Sod to Soybean Considerations

March 10, 2012 10:05 AM
Sod to Soybean Considerations

Don’t be fooled for top yields

With tight hay markets, a raging beef market, $12 soybeans and corn futures near $6, the decision to convert more pasture to grain crops has not been a no-brainer this year. Solid grain demand, expiring Conservation Reserve Program (CRP) contracts and the high cost of herd replacements, however, might have tipped the balance toward more row crop acreage on your farm.

Farmers have always taken advantage of a fertilization bump on fallow land coming back into grain crops, but it’s no slam-dunk to achieve top yields the first year. CRP ground or pasture is typically not the best ground for farming, due to soil types and topography.

It takes fertilization management to match the history of that ground to a proper seedbed for grain crops, but will it be soybeans or corn? Soybeans are the probable first crop choice,
researchers say, in spite of the fact that corn nets more than beans on a per-acre basis.

"Going with soybeans the first year out of CRP is probably a good idea," says Peter Scharf, a University of Missouri (MU) research agronomist.

As one of the authors of an MU research paper comparing no-till corn yield response to nitrogen (N) fertilizer following CRP and following soybeans after sod conversion, Scharf says the choice is clear. The researchers found that it took approximately 205 lb. of N per acre to optimize profit on corn after CRP, but only about 110 lb. of N for corn after soybeans following CRP.

University of Kentucky (UK) research on converted CRP fields shows a 0.5% to 1% increase in soil organic matter. "Organic matter contains about 5% nitrogen, which is equivalent to 1,000 lb. of nitrogen per acre for each 1% of organic matter in the top 6" of soil," says Lloyd Murdock, a UK research agronomist.

The decomposing vegetation the first year might tie up some of that residual N in the soil. "The second year after sod, where soybeans were the first crop, more nitrogen is available, which aids in the decomposition of organic matter and the release of even more nitrogen," Murdock adds.
Andrew Cummins, an east central Missouri grain crop farmer, agrees with the bean choice.

Besides having a more flexible planting period with soybeans, he likes taking advantage of the soybean’s N fixing ability while the carbon to N ratio becomes more balanced throughout the first year from sod decomposition. He will be planting an additional 240 acres of prepared sod ground this year, with only 10 acres going into corn "for experimental purposes" and the remainder in soybeans, he says.

Impacting many agronomic considerations is the choice between a no-till seedbed and plowing/disking. "I’ve been on both sides of the fence on this one," Cummins says. Though aware of the benefits of no-till, he continues, "Plowing has always worked better for me—I’ve always had a much better stand the first year out of sod."

Soil secrets. The first weapon in Cummins’ sod-to-soy arsenal is a proper soil test with 20 probes to the acre. "That’s a $20 investment on ground that could gross $500. Everybody thinks you don’t need fertilizer on this ground the first year because it’s already got plenty of reserves [from decomposing sod], and they’re finding that’s completely wrong," says the Santa Fe, Mo., farmer. "For example, some farmers are finding they need 80 lb. to 100 lb. of monoammonium phosphate [or the equivalent] just to get caught up to where they need to be."

pC22 Sod to Soybean Considerations  2
High pH, like low pH, can affect yields. A pH of 7 to 7.3 in the lower right field yielded
30 bu. beans. The field on top yielded 65 bu.

Phosphorus (P) is not the only limiting fertilization factor to consider. "Most hay and pasture fields are potassium [K] deficient," says Chad Lee, UK researcher and Extension agronomist. "Hay removes a lot of K2O from the soil. Without even conducting a soil test, the odds are good that you will need 60 lb. of K2O per acre."

There are mixed views on the efficacy of fertilizer uptake by soybeans on the first year out of sod.

A University of Missouri Extension guide on CRP land to crop conversion states that "contrary to many beliefs, soybeans do respond to pre-plant fertilization on soils low in available phosphorus or potassium."

"Corn responds more to applied fertility, but soybeans respond to soil fertility," says Ken Ferrie, Farm Journal Field Agronomist. He says the difference lies in the ability to take up P and K. "While soybeans are efficient feeders on P, they are less efficient with K. Soils with more clay content can fix available K to soil particles, making it unavailable to the plants, but it can also leach out of sandy soils." In these cases, Ferrie recommends applying K every year.

How much? "If it’s below 100 parts per million or 200 parts per acre, I’d definitely consider adding some K," says MU’s Scharf. "You don’t have to add a lot; just put 30 lb. on and you’re prob-ably not going to lose a bushel. The bottom line is, beans need P and K—it’s just that many fields have enough and the soybeans are not going to respond."

Given a favorable pH environment, the greater the difference in nutrient concentration between the root zone and the soil, the greater the chance of nutrient uptake, ultimately showing a positive response in the plant.

As a legume, soybeans are not as forgiving as corn when it comes to soil pH. "You need to keep [the pH] in the sweet spot—the 6.3 to 6.5 range," Ferrie says. He points out that acid soil also reduces the soybean’s ability to draw N from the soil and interferes with N fixation from the rhizobium bacteria.

The uptake of P and K, along with micronutrients, is also affected negatively. Since soil acidity is only resolved over time, is there anything a farmer can do for this year’s beans if the pH is low? Scharf says that depending on how the lime is applied, beans will respond, but they won’t receive the full benefit the first year.

Cummins will rely on his soil tests to lead the way, but adds that you can use a P enhancer product like Avail so more P in the soil will be free to move into the plant.

Finally, since soybeans require more than 300 lb. of nitrogen per acre, according to UK research, beans should be inoculated with rhizobium bacteria (Bradyrhizobium japonicum) at planting. Lee recommends a liquid inoculation, but says if you can get a good mix with the dry version, it is
just as effective.

"Down here, we mix a little Coca-Cola with the inoculant and the beans to get it to stick better," he says.

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