Scientists study irrigation practices

Kyle Peveto, Gould, Frances I., Blanchard, Tobie M.  |  9/12/2018 7:58:25 PM

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Water sprays from a pipe into the furrows of a field at Red River Research Station in Bossier City. Photo by Stacia Davis

As irrigation has grown more common on Louisiana farms, LSU AgCenter researchers are studying the most efficient and beneficial ways to apply water to crops.

While farmers implement long-term fixes to improve the soil’s water-holding capacity — such as planting cover crops and experimenting with poultry litter as fertilizer — irrigation is an immediate solution in dry times.

“It is more like an insurance plan for a dry year when the soil can’t hold the little rainwater we get in a dry year,” Davis said.

AgCenter scientists are studying which methods of irrigation can save water without sacrificing crop yields. They also are evaluating if soil moisture sensors can be used to schedule irrigation in Louisiana.

Davis is comparing two major types of soil moisture sensors available to producers. Volumetric water content sensors estimate the amount of water in the soil, and soil water potential sensors estimate the pressure the plant must use to pull water from the soil, Davis said.

“The sensors are basically a tool to tell you how much moisture is in the ground,” Davis said. “It’s up to the person using the sensor to interpret it.”

The study analyzes soybeans irrigated on schedules determined by the two types of sensors and another method, evapotranspiration, which relies on climate data and a mathematical formula to decide when to irrigate and how much water to apply. These three methods are compared against non-irrigated soybeans.

“This is a really good irrigation year because it is so dry in our area,” Davis said. “We are plugging along and getting everything out, so we are going to get some good results.”

In dry years sensors can help farmers save money by running pumps less and using less water, but the sensors have benefits in wet years, too, Davis said.

“Most of the crops we grow, like soybeans, don’t like to stay wet for too long,” Davis said. “It’s a delicate balance — understanding when it’s too wet and when that can affect your root growth.”

Sensors can show where roots are pulling water from, and farmers can use sensors to estimate root growth. They can also indicate soil compaction.

“They are basically your eyes under the soil surface,” she said.

Focused mainly above the soil surface, AgCenter agronomist Syam Dodla is studying the effects of different methods of irrigation and how to manage fertilizers with irrigation.

His research has found that skip-row irrigation, in which farmers only irrigate every other furrow in the field, doubles the area a farmer can irrigate at one time with a well and can help to minimize nutrient losses and soil runoff. The practice had no significant effect on soybean crop yields in clay soils, Dodla said. However, in very fine sandy loam soils, yields dropped from 6 to 46 percent, depending on the number of times the field was irrigated, the duration of dry spells and at which point in the crop growth stage the field was irrigated.

“It is also important to remember that our observations were from 40-inch-wide rows,” Dodla said. “The smaller row width could minimize the yield drop from skip-row irrigation in these light-textured soils.”

Dodla’s data show skip-row irrigation has no significant effect — and might have a small beneficial effect — on nutrient-use efficiency in clay soils. But sandy loam soils had a 10 to 12 percent drop in nutrient use efficiency.

“This result was because of lack of sufficient soil moisture in the non-irrigated furrow, which is essential for nutrient uptake by roots,” Dodla said. “The effect varies based on the length of time between irrigation events.”

In his research Dodla has found that overall soybean nutrient uptake is 22 percent better with irrigation.

“Many times lack of soil moisture leads to poor nutrient-use efficiency,” Dodla said. “This should be similar for any crop. Based on the duration of dry spells and the stage of the crop, the reported number could vary.”

Because irrigation is more common among soybean and feed grains farmers in Louisiana, producers also must adjust their fertilizer applications when irrigating fields, Dodla said.

“High vegetative growth and grain yields under good soil moisture availability necessitate higher nutrient needs,” he said. “Because of this, LSU AgCenter’s current fertilizer recommendations are higher for irrigated agriculture compared to non-irrigated.”

Kyle Peveto

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