Continuing drought and lagging crop development in several areas of Nebraska make it especially important for soybean growers to carefully plan their final irrigations and not stop watering too soon, a University of Nebraska engineer said.

Final irrigations need to be based on soybean plant needs and not dates on the calendar, especially for crops planted late, said Paul Jasa, Cooperative Extension engineer in the university's Institute of Agriculture and Natural Resources.

While irrigation has ended for corn, soybean irrigation likely will be under way for the next couple of weeks in the eastern two-thirds of the state, he said.

"Peak moisture needs of soybeans usually follow those of corn by about three weeks," he said. "Most of the soybean crop's water needs are during pod fill. Irrigation will help with pod fill, but not add any more pods."

Jasa said producers often are surprised at how many inches of water are needed to finish out the soybean crop. Timing also is important.

"Shutting off too early reduces bean size, greatly reducing yield," he said. "Irrigating later than necessary reduces room for storing off-season precipitation, increases the potential for leaching nitrogen and adds to production cots."

Finding a balance between the two requires knowing how much water is available in the root zone and how much more water the crop will need to reach maturity. Water requirements to reach maturity depend on the crop and growth stage, he said.

"It's especially important to note that even through soybeans may seem like they are shutting down when they start turning yellow, they still need about two inches of water to reach maturity," he said.

The last irrigation usually can be applied two to four weeks before soybeans are mature, depending on the water-holding capacity of the soil. This will leave room for storing off-season precipitation.

For more information on predicting final irrigations, including determining soil moisture, consult Nebraska Cooperative Extension NebGuide G82-602-A, Predicting the Last Irrigation for Corn, Grain Sorghum and Soybeans, available online at http://ianrpubs.unl.edu/irrigation/g602.htm or G84-690-A, Estimating Soil Moisture by Appearance and Feel, available online at http://ianrpubs.unl.edu/irrigation/g690.htm.

Both also are available at local extension offices.

More details about scheduling the last irrigation, including a worksheet to help determine last irrigation, are available in Crop Watch, extension's crop production newsletter, on the Web at http://cropwatch.unl.edu/archives/2004/crop04-20.htm#irrigation.