Watch pH levels when switching to surface water irrigation
Jun 16, 2005 11:19 AM, By David Bennett
The increasing use of surface water to irrigate Arkansas crops is a welcome development. But there are issues producers should be aware of when making the switch from wells.
“The big thing with Arkansas groundwater is most — not all — has appreciable amounts of calcium and magnesium bicarbonates,” said Nathan Slaton, agronomist and professor with the University of Arkansas. “When cold irrigation water is pumped onto a field, the calcium and magnesium fall out of the water and form lime. That’s why most of our east Arkansas soils with long-term irrigation by well water are high pH. And by high pH, I mean basically neutral, 6.5 to 7, all the way up to 8.2. You don’t need to lime with that kind of pH being provided by well water.”
However, when using reservoir water, soil pH levels head in the opposite direction.
“Over time, pH will essentially dry up and you’ve got to be careful not to let it get too low,” said Chuck Wilson, Arkansas Extension rice specialist. “Very rarely — because well water was, essentially, adding lime for us — have we ever limed rice soils. We’re afraid of over-liming. But as we move into an era of more reservoirs, we’re going to have to consider applying lime to some rice soils.”
Wilson recently received phone calls from farmers concerned about soil tests recommending lime on rice acreage. In most cases, pH levels have dropped to 5.5 or lower.
Because of Extension’s previous admonitions, “everyone is worried about pulling liming rice ground,” said Wilson. “Because we’ve used reservoirs here for so long, we’ve had this come up on our experiment station in Stuttgart (Ark.). At one point, because we weren’t liming our rice soils, the pH level had dropped to around 4.8.”
A major problem with soil pH levels is rice is often rotated with soybeans. “Soybeans definitely need a higher pH. We’ve got to watch that closely.”
Slaton is watching to see what happens in the Grand Prairie area where, to address shrinking water availability, reservoirs have become common. “What effects will that have on soils?”
How reservoirs are filled varies from farm to farm. For that reason, Slaton said it’s difficult to make definite statements regarding the quality of reservoir water.
“Rainwater is best. Some operations are able to fill their reservoirs with winter rainfall. Usually, that’s quality water.”
Late in the summer when reservoirs are pumped down, they are often filled again — “sometimes using wells, sometimes picking water up from a stream or ditch. Using that kind of water leads to the same situation that occurs in the field. The calcium and magnesium bicarbonates brought from the ditch or well are just held in the reservoir.”
However, the lime falls to the bottom of the reservoir. When the water is released into a field, the lime doesn’t go with it.
“So basically, when farmers are using reservoir water exclusively to irrigate, they’re getting no lime with it. But when they use well water, they’re pumping lime onto their fields constantly.
“Either approach has good and bad points. Well water irrigation can cause our soil’s pH to be too high — 7.5 to 8.2 is higher than you want. But when you use only reservoir water, you’ll see gradually declining pH levels in soils.”
A field with a current pH of 7.5 to 8 will likely be several years from needing lime, said Slaton.
When a producer does apply lime, Slaton suggests extreme care. “It’s very common for lime to be applied improperly or unevenly. A lack of attention could mean the next time rice is grown there will be streaks of sick rice due to the lime’s uneven distribution. To correct those situations, growers have gone to all kinds of trouble that costs extra money and yield.”
Researchers in Arkansas are re-evaluating lime recommendations. “I think in two or three years, we’ll have some new recommendations. We’ve been playing around in the labs with some new methods. We’ve done some preliminary work and narrowed down to the handful of methods we want to use in the research. I’m searching for a variety of low pH soils: sandy, silt loams and heavy-textured clay soils.”
Drift and warm weather
In other rice news, Wilson said until mid-May he thought the spring would have fewer drift complaints than in years past. Then he suddenly received a handful of calls in short order.
“I’ve spoken with some Extension agents about this. It always seems to start out with one call and, by the end of the day there are four or five of them. For some reason, drift complaints seem to come in small batches.”
There are two main problems with drift in rice, said Wilson. “One, obviously, is glyphosate from burndowns or neighboring Roundup Ready soybean or cotton fields. Glyphosate drift has been an issue for several years.”
The second issue is newer: drift from Newpath being sprayed on Clearfield rice. Newpath can injure conventional rice as badly as glyphosate.
“Lots of times, we’ll see drift injury by a farmer who damaged his own crop. That’s a chance you take when planting Clearfield rice next to conventional rice.”
Late May’s warmer weather perked up the state’s rice, said Wilson. “A few fields of early-planted rice are beginning to be fertilized and flooded. I think, overall, the crop is looking better, although there are still struggling fields out there. I think we’ve turned the corner.”
e-mail: dbennett@primediabusiness.com
