By Rich Maples

Rotating rice and soybeans can create a dilemma for farmers. The rice needs soil with a pH of less than 6.5 to avoid a zinc deficiency, while the soybeans need soil with a pH of 5.8 or higher for nitrogen fixation.

Dr. Mike Daniels, agronomist for the Cooperative Extension Service, says rice and soybeans are often rotated on soils with a pH of 5 or less. The trick is to raise the soil pH for soybeans by liming without hurting the rice.

With funding from the Arkansas Soybean Promotion Board, Daniels is examining the effects of different liming rates--zero lime, 1 ton of lime per acre, 2 tons per acre and 4 tons per acre--on rice and soybeans in a rice-soybean-rice rotation and a soybean-rice-soybean rotation.

Daniels said the trial field, which is located at the UofA Rice Research and Extension Center near Stuttgart, had a pH of 4.8 before it was limed in the spring of 1995. The 1-ton application of lime raised the soil's pH to 5.7, the 2-ton rate increased it to 6.4, and the 4-ton treatment raised the pH to 6.9.

"During the growing season, we collected hundreds of plant samples, which are now being analyzed for all of the essential nutrients," said Daniels. "In this first year, we found the biggest response to the different liming rates in the plant growth of soybeans.

"We looked at two varieties, Delsoy 5500 and Holladay, and found that lime, especially the 2 and 4 ton treatments, helped promote nodulation and nitrogen fixation, which affected plant growth. We saw the biggest response with the Holladays."

He said liming the beans, which were planted on 32-inch rows, also ensured canopy closure, which helped reduce moisture evaporation and suppress weeds.

Four rice varieties were included in the liming study: Bengal, Cypress, Kaybonnet and Drew.

"We thought that the 4-ton lime rate would cause zinc deficiencies, but it didn't," said Daniels.

He also looked at ways to prevent nutrient problems in the rice. He included four different combinations of zinc and phosphorus in the liming tests. Daniels said, "We have plots within the lime treatments that either have both zinc and phosphorus, phosphorus and no zinc, zinc and no phosphorus, or no zinc or phosphorus.

"We didn't see a lot of difference in plant growth due to phosphorus and zinc treatments, and the lime rate didn't really affect plant height, at least with Bengal and Cypress. The 4-ton lime rate somewhat diminished the plant height of Kaybonnet and Drew, but plant height in rice isn't always a good indicator of yield, anyway."

Daniels said the liming-rate study will be expanded to two farms in 1997. One site will be in Lonoke County, where the farmer has switched from well water to surface water for irrigation and the pH has dropped.

"Well water often contains appreciable amounts of lime, so you're actually liming when you irrigate," said the agronomist. "You don't get the lime in surface water. It's more acid."

UofA scientists are using grid soil sampling and satellite (GPS) and computer technology to create maps that illustrate what's happening to pH levels in the fields that are levee irrigated with well water.

Daniels added that Extension economist Dr. Tony Windham is analyzing the economics--the costs and returns--of each lime application rate.