People who irrigate commonly make one of two mistakes — sometimes both, says Joe Henggeler, irrigation specialist at the University of Missouri's Delta Center. “They don't start watering early enough. Or they stop too soon.

“Soybeans begin using water quicker than most crops,” Henggeler adds. “That makes early moisture even more critical with soybeans. On the other hand, beans canopy sooner and shade the soil, which slows the evaporation rate.”

Knowing when and how much to water can be a challenge. The most accurate way to schedule irrigation is to use soil moisture sensors such as tensiometers or gypsum blocks. But these can be pricey, ranging from about $250 to more than $500 for one center pivot.

Often, many computer-based scheduling systems seem too complicated to use. But Henggeler has come up with a simple and effective scheduling tool available on the Web. Actually, the Woodruff Irrigation Chart is a computer-based version of a common sense guide developed by C. M. (Woody) Woodruff, Missouri soils scientist, back in the 1960s.

“The computer program is Missouri-based, but growers in adjacent states within the 36-40° latitude belt and with about the same climate can use it,” says Henggeler. “After you open the program, click on the type of crop you're growing. From there, the program walks you through the steps: crop emergence date, relative maturity, soil type, individual field, county (for Missouri growers) and irrigation method.”

Using historical weather data and up-to-date crop coefficients, the program then provides a grower with estimates of optimum irrigation depth and water uptake by that particular crop. The computer builds a chart customized specifically for each field entered.

A green line across the graph paper chart plots the cumulative water usage of the crop. A red line just below signals when irrigation is needed.

You'll also need rain gauges to measure precipitation in each field. To determine when a field needs irrigation, check your gauge and plot the rainfall amount on the chart. The chart acts as a sort of water “bank account,” with rainfalls and irrigation applications entered as credits. When moisture falls short and approaches the chart's red line, turn on the water.

“This chart is sensitive to overwatering on claypan soils, too,” he says. It also allows for drying of the soil before harvest, so combines don't get hung up.

The Woodruff Chart doesn't work as well on sandy soils, where you'll need to irrigate frequently and put on a lot of water each time, he adds.

And, you need to determine the rainfall total that's effective for your field, not necessarily the total rainfall. “If you've just applied an inch of irrigation, then get a 2" rain, you cannot credit that field with 3" of moisture, because some of it will have run off. However, we rarely get rainfalls of 2" or more in the summer,” Henggeler says.

Over the past several years in Missouri, corn has been the biggest winner when it comes to irrigation — nearly a 50-bu/acre increase over dryland corn. Soybeans gain about 11 bu/acre from irrigation.

“Irrigating growers are becoming better water managers,” says Henggeler. “They are watering earlier — and later — in the season. That gives lenders more peace of mind, too.”

The Internet chart, funded in part by the Missouri Soybean Merchandising Council, can be found at http://agebb.missouri.edu/irrigate/woodruff.