by James W. Bauder

Many research projects have been conducted throughout the western United States in an attempt to determine the effects of irrigation water quality and irrigation practices on alfalfa, grass, pasture, and cereal crop production. These studies have taken a look at many factors. The following have repeatedly surfaced as affecting crop production:

  • Salinity (salt level) of the irrigation water and salt build-up in the soil
  • Length of time the field is flooded and amount of water applied
  • Temperature of the irrigation water, soil, and air
  • Growth stage and age of alfalfa
Salinity of Irrigation Water and Salt Build-up in the Soil
Salinity is the term commonly used to refer to the salt content of either irrigation water or soil water. It can also refer to salt accumulation in the soil. Measuring salinity entails measuring either the amount of salt dissolved in water or the electrical conductance of the water. The saltier the water, the greater its ability to conduct electricity. The commonly used measure for salt dissolved in water is "parts per million" (ppm). If you were to dissolve one gallon of salt in one million gallons of water, the salt concentration would be 1 ppm.

So, what happens to alfalfa under irrigation with salty water? In one study, both inoculated and non-inoculated alfalfa were grown with irrigation waters of progressively increasing salinity levels. In summary, alfalfa nodulation (ability to fix atmospheric nitrogen), number of nodules produced, and actual weight of the nodules was reduced when plants were irrigated with moderately salty water. Growth of the alfalfa plants was also inhibited, decreasing linearly with each increase in salinity, even when nitrogen fertilizer was applied to the crop. Increasing salinity of the irrigation water caused a greater decrease in yield for the non-inoculated alfalfa than for the inoculated alfalfa.

 

Length of time in the Field is Flooded and Amount of Water Applied
Does temperature and the length of flooding time affect the growth and survival of irrigated alfalfa? Yes it does, and the conditions are pretty well known. Question has frequently been raised about the effect of extended periods of flooding on alfalfa growth. "Poor quality" water is not the only cause of alfalfa stand deaths; standing water and excessively wet soils can also be at fault. The results of one study by Thompson and Fick are helpful in determining one cause for dying alfalfa stands.

Generally, in flood or sprinkler irrigated fields where alfalfa stands die out, there are some common characteristics:

  • Relatively old stands of alfalfa
  • Dying alfalfa replaced by grass
  • Flood irrigated fields
  • Alfalfa most severely affected where water was turned into the field or in low spots where water stands for extended periods of time
  • Poorly drained, "heavy" silt loam or silty clay loam soils
  • Recent flood irrigation with relatively warm water

Alfalfa is very sensitive to excessive soil water. In one study, growth of roots and tops was measured for different durations and temperatures of flooding. Alfalfa was flooded for periods of 4, 8, 12, 16 and 20 days at temperatures of 60, 70, and 800F. In another study, alfalfa was flood irrigated for 3, 6, 9, or 12 days at 900F. Root and top growth were measured after the soil was drained and then again three weeks later. This three week wait and cut period would be similar to irrigating three weeks before harvesting the alfalfa. The following results were observed:

  • Root growth stopped during flooding.
  • Three weeks following irrigation, the rate of top growth was reduced by 50% with 8 days of flooding at 600F.
  • Top growth was reduced by 50% with 4 days of flooding at 700F.
  • Top growth was reduced by 50% with 3 days of flooding at 800F.
  • Top growth was reduced by 50% with 2 days of flooding at 900F.
  • Plants stopped growing or died under the conditions of 14 days flooding at 600F, 10 days flooding at 700F, 8 days flooding at 800F, and 6 days flooding at 900F.

As the previous studies show, yields of alfalfa can be significantly reduced by wet or flooded soil conditions caused by irrigation, high water tables accompanying spring thaws, and exceptionally heavy rains. Wet soil conditions reduce stand vigor and promote the occurrence of fungal root infections, and periodic inundation can create short-term flooded conditions which inhibit growth. The associated reduced yields are the results of direct interference with normal plant physiological processes.

 

Soil, Air and Irrigation Water Temperature
Soil temperature appears to be more critical than air temperature when irrigating during mid-summer. Commonly, you will hear irrigators say that it is not good to irrigate when irrigation water gets warm, but it's okay when the water is cold. There are two reasons for this. Colder water holds more dissolved oxygen, and colder temperatures keep the alfalfa's demand for oxygen relatively low. Colder water usually contains less dissolved salt.

A study by Thompson and Fick revealed that three factors significantly affected vigor and yield of alfalfa plants. They are flooding itself, duration of flooding, and temperature during flooding. The visible pattern of development of symptoms of flooding damage started with yellowing and subsequent death of leaves, beginning with the leaves lowest on the stem and progressing up the plant.

 

At 600F and 700F, the progress was gradual; at higher temperatures, the whole plant turned yellow only a few days after the first signs of yellowing had appeared. Yellowing continued to appear even after the soil was drained. In all cases, the plants had a drastically reduced future yield potential by the time yellowing appeared.

Flooding also caused death and rotting of the taproot. The taproot was generally intact immediately following drainage, but the root harvest 5 weeks after drainage showed that rotting had occurred in the interim on all flooding treatments. In severely damaged plants, the taproot was completely gone rather than partially rotted, and the root system consisted of adventitious (surface, branching) roots sprouting from the crown.

Alfalfa yields after 3 weeks of recovery from flooding showed that while top growth may have been normal for some time during the flooding treatment, the plants were being damaged by the flooding from the beginning. All flooded plants at all temperatures showed yield loss compared to non-flooded plants. The actual yields decreased proportionally as the period of flooding increased.

Several significant findings came out of this study. Flooding did not appear to kill the alfalfa directly; but it did reduce plant vigor so as to make the plants susceptible to other disturbances. Flooding stops growth and causes leaf loss; the crop may have no net growth for several weeks following flooding. By definition of this study, the period of flooding which a stand can endure (time before leaves start to yellow) is approximately 14 days at a soil temperature of 600F, 10 days at 700F, 7 to 8 days at 800F, and 6 days at 900F. Leaf yellowing appearing while the plant is still flooded is indicative of eventual stand loss.


What to Look For and What to Do
What signs indicate that too much water, high salinity, high soil and water temperatures, or extended periods of flooding are damaging an alfalfa stand? Look for the following:

  • Replacement of alfalfa by grass, especially where water is turned into the field or in low spots in basins
  • Yellowing and loss of lowest leaves on the plant, followed by progressively more loss of leaves up the stem
  • Rotted taproot and/or development of many fine, short, lateral surface roots
  • Progressive reduction in alfalfa tonnage
  • Development of tiny, new leaves at the axil on the stem where leaves have fallen off

What does one do about the problem of alfalfa stand loss due to flooding with high temperature water or soils? Several things can be done to minimize the potential damage caused by long duration flooding.

  • Irrigate early in the season to take advantage of cooler water and soil temperatures and less vigorous plant growth.
  • Move water across fields quickly, and avoid prolonged ponding, particularly at low spots and turnouts. Insure uniform water distribution, and apply only the amount needed to fill the profile.
  • Remember that even under the driest conditions, few soils will hold and store more than 6 to 8 inches of water.
  • Obtain a detailed map of the soils in your field and manage water to minimize ponding on fine textured, poorly drained soils, such as silty clays and silt loams.
  • Divert water away from low spots that pond easily.
  • If you must irrigate when the soil or water temperature is high (above 65-700F), do it immediately after cutting, and put on only the necessary amount of water.
  • Concentrate your irrigated alfalfa production on fields with a slight slope and good drainage, such as fields with fine sandy loam soils.
Hopefully, these tips will lead to a healthy, productive alfalfa stand.