A 3.5 acre pomegranate orchard at the Kearney Ag Center in Parlier, Calif. has borne more fruit than the big red orbs that pull branches downward and reflect the sun falling on them.
By studying what has gone on there for six years using a lysimeter and other complex equipment to measure water loss and nutrient applications, researchers can point to decided advantages of high frequency subsurface drip irrigation (SDI) over high frequency surface drip.
U.S. Department of Agriculture researcher James Ayars said he and others made these and other findings in a study on improving pomegranate fertigation and nitrogen use efficiency in which both systems of irrigation were used:
- The SDI system minimized nitrate leaching;
- SDI showed higher nitrogen use efficiency than drip;
- SDI used water more efficiently than drip;
- SDI resulted in lower weed populations than drip; and
- The two systems showed no significant differences in fruit and juice quality.
The overall objective of research was to optimize water-nitrogen interactions to improve fertilizer use efficiency of young and maturing pomegranate trees.
Other objectives were aimed at determining the real time seasonal nitrogen requirements without reducing yields. Three nitrogen injection treatments were followed: a 50 percent level, 100 percent, and 150 percent.
Another goal was determining if concentrations of macronutrients (phosphorous, potassium, calcium, and magnesium) and micronutrients (zinc, copper, manganese, iron, boron, and selenium) in the soil, peel, and fruit were influenced by precise irrigation and fertigation management.
Claude Phene, an irrigation and soil physics consultant from Clovis, played a significant role in guiding the six-year research effort.
It started with the first two years spent installing monitoring and control equipment, establishing the trees and working out problems with the operation of the fertilizer injection equipment, and the operation of the lysimeter.
During the field day, Phene opened the cover of the lysimeter, brushed away some cobwebs and invited anyone who wanted to climb down a ladder leading about 12 feet to the bottom.
There were no takers.
The 82-year Phene, who retired from the USDA after serving as director of the Water Management Research Laboratory in the region, has climbed down into the lysimeter to check instruments about a dozen times a year since the project started.
Phene said the lysimeter is one of very few in the state. Essentially a truck scale, it reads weight loss that shows evaporation and drainage.
“In the past five years, it has shown no leaching,” Phene said, adding that – given it is home to a single tree watered in an area just under half the size of areas that harbor other trees – “that means there was no leaching for the other trees.”
Leaching has taken on greater significance as the state moves to cut down on the amount of nitrates leaching through the soil and into the underground water supply, Phene said.
The lysimeter, which has eight soil moisture sensors at various depths, traces evaporation to within a fraction of a millimeter, Phene said. He had a hand in its installation in 1986.
All drip lines – subsurface as well as surface – were placed within 3 and 3.5 feet of each side of trees. Subsurface drip was placed 20-22 inches underground.
Among researcher’s findings was the fact that watering frequently is more effective than watering for long periods because it cuts down on “gravitational water loss,” Phene said.
Water's matric potential
He believes it is important for growers to understand “matric potential of soil water,” the movement of water within the soil and the differences between gravitational force (whereby water moves downward) and capillary force (in which water moves upward, downward, and to both sides).
To combat salinity, Phene said growers should be wary of putting on the cheapest source of potassium - potassium chloride. “It adds to salinity,” he said.
It is better, he added, to use potassium chloride or potassium thiosulfate.
Phene is also wary of recycling water.
“Every time you do that, you lose quality,” he said. “If you recycle four or five times, salts go up four or five times.”
A better solution on salinity, he explained, is to minimize the amount of water put into the soil.
“If you put in twice as much, you get twice as much salt,” he said. “Also, watch what kind of fertilizer you are putting into the soil.”
About 35 inches of water was applied through surface drip on the plot, 32 inches through subsurface.
Surface drip more vulnerable
Phene said among challenges of using surface drip is that it’s more vulnerable to damage that can come when equipment passes over it or when gophers, squirrels, and other critters chew on it.
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Over the six years for the project, precipitation in inches has dropped significantly, from 17.34 inches in 2010 to 3.7 inches by mid-October 2015. In 2013, precipitation was still lower at 3.21 inches.
Paramount Farming Company, which provided trees for the research plot, was among the contributors for the project.
Dennis McCoy, a research specialist for Wonderful Orchards in Shafter, which grows pomegranates for Paramount, which recently rebranded itself as The Wonderful Company, said he was impressed with the precision of the systematic measurements of fertilizer and water applied at the test plot.