understanding Sodic versus Saline Soil Management

Meet Grant Cardon

Websites

Salinity Issues in Colorado

Salinized fields may appear white in extreme cases.

Irrigated Colorado land is increasingly affected by excess salts.

Salinity is an increasingly important problem in many irrigated areas of Colorado. Batie and Healy (1993) describe excess salinity as the most pervasive problem associated with irrigated agriculture. It has been estimated that 25 to 35% of the irrigated land in the western U.S. is affected by salinity. Colorado is no exception to this: statewide, it is estimated that almost 1 million acres are impacted by excess salts. This month's newsletter focuses on salinity issues in Colorado and will attempt to clear up some commonly held misperceptions.

Simply stated, salinity problems are caused by the accumulation of soluble salts in the root zone. In high amounts, these excess salts reduce plant growth and vigor by Salinized fields may appear white in extreme cases. Altering water relations or by causing ion-specific toxicities or imbalances. Establishing good drainage is the universal cure for these problems, but as we will see in this newsletter, salinity problems are much more complex than these generalities would indicate.

Salinity Facts An estimated 980,000 acres of irrigable land in Colorado are affected by salts. Crop losses may occur with irrigation water containing as little as 700-850 mg /l TDS orEC ~1 .2 dS/m. Salinity is often measured by electro-conductivity (EC) and reported as millimhos per centimeter(mmhos/cm) or deciSiemens per meter (dS/m). Generally, salt is thought of as ordinary table salt or sodium chloride. However, many types of salt are common in Colorado soils (see Common Salt Compounds box below)

Salt Sources
Saline soils and poor quality irrigation water can be found in many areas of Colorado. These salts originate mainly from the natural weathering of minerals or from fossil salt deposits left from ancient sea beds. Salts tend to accumulate in soils of arid environments as irrigation water or groundwater seepage evaporates, leaving minerals behind. Irrigation water may contain salts picked up as the water moves across the landscape, or the salts may come from man-induced sources such as municipal runoff or water treatment. As water is used and reused, salts levels tend to increase as the water is consumed, transpired or evaporated. Irrigation water containing 750 mg/L salt (EC~1.2dS/m) would carry about 1 ton of salt for every acre foot of water applied to the land. Over time, this salt builds up to damaging levels if it is not leached from the root zone.

In Colorado, we have saline soils, sodic soils, and saline sodic soils, often in close proximity. Because these problems must be managed differently, it is important to understand their cause, effect, and the best management options. Visual symptoms of crop stress are not diagnostic of which type of salinity is present. In fact, yield reductions of 25 to 30% due to salinity have been reported without any visible crop symptoms. The most widespread salinity problems in our state are found in the Arkansas River Valley, the South Platte River Valley, and in the Colorado River Basin. Interestingly, the salinity problems and salt sources are distinctly different in each of these three regions.

The Arkansas River is one of the most saline rivers in the U.S. Average total dissolved solids (TDS) in the river ranges from about 500 mg/L at Pueblo to 3500mg/L at the state line near Holly. Salinity in the alluvial aquifer is even higher. These salts are primarily calcium and sodium bicarbonate and sulfate and originate mainly from minerals leached from sedimentary rock deposits in the foothills to the east of Canon City. Due to siltation of the river bed and a subsequent rise in the water table, poor drainage is impeding leaching and in some cases causing salts to further concentrate as groundwater seeps to the surface and evaporates. The Patterson Hollow HUA project, headed by Jim Valliant, is dedicated to addressing these problems through improved irrigation system sand water management.

The Colorado River, by contrast, typically contains about 500 ppm (or mg/L) TDS on the West Slope near the Utah line. Plentiful supplies of good quality irrigation water limit salinity problems on irrigated fields in the region. In spite of this, there has been a strong focus on salinity control in this basin due to a treaty with Mexico obligating the U.S. to reduce salt loading to the river. Salinity problems in this area are due in part to deep percolation of irrigation water picking up fossil salts from the Mancos Shale, an ancient sedimentary marine deposit. Irrigation return flows carry sodium, magnesium and calcium chloride and sulfate back to the river, degrading water quality. Cooperative Extension has a long history of work on salinity control projects in the basin. Currently, Dick Bartholomay and Dan Champion are heading CSU salinity efforts on the West Slope. Allowing for seasonal and annual river flow fluctuations, they report that there has been a slight downward trend in salt loading to the Colorado River since1970. The salinity problems on the S. Platte River increase as the water moves eastward to the state line. The two causes for the increase in dissolved solids are salt concentration and salt pickup. The surface water in the basin picks up both naturally occurring and human induced salts as it moves downstream. These salts are concentrated as the water is consumptively used and evaporated. While drainage problems exist in isolated areas, they are not as pervasive as in the Arkansas River Valley. High pH(alkaline) soils also seem to be causing producers more problems, likely due to an accumulation of sodium. At present, no formal programs are in place in the S. Platte basin to address these issues, in spite of increasing reports of salt problems. Mahdi Al-Kaisi is working with producers on water management in the basin and Israel Broner is interested in organizing amore focused research and extension program on salinity. In the meantime, producers are left to cope with declining soil quality and reduced crop yields.

Reagan Waskom
Extension Water Quality Specialist
Colorado State University

Table 1. Terms, units, and useful conversions for understanding water quality analysis reports.

Symbol	Meaning	Units
Total Salinity
EC	Electric condicutivity	mmhos/cm µmhos/cm dS/m
TDS	Total dissolved solids	mg/L ppm
Sodium Hazard
SAR	Sodium adsorption ratio	---
ESP	Exchangeable sodium percentage	---

Conversions 1 dS/m = 1 mmhos/cm = 1000 µmhos/cm 1mg/L = 1 ppm TDS (mg/L) = EC (dS/m) x 640 for EC < 5 dS/m TDS (mg/L) = EC (dS/m) x 800 for EC > 5 dS/m TDS (lbs/ac-ft) = TDS (mg/L) x 2.72

Key mg/L = milligrams per liter ppm = parts per million dS/m = deci Siemens per meter at 25E° C mmhos/cm = millihos per centimer at 25° C

FROM THE GROUND UP agronomy news is a monthly publication of Cooperative Extension, Department of Soil & Crop Sciences, Colorado State University, Fort Collins, Colorado.

Web Site: http://www.colostate.edu/Depts/SoilCrop/extension/Newsletters/news.html

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