Promising precision irrigation of spuds needs more study to work out economic kinks



By Marni Katz

Site-specific precision agriculture has been successfully used to improve efficiency of several inputs, from fertility to pest control, in the Western United States. At the University of Idaho, researchers would like to extend those benefits to the number one input for high-value crops grown in arid regions--water.

Associate professor Bradley King has been studying site-specific irrigation management on potatoes under center-pivot irrigation since he arrived at the University of Idaho's Aberdeen Research and Extension Center in 1992.

He says irrigation water and water-run inputs are the most logical use of precision ag technology in potatoes. And he continues to study ways to make site specific irrigation management pay for growers on a commercial level.

Center pivots are the most commonly used irrigation system for potato production in the Pacific Northwest and Intermountain West. However, conventional center-pivot systems irrigate fields uniformly without consideration for variability in crop water requirements throughout the field.

That variability, which could result from factors such as soil type, slope or crop vigor, can ultimately lead to spatial differences in reduced tuber quality and yield. The goal of site-specific water management, King says, is not so much to influence the amount of water applied, but to influence the timing of applications so that they match crop water requirements under different variables and real-time moisture availability.

Logical first step

One of the challenges to researching the use of variable-rate water and fertilizer applications in high-value crops, such as potatoes, is that most of the applications are done in-season, unlike most current precision ag applications, such as preplant fertilizer and soil amendments, that are preseason. That makes the irrigation system a logical first step in studying precision ag in crops like potatoes.

"If you are going to use precision ag in potatoes, it is going to have to be through the irrigation systems," King says. "We are trying to achieve a system where you could site-specific manage water and hopefully cause an effective increase in quality and perhaps yield. The problem is you are working on a pretty thin margin with water management, and there are so many other things that affect crop quality and yield."

Potatoes suited to site-specific irrigation

Idaho accounts for about 25 percent of the U.S. potato production in the fall, and growers typically grow potatoes under continuous-run center-pivot irrigation systems. King says most Idaho potatoes are produced under sprinklers, and at least 60 percent of that sprinkler acreage is irrigated with center-pivot systems.

He says potatoes could be well suited to precision ag benefits, because they are highly responsive to water and fertilizer inputs. Center-pivot irrigation systems are also well suited to variable-rate applications, because they are already automated and convert fairly easily to site-specific irrigation.

In King's study, the center-pivot irrigation system was equipped with a variable-rate irrigation control system consisting of two sprinkler nozzles with individual solenoid valves along the center-pivot lateral.

The valves, which irrigated at exponentially stepped rates of 0X, 1X, 2X and 3X were opened and closed based on data from soil moisture, water application and environmental data gleaned from in-field probes, crop history and estimated water use. Information is sent to an onboard computer and is used to calculate estimated water needs on a real-time basis in individual irrigation management zones.

The computer then helps determine how much irrigation water to apply via the two sprinkler heads to each individual management zone.

Compared to the continuous-irrigation management systems done side by side, the variable-rate technology allowed King to fine tune irrigation water applications within 18 irrigation management zones of a 7-acre quadrant.

Questionable return

After several years of study, King says the jury is still out on whether site-specific irrigation management can be economically viable on a commercial scale in potatoes. While the 2001 study did show that total yields, marketable yields and gross income increased under site-specific irrigation compared to conventional uniform center-pivot irrigation, it remains to be seen whether growers can realize enough economic return from site-specific irrigation management to justify the investment in technology commercially.

According to King's 2001 study, there was a slight trend in improved gross income of about $65 per acre using site-specific management to irrigate potatoes. While not statistically significant, King says that consistent improvement in gross income is encouraging. However, continued research and development to help reduce the capital and operational costs of site-specific irrigation management could make the practice more economically viable.

"So far, irrigation recommendations haven't resulted in significant differences in yield and quality," King says. "The trend is there, but I'm not sure it is statistically different or ever will be because you've got so many other factors beside water that can affect yield and quality."

Still, he believes there could be benefits to precision ag in potatoes if growers and researchers can find a way to boost gross return beyond the cost of initial investment either by finding cheaper ways to variably apply inputs or increasing the quality benefits and overall returns.

"Overall, depending on the field and how much variability there is in the field, there are some advantages of site specific application, but it comes at a high cost," King says. "Whether it will ever be economically feasible I have doubts, although from an environmental point of view it would be beneficial."

King is expanding the study this year to explore the benefits of variable-rate application of nitrogen and other fertilizers through the center pivot system. He says it's an easy step to convert site-specific irrigation systems to fertigation management since most of the hardware is already in place.

Water-soluble nitrogen applications will be applied site specific based on petiole analysis through the field. Commercially, growers would likely use remote sensing maps to fine-tune fertilizer applications.

"Our hope is to have more precise control of the two largest things that affect plant growth--water and nitrogen--to see if we can leverage some return out of the cost of precision agriculture equipment," King says.