ArkValley Irrigation Grant Final Report

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I I I I I I I I I I I I I I I I I I I Tests done by Dr. Jim Rhoades, Director of the US Salinity Lab and his field team in April, 1996 confmned the high salt levels, however, these high salt levels were point located using readings from an EM-38, a senSQr which produces an electromagnetic field that penetrates the soil causing an electrical current flQW within the soil profile. These readings indicated that the most saline part Qfthe field was located in the northwest quarter of Pivot #7. The average was 7.6 dS/m with readings up to 10.3 dS/m or mmhosJcm in the surface foot. These readings were sUPPQrted by ground site soil samples. Readings in Pivot #8 averaged 4.4 dS/m ranging up to 7.1 dS/m in the surface foot. Again in 1998, readings with an EM-38 by Dr. Tim Gates of the Colorado State University Civil Engineering Department showed salinity readings similar in location to those of Dr. Rhoades but it appears that some of the salts in the middle part of the circle have been leached down on Pivot #7. SQil salinity levels, ECe, are below 3.0 dS/m in a large nQrth/south area just east of the center of the circle. Readings up to 10.0 dS/m are stiJI present in the northwest quarter of the circle. Pivot #8 appears to have increased slightly in soil salinity, especially in the northeast quarter of the circle. About 90% of Pivot #8 in 1998 had a salinity level of3.5 dS/m or greater and, at these levels, crop production would be expected to be reduced 10 to 40% when compared to areas ofless that 2.0 dS/m. Salinity was also high in the water from wells being used to irrigate the two center pivots. Water in pump #14 which supplied PivQt #7 was tested in December, 1994 at 4.6 dS/m and 4.2 dS/m when tested in July of 1995 and up tQ 5.2 dS/m in August 1995. Other wells, used Qn Pivot #8, tested in the 1.6 dS/m range. Salinity levels continued in this range during the demonstration period. Nitrate-nitrogen was average Qn both fields except in the Bloom silt loam on Pivot #8 where it ranged up to 164 ppm. SmalJ grains and fQrage crops had been grQwn in each of the fields so amounts Qfnitrogen applied were IQwer than would have been applied tQ CQm or Qther high nitrQgen use crops. Alfalfa and Qther legumes and grasses and/or grass-legume mixtures were used in the demQnstratiQn as they are mQre salt tolerant than SQme Qther crops and nQnnally have lower water and/Qr nitrate use which should reduce salt and nitrate loading. Alfalfa was planted Qn Pivot #7 in the late summer of I 995 but excessive rains in August and the high salt CQntent Qf the soil and irrigation water caused a total crop failure. LOQking for forage crops with more salt tQlerance, grasses and sorghum-sudangrass were planted in 1996, again resulting in a crQP failure as adequate stands were not achieved. In 1997, the area was again planted to sQrghum- sudangrass. The crop came up to a excellent stand but, Qnce the roots reached the second fQot, the plants became stunted and the crop was overtaken by weeds The high salts in the water and in the lower root zone was cQnsidered tQ be the reason fQr the cQntinuing crop failure. This was confirmed by a greenhouse study conducted in cooperation with Dr. Gary Banuelos of the USDA-ARS Water Management Research Laboratory in Fresno, CA. SQils from Pivot #7 were sent to the laboratory, potted and irrigated with waler synthetically constructed based on analysis from samples of river and well water. Tall fescue and Birdsfoot trefoil, both broad leaf and narrow leat; were tested. "Salt toxicity as burning of the leaf margins and stunting of growth began to appear in all three species shQrtly after applying the poor II quality water" (Ref. #1) from the well. HQwever, the tall fescue produced acceptable amounts of dry matter, nearly twice the dry matter produced by the Birdsfoot trefoil. 2