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Low pH: Big Issue for Some High Plains Soils

Tuesday, February 1, 2011
filed under: Fertility

Dealing with low-pH soils is not, fortunately, on the “need-to-do” list for most sunflower producers. Acid soils — those with pH levels below 6.0 — are relatively uncommon across the Great Plains, where most sunflower is grown.

Nationally, low-pH soils are a yield-impacting issue for many ag producers. A recent report from the International Plant Nutrition Institute (IPNI) titled “The Fertility of North American Soils, 2010” summarized findings on about 4.4 million soil samples tested by some 60 private and public laboratories (making it the largest summary of soil samples ever conducted in the U.S. and Canada). Soil pH was one of the categories summarized. The median pH for North America was 6.4, with 27% of samples testing below 6.0. “Median pH is lowest in the southeastern U.S. and generally increases toward the west,” the report states.

Where acid soils do exist, affected growers need to be aware, concerned and, sometimes, take steps to correct the issue. Reduced-tillage systems may be especially prone. “Repeated surface applications of nitrogen in conservation-till systems can lead to pH stratification,” exemplifies IPNI North Central regional director Scott Murrell. “As ammonium and urea forms of nitrogen convert to nitrate, acidity is produced. With reduced tillage, this acidity may become concentrated in the upper [two to three inches] of the soil profile.”

“Symptoms of low pH include stunted plant development, uneven crop growth, faster aging of lower leaves and yellowing between veins on upper leaves,” relates Steve Barnhart, a regional agronomist with Croplan Genetics. “Additionally, bacterial activity is reduced under low-pH conditions, slowing the breakdown of soil organic matter, crop residues and organic fertilizer sources like manures. As a result, nutrient release from these sources is lessened.”

Within the sunflower world, low pH soils are perhaps found most commonly in parts of Kansas and Oklahoma. A review of soil test results in 2005 by the Potash & Phosphate Institute (IPNI’s predecessor), indicated that 46% of the Oklahoma tested samples and 22% of those from Kansas had a pH below 6.0. For Nebraska and Texas, the low-pH numbers were 31% and 23%, respectively.

“Soil acidity has a great impact on nutrient availability and nutrient use efficiency, [because] as the pH drops, so does the ability of the plant to uptake and use the applied nutrients,” explains Brian Arnall, soil scientist with Oklahoma State University. His state’s wheat producers have had access to aluminum-tolerant wheat varieties to help counter the effect of acid soils; but as more and more Oklahoma farmers look to expand their rotations by including crops like sunflower and sorghum, they find little or no aluminum tolerance.

The results can be dramatic, in a bad way. “The low pH releases aluminum — which is toxic to the plant and prevents root growth,” Arnall notes. Even with a pH of 5.0, root growth is retarded. “So if we go into a dry period, you’ll see drought stress moreso than you would with a healthy root system.”

The answer for low-pH soils, other than tolerant crop varieties, is liming. Adding appropriate quantities of lime will raise pH levels and maintain soils for healthy crop production. “Lime is recommended for sunflower on all soils with a pH of 6.0 or less,” notes USDA-ARS soil scientist Merle Vigil. “If sunflower is grown in a cropping system that includes legumes, liming to obtain a higher pH (6.2 to 6.5) should be maintained.”

“Lime is a slow-acting soil amendment,” OSU’s Arnall notes. “So it’s going to take weeks or months (or even longer in Northern Plains states) before it becomes effective. You want that soil pH up to a good level before you put your seed in the ground, so it can germinate under the best possible conditions.”

Soil pH also has a big impact of phosphorus availability, Arnall adds, with soil P being most available to plants when the pH range is between 6.0 to 6.7.

The main liming activity in North Dakota to date has been among sugarbeet producers in the Red River Valley, notes Dave Franzen, North Dakota State University soil fertility specialist. These growers actually use spent lime from the region’s sugarbeet factories, where it is a byproduct of the beet sugar purification process. Along with raising pH levels of acidic soils, beet growers benefit from the lime’s nutritional value (especially phosphorus) and its proven ability to help reduce Aphanomyces root rot, a serious disease for the region’s sugarbeet industry.

Franzen says very little liming has been carried out in other parts of North Dakota to date, although some locales do have low soil pH levels. He believes sunflower has an advantage over other vulnerable crops because it roots so deeply “that maybe it’s tapping into some subsoil carbonates.”

Croplan’s Steve Barnhart says the best time to be liming “is right before fall tillage, so you can work the lime into the soil profile. Another benefit to liming prior to tillage is that the winter winds won’t blow it off your fields. This is also a good time to treat no-till acres, and blowing won’t be as much of a concern on these fields because it will adhere to the crop residue that’s left over.” The lime’s acidity-neutralizing effect takes longer to accomplish in no-till fields as compared to field where it is incorporated through tillage, he adds. - Don Lilleboe
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