Drop by Drop

The 20,000-square-foot test plot of Gardenia augusta ‘Heaven Scent’ at McCorkle Nurseries.

Savings of $1 per square foot realized in a “problem crop” through the use of precision, soil-moisture based irrigation.

While it may sound like a late-night commercial, the title of this article is in fact a reality that has been documented as part of a multi-state USDA Specialty Crops Research Initiative (SCRI) project (www.smart-farms.net/).

Precision irrigation can boost profits in several ways including: better control over crop growth and quality; 2) reduced fertilizer application and leaching; 3) lowered disease pressure; and reduced irrigation application.

Researchers at the University of Georgia, led by Marc van Iersel, are working with colleagues from four other universities and two industry partners to better understand how precision irrigation can lead to increased efficiencies in growing, and in particular irrigating, ornamental crops.

Precision irrigation refers to the ability of a grower to monitor and precisely control when irrigation is applied to crops in real-time, based on a minimum amount of water in the substrate. Basically, when plants have used a certain amount of water, the irrigation is automatically turned on via a solenoid valve to resupply the crop. Using this approach, plants only get watered when needed, and only with the amount of water required to rewet the substrate. For example, if a crop is prone to root pathogens, such as Gardenia augusta, a grower can more precisely keep the substrate on the “dry side” to avoid an outbreak of Phytopthora in the crop.
 

 
Now let’s talk dollars and cents. In 2010-11, van Iersel and colleagues at UGA implemented a precision irrigation system at McCorkle Nurseries in Dearing, Ga., on a “problem crop.” Many growers in the Southeastern United States suffer significant losses in G. augusta, with typical shrinkage of 10-30 percent — with some losses as high as 70 percent — due to root pathogens, associated mortality and reductions in quality. Our hope was that precision irrigation would prevent overwatering and therefore reduce shrinkage due to root pathogens. This study was conducted on a block of 23,400 No. 2 G. augusta ‘Heaven Scent,’ divided into two treatments grown in a 20,000-square-foot area. The first treatment was irrigated using the nursery’s standard irrigation practices. The second treatment utilized a precision irrigation control system that irrigated the crop using a set point of 18 percent, meaning that 18 percent of the volume of the container is taken up by water. Based on previous studies with Hydrangea macrophylla in the same production facility, researchers expected to see water savings as high as 83 percent over standard irrigation practices. Yet somehow irrigation volumes were within 5 percent between the two treatments. The reason: the irrigation technician observed when and how long the precision irrigation control system irrigated, and mimicked the precision control system in the standard irrigation practices treatment he was charged with irrigating. While not the best scenario for determining differences between standard irrigation practices and a precision irrigation control system, there were still some amazing outcomes. There was zero loss across the entire 23,400 units from pathogen pressure. The projected losses were 2,000 plants; therefore at an industry standard sales price of $6.50 per plant, profits increased $13,000 due to a lack of shrinkage. Even more impressive is that the production period was shortened to 8-11 months rather than the typical 11-22 months. On average, plants were sold six months earlier than projected. The reduction in production time resulted in plants not requiring the final controlled release fertilizer treatment. Simply shortening the production period reduced production costs, including fertilizer, pesticides and routine plant maintenance, by $7,700. Finally, the reduction in growing time lowered the interest cost associated with growing plants in inventory by approximately $500, assuming simple interest at a rate of 8 percent per year. The total increase in profits from reduced production time and elimination of shrinkage thus totaled $21,200, corresponding to $1.06 savings per square foot per production cycle, or $0.90 per plant. This calculation does not include the value of the opportunity associated with being able to start a new crop in the same growing area six months earlier than projected; meaning benefits would be even greater than we calculated. At this level of savings, the $3,000 precision irrigation system we used in this study would have a payback period of less than two months. When applied at a larger scale, the cost of the system is likely to be substantially lower when amortized on a per square foot or per unit basis, so better control irrigation clearly can be cost-effective.

TOP: Consider the value of the opportunity associated with being able to start a new crop in the same growing area six months earlier than projected. BOTTOM: Researchers are testing hardware associated with precision irrigation.

Make it happen
So how can you apply this to your nursery? As part of this project, Decagon Devices Inc., our industry partner, is continuing to develop and improve the hardware that is being used in this project, while Carnegie Mellon University is developing the software interface for wireless sensor networks. These wireless networks can measure a wide range of environmental conditions, including PAR, weather information and substrate moisture content. The sensors open and close irrigation valves based on substrate moisture readings. Instead of a grower visually inspecting a plant or block of plants to determine when irrigation is necessary, the sensors placed in the substrate determine when to irrigate. The goal is that these networks will be; 1) economical, 2) durable in indoor and outdoor production environments, 3) easy to set up and operate, 4) easily moved from one block to another in the production area, 5) able to connect to a computer or handheld device wirelessly and 6) reliable and provide accurate measurements in real-time. We are currently testing prototypes of the hardware and early versions of the supporting software. A nice feature of the software is that it is web-based, allowing users to access the system from anywhere with internet access, including smartphones.

Matthew Chappell is assistant professor and extension horticulturist, nursery production, UGA, hortprod@uga.edu; Marc van Iersel is professor, plant nutrition and physiology, UGA, mvanier@uga.edu; John Ruter is horticulture professor, UGA, ruter@uga.edu; Erik Lichtenberg is profeesor, agricultural and resource economics, University of Maryland, College Park, elichtenberg@arec.umd.edu; John Majsztrik is plant science grad student, University of Maryland, jcmajsz@umd.edu; and Paul Thomas is horticulture professor, UGA, pathomas@uga.edu.